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What did he do?
Paracelsus introduced the use of mineral substances, as well as chemical substances into medicine. He distinguished the difference between toxicity and treatment, and theorized the constitution of matter. This was based on three elements: salt, sulphur, and mercury.

Why was he important?
He was important because he heavily contributed to the substantial rise of modern day medicine.

Why should we remember him?
We should remember Paracelsus because he made an attempt to completely overthrow tradition and try to find actual causes and cures for harmful diseases.

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Anders Celsius was a Swedish astronomer who is most well known for his observations and research on aurora borealis. He also developed the temperature scale that would go on to be known as the Celsius Scale. He is very important to study and learn from, as he revolutionized temperature scaling by developing the scale that almost everyone around the world uses.

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Rene Descartes was a French Philosopher, scientist, and mathematician. He discovered the possibility of analytic geometry as well as others. He was known best for his famous phrase, "I think, therefore I am".

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Watch: The legacy of William Harvey, a forgotten pioneer in human anatomy

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A video from the World Science Festival tells the story of William Harvey, a forgotten pioneer in human anatomy

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Sir Isaac Newton was an English mathematician who was a key figure in the Scientific Revolution and the Enlightenment that followed, Newton discovered the 3 laws of motion but he is most famous for discovering the law of gravity when he was sitting under an apple tree and one apple fell beside him. He also invented many other little things like the reflecting telescopes, cat doors, and calculus things we still use today.

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La femme derrière Lavoisier – Marie-Anne Lavoisier – #Mini-bio 39

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The Man Who Discovered Oxygen - mysimpleshow.com

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First published Wed Dec 3, 2008; substantive revision Mon Oct 23, 2023
René Descartes (1596-1650) was a creative mathematician of the first order, an important scientific thinker, and an original metaphysician. During the course of his life, he was a mathematician first, a natural scientist or "natural philosopher" second, and a metaphysician third. In mathematics, he developed the techniques that made possible algebraic (or "analytic") geometry. In natural philosophy, he can be credited with several achievements: the first to publish the sine law of refraction; developer of an important empirical account of the rainbow; and proposer of a naturalistic account of the formation of the earth and planets (a precursor to the nebular hypothesis, that the planets formed from loose matter orbiting the sun). More importantly, he offered a new vision of the natural world, which shaped modern physics: a world of matter possessing a few fundamental properties and interacting according to a few universal laws. This natural world included an immaterial mind that, in human beings, was directly related to the brain, a position that led to the modern mind-body problem. In metaphysics (the search for the basic principles of everything there is), Descartes provided arguments for the existence of God and to show that the essence of matter is to be spatially extended, and that the essence of mind is thought (where "thought" includes sensory images as well as rational discourse). Descartes claimed early on to possess a special method, which was variously exhibited in mathematics, natural philosophy, and metaphysics, and which came to include, or to be supplemented by, a method of doubt.

Descartes presented his results in major works published during his lifetime: the Discourse on the Method (in French, 1637), with its essays, the Dioptrics, Meteorology, and Geometry; the Meditations on First Philosophy (i.e., on metaphysics), with its Objections and Replies (in Latin, 1641, 2nd edn. 1642); the Principles of Philosophy, covering his metaphysics and much of his natural philosophy (in Latin, 1644); and the Passions of the Soul, on the emotions (in French, 1649). Works published posthumously included his Compendium of Music (in Latin, 1650), Letters (in Latin and French, 1657-67); World, or Treatise on Light, containing the core of his natural philosophy (in French, 1664); Treatise on Man (in French, 1664), containing his physiology and mechanistic psychology; and the Rules for the Direction of the Mind (in Latin, 1701), an early, unfinished work attempting to set out his method.

Descartes' works have been variously received and valued. Among the learned of his day he was considered to be a top mathematician, the developer of a new and comprehensive physics or theory of nature (including living things), and the proposer of a new metaphysics. In the years following his death, his natural philosophy was especially valued and discussed. His works were invoked in debates over the equality of women. In the eighteenth century, aspects of his science remained influential, as did his project of investigating the cognitive capacities of the knower in assessing the possibility and extent of human knowledge. He was also remembered for his skeptical arguments and for failing to provide a successful response to them in his metaphysics. In the nineteenth century, he was revered for his mechanistic physiology and theory that animal bodies are machines (are constituted by material mechanisms, governed by the laws of matter alone). The twentieth century variously celebrated his famous "cogito" starting point, reviled the sense data that some alleged to be the legacy of his skeptical starting point, and looked to him as a model of the culturally engaged philosopher. He has been variously seen as a hero and a villain; as a brilliant theorist forging new directions in thought and as harbinger of a cold, rationalistic, and calculative conception of human beings. Those new to the study of Descartes should engage his own works in some detail prior to developing a view of his legacy.

1. Intellectual Biography

1.1 Early life and education

Descartes was born on 31 March 1596 in his maternal grandmother's house in La Haye, in the Touraine region of France. His father Joachim, a lawyer who lived in Châtellerault (22 kilometers southwest of La Haye, across the Creuse River in the Poitou region), was away at the Parliament of Brittany in Rennes. The town of La Haye, which lies 47 kilometers south of Tours, has subsequently been renamed Descartes.

When Descartes was thirteen and one-half months old, his mother, Jeanne Brochard, died in childbirth. The young René spent his early years with his grandmother, Jeanne Sain Brochard, in La Haye, together with his older brother Pierre and older sister Jeanne. It is likely that he then moved to the house of his great uncle, Michel Ferrand, who, like many of René's male relatives, was a lawyer; he was a Counselor to the King and held the royal office of provincial Lieutenant General in Châtellerault. When Descartes met Isaac Beeckman in 1618, he introduced himself as "Poitevin," or from Poitou ( 10:46, 51-4; Rodis-Lewis 1998, 3, 26; see also 2:642). At this time (and now and again later on), he also signed letters as "du Perron" and called himself "sieur du Perron" (Lord of Perron), after a small farm in Poitou he inherited from his mother's family (Watson 2007, 81, 230). But he did not neglect his birthplace in La Haye: in a letter of 1649, he described himself as "a man who was born in the gardens of Touraine" (5:349).

In 1606 or 1607, Descartes, a Roman Catholic, entered the newly founded Jesuit College of La Flèche, where he remained until 1614 or 1615. He followed the usual course of studies, which included five or six years of grammar school, including Latin and Greek grammar, classical poets, and Cicero, followed by three years of philosophy curriculum. By rule, the Jesuit philosophy curriculum followed Aristotle; it was divided into the then-standard topics of logic, morals, physics, and metaphysics. The Jesuits also included mathematics in the final three years of study.

Aristotle's philosophy was taught through textbooks and printed commentaries on his works. Aristotle himself frequently discussed the positions of his ancient predecessors. The most extensive commentaries also elaborated in some detail on positions other than Aristotle's. Within this framework, and taking into account the reading of Cicero, Descartes would have been exposed in school to the doctrines of the ancient atomists, Plato, and the Stoics, and he would have heard of the skeptics. Further, important intellectual events were celebrated at La Flèche, including Galileo's discovery of the moons of Jupiter in 1610. Although scholastic Aristotelian philosophy was dominant in Descartes' school years, it was not the only philosophy that he knew.

Famously, Descartes wrote in the autobiographical portion of the Discourse (1637) that, when he left school, "I found myself beset by so many doubts and errors that I came to think I had gained nothing from my attempts to become educated but increasing recognition of my ignorance" (6:4). And yet, he continued, he did not "cease to value the exercises done in the schools" (6:5), for languages, fables, oratory, poetry, mathematics, morals, theology, and philosophy all had their value, as did jurisprudence, medicine, and other sciences (including engineering) that serve as professions and which one might study after attending a school such as La Flèche. He noted the contradiction and disagreement that beset philosophy and so infected the higher sciences (including medicine) "insofar as they borrow their principles from philosophy" (6:8). A year later, in 1638, he told an inquiring father that "nowhere on earth is philosophy taught better than at La Flèche," where he advised his correspondent to send his son even if he wanted him subsequently to transcend the learning of the schools-while also suggesting that the son might study at Utrecht with Henry le Roy, a disciple of Descartes (2:378-9). According to the Discourse, it was not surprising that philosophy as taught at La Flèche was uncertain: it had to be, since he (Descartes) was now offering a first glimpse of the one true philosophy, only recently discovered. Until it could be promulgated, La Flèche, or another good school, would do.

His family wanted Descartes to be a lawyer, like his father and other relatives. To this end, he obtained a law degree from Poitiers in 1616. But he never practiced law or entered into the governmental service that such practice would have made possible (Rodis-Lewis 1998, 18-22). Instead, he became a gentleman soldier, moving in 1618 to Breda, to support the Protestant Prince Maurice against the Catholic parts of the Netherlands (which later constituted Belgium), which were controlled by Spain-a Catholic land, like France, but at this point an enemy.

1.2 First results, a new mission, and method

Perhaps at this time, though certainly by 1628, Descartes had the fundamental insight that makes analytic geometry possible: the technique for describing lines of various sorts by using mathematical equations involving ratios between lengths in relation to coordinate lines. (Descartes did not require that the lines be perpendicular to one another.) Descartes himself did not foresee replacing geometrical constructions with algebraic formulas; rather, he viewed geometry as the basic mathematical science and he considered his algebraic techniques to provide an extension beyond compass-and-ruler constructions. When the right-angled coordinate system of algebraic geometry was subsequently developed, the name "Cartesian coordinates" honored Descartes' discovery. (See Boyer 1968, Ch. 17, Shea 1991, Ch. 3, and, more generally, Domski 2022.)

Descartes left Breda in 1619 to join the Catholic army of Maximilian I (Duke of Bavaria and ally of France). The war concerned the authority of Ferdinand II, a Catholic, who in September had been crowned emperor of the Holy Roman Empire (located in Central Europe and including Austria and parts of northern Italy). Descartes attended the coronation and was returning to the army when winter caught him in the small town of Ulm (or perhaps Neuburg), not far from Munich. On the night of 10 November 1619, he had three dreams that seemed to provide him with a mission in life. The dreams themselves are interesting and complex (see Sebba 1987). Descartes took from them the message that he should set out to reform all knowledge. He decided to begin with philosophy, since the principles of the other sciences must be derived from it ( Disc. II, 6:21-2).

Descartes' activities during the early 1620s are not well-documented. He was in France part of the time, visiting Poitou to sell some inherited properties in 1622 and visiting Paris. He also went to Italy (1623-25). Upon his return he lived in Paris, where he was in touch with mathematicians and natural philosophers in the circle of his long-time friend and correspondent Marin Mersenne (1588-1648). While in Paris, he worked on some mathematical problems and derived the sine law of refraction, which facilitated his work on formulating mathematically the shapes of lenses (later published in the Dioptrics). His major philosophical effort during these years was on the Rules.

In the Rules, he sought to generalize the methods of mathematics so as to provide a route to clear knowledge of everything that human beings can know. His methodological advice included a suggestion that is familiar to students of elementary geometry: break your work up into small steps that you can understand completely and about which you have utter certainty, and check your work often. But he also had advice for the ambitious seeker of truth, concerning where to start and how to work up to greater things. Thus, Rule 10 reads: "In order to acquire discernment we should exercise our intelligence by investigating what others have already discovered, and methodically survey even the most insignificant products of human skill, especially those which display order" (10:403). As examples of "simple" arts "in which order prevails" (10:404), he offered carpet-making and embroidery, and also number-games and arithmetic games. He went on to discuss the roles of the "cognitive faculties" in acquiring knowledge, which include the intellect, imagination, sense perception, and memory. These faculties allow the seeker of knowledge to combine simple truths in order to solve complex problems as found in optics (10:394) or to discover how a magnet works (10:427).

By the end of 1628, Descartes abandoned work on the Rules, having completed about half of the projected treatise. He moved to the Dutch Netherlands, returning to France only infrequently prior to moving to Sweden in 1649. While in the Netherlands, he tried to keep his address secret and changed locations often, according to his motto, "who lives well hidden, lives well" (1:286*).

Descartes spent his first year in the Netherlands in Franeker, where he registered at the University in April, 1629. Initially, he worked on two topics: mathematical science and metaphysics. In Summer, 1629, an impressive set of parhelia, or false suns, were observed near Rome. When Descartes heard of them, he set out to find an explanation. (He ultimately hypothesized that a large, solid ice-ring in the sky acts as a lens to form multiple images of the sun [ Met. 6:355].)

While working on the parhelia, Descartes conceived the idea for a very ambitious treatise, about which he wrote Mersenne on 16 November 1629 from his new residence in Amsterdam. He now intended to explain not "just one phenomenon" (the parhelia), but "all the phenomena of nature, that is to say, the whole of physics" (1:70). This work eventually became The World, which was to have three parts: on light (a general treatise on visible, or material, nature), on man (a treatise on physiology), and on the soul. Only the first two survive (and perhaps only they were ever written), as the Treatise on Light and Treatise on Man. In these works, which Descartes decided to suppress upon learning of the condemnation of Galileo (1:270, 305), he offered a comprehensive vision of the universe as constituted from bare matter possessing the properties length, breadth, and depth (three-dimensional volume) and carved up into particles with size and shape, which may be in motion or at rest, and which interact through quantitative laws of motion that are formulated and sustained by God ( Light, 11:33-4). These works described the visible universe as a single physical system in which all operations, from the formation of planets and the transmission of light from the sun to the physiological processes of human and nonhuman animal bodies, can be explained through the mechanism of matter arranged into shapes and structures and moving according to three laws of motion. In fact, his explanations in the World and the subsequent Principles made little use of the three laws of motion in other than a qualitative manner. The laws sustained the notion that matter moves regularly (in a straight line) and that upon impact bits of matter alter their motions in regular ways-something that happens constantly in the full universe (the "plenum") conceived by Descartes. (At this time, Descartes received an offer of appointment to the medical faculty in Bologna-even though he had not yet made public any of his results-which he declined; see Manning 2014.)

After suppressing his World, Descartes decided to put forward, anonymously, a limited sample of his new philosophy, in the Discourse with its attached essays. The Discourse recounted Descartes' own life journey, explaining how he had come to the position of doubting his previous knowledge and seeking to begin afresh. It offered some initial results of his metaphysical investigations, including mind-body dualism. It did not, however, engage in the deep skepticism of the later Meditations, nor did it claim to establish, metaphysically, that the essence of matter is extension. This last conclusion was presented merely as a hypothesis whose fruitfulness could be tested and "proven," or perhaps merely confirmed (see McMullin 2008), by the results contained in the attached essays on Dioptrics and Meteorology (the latter covering "atmospheric" phenomena).

In his Meteorology, Descartes described his general hypothesis (or supposition) about the nature of matter, before offering accounts of vapors, salt, winds, clouds, snow, rain, hail, lightning, the rainbow, coronas, and parhelia. His hypothesis was as follows:

that the water, earth, air, and all other such bodies that surround us are composed of many small parts of various shapes and sizes, which are never so properly disposed nor so exactly joined together that there do not remain many intervals around them; and that these intervals are not empty but are filled with that extremely subtle matter through the mediation of which, I have said above, the action of light is communicated. (6:233)

He presented a corpuscularian basis for his physics that denied the atoms-and-void theory of ancient atomism and affirmed that all bodies are composed from one type of matter, which is infinitely divisible into particles or corpuscles (6:239). He here also proclaimed that his natural philosophy had no need for the "substantial forms" and "real qualities" that other philosophers (i.e., Aristotelians) "imagine to be in bodies" (6:239). He had taken the same position in the Treatise on Light, where he said that in conceiving his new "world" (i.e., his conception of the universe), "I do not use the qualities called heat, cold, moistness, and dryness, as the Philosophers do" (11:25).

In effect, he was denying the then-dominant scholastic Aristotelian ontology, which explained all natural bodies as comprised of a "prime matter" informed by a "substantial form," and which explained qualities such as hot and cold as really inhering in bodies in a way that is "similar" to the qualities of hot and cold as we experience them tactually. According to the Aristotelian explanation, the qualities that are proper to each sense-color, sound, odor, tastes, and tactual qualities-are really inherent in things, and our experienced sensations resemble these qualities in things. As regards prime matter, many scholastic Aristotelians held that it cannot exist on its own; to form a substance, or something that can exist by itself, prime matter must be "informed" by a substantial form (a form or active principle that gives a substance its essential properties). The four Aristotelian elements, earth, air, fire, and water, had substantial forms that combined the basic qualities of hot, cold, wet, and dry: earth is cold and dry; air is hot and wet; fire is hot and dry; and water is cold and wet. These elements can themselves then serve as "matter" to higher substantial forms, such as the form of a mineral, or a magnet, or a living thing. Whether in the case of earth or of a living thing, such as a rabbit, the "form" of a thing directs its characteristic activity. For earth, that activity is to approach the center of the universe; water has the same tendency, but not as strongly. Accordingly, Aristotelians explained, the planet earth exists at the center, with water on its surface. A new rabbit is formed when a male rabbit contributes, through its seed-matter, the "form" of rabbithood to the seed-matter of the female rabbit. This form then organizes that matter into the shape of a rabbit, including its various organs and physiological processes. The newborn rabbit's behavior is guided by its "sensitive soul," a rabbit-specific instance of the type of substantial form possessed by all animals. Other properties of the rabbit, such as the whiteness of its fur, are explained by the "real quality" of white inhering in each strand of hair.

Although in the World and Meteorology Descartes avoided outright denial of substantial forms and real qualities, it is clear that he intended to deny them ( Corr. 1:324; 2:200; 3:420, 500, 648). Indeed, he claimed that he could explain these qualities themselves through matter in motion ( Light 11:26, Met. 6:235-6). As an example, he explained color in things as a property of surfaces that puts a spin on particles of light, which in turn affect the nerves in the retina, which then affect the brain, causing a sensation in the mind (via matter's effect on the immaterial mind). The sensation of a color (such as red) does not resemble the surface property that causes the effect on the nerve that produces the sensation. The experienced red is an apparently arbitrary sign or signal for a surface property in objects ( Light I; see also Dioptrics VI, 6:130, and Princ. I.68-70).

Two considerations help explain Descartes' tentative language with respect to substantial forms and real qualities. First, when he wrote these works in the 1630s, he was not yet prepared to release his metaphysics, which would support his hypothesis about matter and so rule out substantial forms (1:563). Second, he was sensitive to the prudential value of not directly attacking the scholastic Aristotelian position (3:298), since it was the accepted position in university education (3:577) and was strongly supported by orthodox theologians, both Catholic and Protestant (1:85-6; 3:349).

In 1635, Descartes fathered a daughter named Francine. Her mother was Descartes' housekeeper, Helena Jans. They lived with Descartes part of the time in the latter 1630s. He was arranging for the daughter to live with a female relative of his for the sake of her education when he learned of her death in September 1640, which saddened him greatly (Rodis-Lewis 1998, 140). Subsequently, in 1644, he contributed a dowry for Helena's marriage (Watson 2007, 188).

Recall that Descartes considered the Meditations to contain the principles of his physics. But there is no Meditation labeled "principles of physics." The principles in question, which are spread through the work, concern the nature of matter, the activity of God in creating and conserving the world, the nature of mind (that it is an unextended, thinking substance), mind-body union and interaction, and the ontology of sensory qualities.

The Principles appeared in Latin in 1644, with a French translation in 1647. Descartes added to the translation an "Author's Letter" as a preface. The letter explained important aspects of his attitude toward philosophy, including the view that, in matters philosophical, one must reason through the arguments and evaluate them for one's self (9B:3). He also presented an image of the relations among the various parts of philosophy, in the form of a tree:

Thus the whole of philosophy is like a tree. The roots are metaphysics, the trunk is physics, and the branches emerging from the trunk are all the other sciences, which may be reduced to three principal ones, namely medicine, mechanics and morals. By "morals" I understand the highest and most perfect moral system, which presupposes a complete knowledge of the other sciences and is the ultimate level of wisdom. (9B:14)

The extant Principles offers metaphysics in Part I; the general principles of physics, in the form of his matter theory and laws of motion, as following from the metaphysics, in Part II; Part III concerns astronomical phenomena; and Part IV covers the formation of the earth and seeks to explain the properties of minerals, metals, magnets, fire, and the like, to which are appended discussions of how the senses operate and a final discussion of methodological issues in natural philosophy. His intent had been also to explain in depth the origins of plants and animals, human physiology, mind-body union and interaction, and the function of the senses. (Descartes and his followers variously included topics concerning the nature of the mind and mind-body interaction within physics or natural philosophy, on which, see Hatfield 2000.) In the end, he had to abandon the discussion of plants and animals ( Princ. IV.188), but he included some discussion of mind-body union in his abbreviated account of the senses.

1.5 Theological controversy, Passions, and death

From early in his correspondence with Mersenne, Descartes expressed concern to avoid becoming embroiled in theological controversy or earning the enmity of church authorities (1:85-6, 150, 271). Aside from such prudential concerns, he strictly separated theological doctrines based in faith (e.g., the trinity) from his claims about God based solely in reason ( Corr. 1:44, 150, 153; 4:117). Descartes was a Roman Catholic, but it is clear that the God of his metaphysics is nondenominational, tending toward the deistic. If Descartes were fully consistent, he would make no claims about God's plans or his Providential acts and there would be no miracles in his world ( Light VII, 11:48; Princ. III.2-3; see also Corr. 2:558, 3:214).

In the mid-1640s, Descartes continued work on his physiological system, which he had pursued throughout the 1630s. He allowed the unpublished manuscript of the Treatise on Man to be copied (4:566-7) and he began a new work (5:112), Description of the Human Body, in which he sought to explain the embryonic development of animal bodies. During this period he corresponded with Princess Elisabeth of Bohemia (daughter of Frederick V, a Protestant, and briefly King of Bohemia), now living in the Netherlands, at first on metaphysical topics from the Meditations and then on the passions and emotions. Eventually, he wrote the Passions of the Soul (1649), a comprehensive and original theory of the passions and emotions that presented the most extensive account of his behavioral physiology published during his lifetime.

In 1649, Descartes accepted the invitation of Queen Christina of Sweden to join her court. At her request, he composed the Statutes of the Swedish Royal Academy. On the day he gave them to her, he became ill. He never recovered, and died on 11 February 1650. (On Descartes' intellectual biography, see Clarke 2006, Gaukroger 1995, and Rodis-Lewis 1998.)

2. Philosophical Development

In general, it is rare for a philosopher's positions and arguments to remain the same across an entire life. This means that, in reading philosophers' works and reconstructing their arguments, one must pay attention to the place of each work in the philosophical development of the author. Readers of Immanuel Kant are aware of the basic distinction between his critical and precritical periods. Readers of G. W. Leibniz are also aware of his philosophical development, although in his case there is less agreement on a developmental scheme.

Scholars have proposed various schemes for dividing Descartes' life into periods. This entry adopts a relatively simple division between the era when mathematics provided the model for his method (in the Rules) and the period after the "metaphysical turn" of 1629, when his conception of the role of the intellect in acquiring knowledge changed to privilege purely intellectual intuitions. At this time, he also recognized that the truth of his special or particular hypotheses in natural philosophy was less than certain and so was subject to confirmation through consequences (as mentioned in Sec. 1.3, above). In effect, he adopted what is now called a hypothetico-deductive scheme of confirmation, but with this difference: the range of hypotheses was limited by his metaphysical conclusions concerning the essences of mind and matter, their union, and the role of God in creating and conserving the universe. Consequently, some hypotheses, such as the "substantial forms" of the scholastics, were ruled out. Argumentative differences among the World, Discourse, and Meditations and Principles may then be ascribed to the fact that, in the 1630s, Descartes had not yet presented his full metaphysics and so adopted an empirical mode of justification for even the first principles of his physics, whereas, after 1641, he appealed to his published metaphysics in securing those principles.

Other scholars see things differently. John Schuster (1980) finds that the epistemology of the Rules lasted into the 1630s and was superseded (unhappily, in his view) only by the metaphysical quest for certainty of the Meditations. Daniel Garber (1992, 48) also holds that Descartes abandoned his early method only after the Discourse. In contrast, Peter Machamer and J. E. McGuire (2006) believe that Descartes held natural philosophy to the standard of absolute certainty through the Meditations but admitted defeat on that score at the end of Principles IV, adopting a lower standard of certainty for his particular hypotheses (such as the explanation of magnetism by corkscrew-shaped particles). They see the Principles as marking Descartes' "epistemic turn" away from the methodological stance of realism about intuitive knowledge of substances that they find in the Rules, Discourse, and Meditations.

Some scholars who emphasize epistemology find that the main change in Descartes' intellectual development is the introduction of skeptical arguments in the Discourse and Meditations. Some interpreters, perhaps inspired by Richard Popkin (1979), believe that Descartes took the skeptical threat to knowledge quite seriously and sought to overcome it in the Meditations (e.g., Curley 1978). By contrast, in the interpretive thread followed here, skeptical arguments were a cognitive tool used to guide the reader of the Meditations into the correct cognitive frame of mind for grasping the first truths of metaphysics. An answer to skepticism was a side-effect of this fundamental aim.

Descartes presented his metaphysics first in the Meditations and then, in textbook-format, in Principles I. His metaphysics sought to answer such questions as: How does the human mind acquire knowledge? What is the mark of truth? What is the actual nature of reality? How are our mental experiences related to our bodies and brains? Is there a benevolent God, and if so, how can we reconcile his existence with the facts of illness, error, and immoral actions?

3.1 How do our minds know?

Descartes had no doubt that human beings know some things and are capable of discovering others, including (at least since his metaphysical insights of 1629) fundamental truths about the basic structure of reality. Yet he also believed that the philosophical methods taught in the schools and used by most of his contemporaries were deeply flawed. Accordingly, the doctrines of scholastic Aristotelian philosophy contained a basic error about how fundamental truths, such as those in metaphysics, are obtained. He articulated this error in the First Meditation, by saying (not in his own voice, but in that of the reader): "Whatever I have up till now accepted as most true I have acquired either from the senses or through the senses" (7:18). He then challenged the veridicality of the senses with the skeptical arguments of the First Meditation, including the dream argument (that we might experience sensory images while dreaming that are indistinguishable from waking experience) and the argument that a deceptive God or an evil deceiver is causing our sensory experience or causes us to err even when reasoning.

In the Aristotelian scheme against which Descartes was moving, all knowledge arises through the senses, in accordance with the slogan "There is nothing in the intellect that was not previously in the senses" (7:75, 267). Similarly, orthodox scholastic Aristotelians agreed that there is "no thought without a phantasm," or an image. Descartes explained these convictions as the results of childhood prejudice (7:2, 17, 69, 107; Princ. I.71-3). As children, we are naturally led by our senses in seeking benefits and avoiding bodily harms. As a result, when we grow into adults we are "immersed" in the body and the senses, and so we accept the philosophical view that the senses reveal the nature of reality (7:38, 75, 82-3).

Descartes unfolds a sequence of clear and distinct perception in Meditations III-VI and again in Principles I-II. We consider these results in Secs. 3.3-3.5. For now, let us examine Descartes' thoughts about the senses as a source of knowledge, different from pure intellect.

Descartes assigned two roles to the senses in the acquisition of human knowledge. First, he acknowledged that the senses are usually adequate for detecting benefits and harms for the body. Their natural function is "to inform the mind of what is beneficial or harmful for the composite of which the mind is a part" (7:83), that is, for the composite of mind and body. Here, he adopted a widely held conception of sensory function within natural philosophy, also found in the Aristotelian and medical literatures.

Second, he acknowledged an essential role for the senses in natural philosophy. Older interpretive literature sometimes has Descartes claiming to derive all natural philosophical or scientific knowledge from the pure intellect, independent of the senses. But Descartes knew full well that he could not do that. He distinguished between the general principles of his physics and the more particular mechanisms by which he explained natural phenomena such as magnetism or the properties of oil and water. He claimed to derive the general principles "from certain seeds of truth" that are innate in the mind ( Disc. 6:64). These include the fundamental doctrine that the essence of matter is extension ( Princ. II.3-4, IV.203). For particular phenomena, he relied on observations to determine their properties (such as the properties of the magnet), and he acknowledged that multiple hypotheses about subvisible mechanisms could be constructed to account for those phenomena. The natural philosopher must, therefore, test the various hypotheses by their consequences, and consider empirical virtues such as simplicity and scope ( Disc. VI; Princ. IV.201-6). Further, Descartes knew that some problems require measurements that rely on the senses, including determining the size of the sun ( Med. 7:80, Princ. III.5-6) or the refractive indexes of various materials ( Met. VIII).

Although Descartes recognized an important role for the senses in natural philosophy, he limited that role by comparison with Aristotelian epistemology. Many scholastic Aristotelians held that all intellectual content arises through a process of intellectual abstraction that starts from sensory images found in the faculty of imagination. Mathematical objects are formed by abstraction from such images. Even metaphysics rests on knowledge derived by abstraction from images. Of course, they held that the intellect plays a crucial role in abstracting mathematical objects or the essences of natural things. By contrast, Descartes affirmed that the truths of mathematics and metaphysics are grasped by the pure intellect operating independently of the senses and without assistance from the imagination.

As regards knowledge, Descartes accepted that in order to know something you must not only represent it as true (e.g., "the essence of matter is extension"), but you must also affirm its truth and do so with some justification. In his scheme of mental capacities, the intellect, as the faculty of representation, offers content for judgment. A second mental faculty, the will ( Med. IV, Princ. I.32-4), affirms or denies the truth of that content (e.g., asserts that the essence of matter is extension).

Not all content deriving from the intellectual faculty is "pure." Purely intellectual content arises from innate ideas without any accompanying brain processes. Other intellectual acts require the presence of the body: sense perception, imagination, and corporeal (body-involving) memory. The content of these acts is less clear and distinct than that of pure intellect, and may indeed be obscure and confused (as in the case of color sensations). Nonetheless, the will can affirm or deny such intellectual content. With clear and distinct purely intellectual perceptions, the will is justified in affirming their truth. (As discussed in the next subsection, no error can arise in these judgments.) For lesser degrees of clarity and distinctness, care must be taken in what the will affirms ( Med. IV, Princ. I.66-70, IV.205-6). (See Newman 2019.)

In sum, in considering Descartes' answer to how we know, classes of knowledge differ in the degree of expected certainty. Metaphysical first principles as known by the intellect acting alone should attain absolute certainty. Practical knowledge concerning immediate benefits and harms is known by the senses. Such knowledge needn't attain absolute certainty; even so, it is usually accurate enough. Objects of natural science are known by a combination of pure intellect and sensory observation: the pure intellect tells us what properties bodies can have, and we use the senses to determine which particular instances of those properties bodies do have. For submicroscopic particles, we must reason from observed effects to potential cause. In these cases, our measurements and our inferences may be subject to error, but with care we may hope to arrive at the truth.

3.2 The mark of truth and the circle

At the beginning of the Third Meditation, Descartes declares "I now seem to be able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true" (7:35). Clarity and distinctness of intellectual perception is the mark of truth.

In the fifth set of Objections to the Meditations, Gassendi suggests that there is difficulty concerning

what possible skill or method will permit us to discover that our understanding is so clear and distinct as to be true and to make it impossible that we should be mistaken. As I objected at the beginning, we are often deceived even though we think we know something as clearly and distinctly as anything can possibly be known. (7:318)

Gassendi has in effect asked how it is that we should recognize clear and distinct perceptions. If clarity and distinctness is the mark of truth, what is the method for recognizing clarity and distinctness?

In reply, Descartes claims that he has already supplied such a method (7:379). What could he have in mind? It cannot be the simple belief that one has attained clarity and distinctness, for Descartes acknowledges that individuals can be wrong in that belief (7:35, 361). He offers this criterion: we have a clear and distinct perception of something if, when we consider it, we cannot doubt it (7:145). That is, in the face of genuine clear and distinct perception, our affirmation of it is so firm that it cannot be shaken, even by a concerted effort to call the things thus affirmed into doubt.

As mentioned in Section 3.1, Descartes held that any act of judgment, such as the affirmation "I think, therefore I am," involves both the intellect and will. The intellect perceives or represents the content of the judgment; the will affirms or denies that content. In the face of genuine clarity and distinctness, "a great light in the intellect" is followed by "a great inclination of the will" (7:59). The inclination of the will is so strong that it amounts to compulsion; we cannot help but so affirm. Descartes maintains that unshakable conviction provides the criterion of genuinely clear and distinct perception. Can't someone be unshakable in their conviction merely because they are stubborn? Assuredly. But Descartes is talking about a conviction that remains unshakable in face of serious and well-thought out challenges (7:22). To be immune from doubt does not mean simply that you do not doubt a proposition, or even that it resists a momentary attempt to doubt; the real criterion for truth is that the content of a proposition is so clearly perceived that the will is drawn to it in such a way that the will's affirmation cannot be shaken even by the systematic and sustained doubts of the Meditations. Perhaps because the process for achieving knowledge of fundamental truths requires sustained, systematic doubt, Descartes indicates that such doubt should be undertaken only once in the course of a life (7:18; 3:695).

Even so, problems remain. Having extracted clarity and distinctness as the criterion of truth at the beginning of the Third Meditation, Descartes immediately calls it into question. He re-introduces an element of the radical doubt from the First Meditation: that a powerful God might have created him with "a nature such that I was deceived even in matters which seemed most evident" (7:36). Descartes therefore launches an investigation of "whether there is a God, and, if there is, whether he can be a deceiver" (7:36).

In the course of the Third Meditation, Descartes constructs an argument for the existence of God that starts from the idea of an infinite being. The argument is intricate. It invokes the metaphysical principle that "there must be at least as much reality in the efficient and total cause as in the effect of that cause" (7:40). This principle is put forward as something that is "manifest by the natural light" (7:40), which itself is described as a cognitive power whose results are indubitable (7:38), like clear and distinct perception (7:144). Descartes then applies that principle not to the mere existence of the idea of God as a state of mind, but to the content of that idea. Descartes characterizes that content as infinite, and he then argues that a content that represents infinity requires an infinite being as its cause. He concludes, therefore, that an infinite being, or God, must exist. He then equates an infinite being with a perfect being and asks whether a perfect being could be a deceiver. He concludes: "It is clear enough from this that he cannot be a deceiver, since it is manifest by the natural light that all fraud and deception depend on some defect" (7:52). (If you are concerned by the move from infinity to perfection, consider that in Descartes' philosophical landscape perfect meant complete, and infinitude is the highest degree of completeness.)

The second and fourth sets of objections drew attention to a problematic characteristic of this argument. In the words of Arnauld:

I have one further worry, namely how the author avoids reasoning in a circle when he says that we are sure that what we clearly and distinctly perceive is true only because God exists. But we can be sure that God exists only because we clearly and distinctly perceive this. Hence, before we can be sure that God exists, we ought to be able to be sure that whatever we perceive clearly and evidently is true. (7:214).

Arnauld here raises the well-known problem of the Cartesian circle, much discussed in recent years.

In reply to Arnauld, Descartes claims that he avoided this problem by distinguishing between present clear and distinct perceptions and those that are merely remembered (7:246). He is not here challenging the reliability of memory (Frankfurt 1962). Rather, his strategy is to suggest that the hypothesis of a deceiving God can only present itself when we are not clearly and distinctly perceiving the infinity and perfection of God, because when we are doing that we cannot help but believe that God is no deceiver. It is as if this very evident perception is to be balanced against the uncertain opinion that God might be a deceiver (7:144). The evident perception wins out and the doubt is removed.

Scholars have debated whether the above response is adequate. Some have constructed other responses on Descartes' behalf or have found them embedded in his writings. One type of response appeals to a distinction between the natural light and clear and distinct perception, and seeks to vindicate the natural light without appeal to God (Jacquette 1996). Another response suggests that, in the end, Descartes was not aiming at metaphysical certainty concerning a mind-independent world but was merely seeking an internally coherent set of beliefs (Frankfurt 1965). A related response suggests that Descartes was after mere psychological certainty (Loeb 1992). The interested reader can follow up this question by turning to the literature just cited (as also to Carriero 2008, Doney 1987, Hatfield 2006, and Newman 2019).

Building on his claim that clear and distinct perceptions are true, Descartes seeks to establish various results concerning the nature of reality, including the existence of a perfect God as well as the natures of mind and matter (discussed more fully in Sec. 3.3). Here we must ask: What is the human mind that it can perceive the nature of reality? Descartes has a specific answer to this question: the human mind comes supplied with innate ideas that allow it to perceive the main properties of God (infinity and perfection), the essence of matter, and the essence of mind. For readers in Descartes' day, this claim would naturally raise a further question: assuming that these innate ideas concern "eternal truths" about God, matter, and mind, do these truths hold independently of God, or do they instead derive from the original contents of God's own intellect?

Descartes rejected both alternatives. He denied, along with many of his contemporaries, that there are eternal truths independent of the existence of God. But he departed from many of his contemporaries in also denying that the eternal truths are fixed in God's intellect. Some Neoplatonist philosophers held that the eternal truths in the human mind are copies, or ectypes, of the archetypes in the mind of God. Some Aristotelian philosophers just prior to Descartes, including Francisco Suárez (1548-1617), held that the eternal truths reflect God's own understanding of his creative power; God's power includes that, if he creates a rabbit, it must be an animal. Eternal truths are latent in God's creative power, and he understands this, so that if human beings understand the eternal truths as eternal, they do so by understanding the creative power of God, which may be something beyond human capacity (Hatfield 1993).

Descartes had a different account. He held that the eternal truths are the free creations of God ( Corr. 1:145, 149, 151; Med. 7:380, 432), originating from him in a way that does not distinguish among his power, will, and intellect. God decides what the essence of a circle is, or to make 2 + 3 = 5. He might have created other essences, but we can't conceive what they might have been. Our conceptual capacity is limited to the innate ideas that God has implanted in us, and these reflect the actual truths that he created. God creates the eternal truths (concerning logic, mathematics, the nature of the good, the essences of mind and matter), and he creates the human mind and provisions it with innate ideas that correspond to those truths. Even in this scheme there must be some eternal truths not created by God: those that pertain to the essence of God himself, including his existence and perfection (see Wells 1982).

3.3 The nature of reality

Descartes' arguments to establish the essences of these substances appeal directly to his clear and distinct perception of those essences. God is an infinite substance and the idea of God includes necessary existence ( Med. III, V, Princ. I.14, 19). Descartes used this idea of God to fashion an argument for the existence of God, now called the Ontological Argument (see Nolan 2021).

The essence of matter is extension in length, breadth, and depth. One might speak here of "spatial extension," but with this proviso: that Descartes denied the existence of space separate from matter. Cartesian matter does not fill a distinct spatial container; rather, spatial extension is constituted by extended matter (there is no void, or unfilled space). This extended substance possesses the further "modes" of size, shape, position, and motion. Modes are properties that exist only as modifications of the essential or principal attributes of a substance. In addition to its essence, extension, matter also has the general attributes of existence and temporal duration (these are shared with mind).

The essence of mind is thought. Minds have the principal attribute of thinking, divided into the two chief powers or faculties previously mentioned: intellect and will. The intellectual, or perceiving power is further divided into the modes of pure intellect, imagination, and sense perception. As mentioned in Section 3.3, pure intellect operates independently of the brain or body, while imagination and sense perception require the body for their operation. The will is also divided into various modes, including desire, aversion, assertion, denial, and doubt. These always require some intellectual content (whether pure, imagined, or sensory) upon which to operate. Perhaps for that reason, Descartes describes the mind as an "intellectual substance" ( Med. VI, 7:78; also, 7:12). The mind essentially has a will, but the intellectual (or perceptive, or representational) power is more basic, as the operations of the will depend on it.

What role does consciousness play in Descartes' theory of mind? Many scholars believe that, for Descartes, consciousness is the defining property of mind (e.g., Rozemond 2006). There is some support for this position in the Meditations (Second Replies). Descartes defines mind as "the substance in which thought immediately resides" (7:161), and he says that the term "thought" extends to "everything that is within us in such a way that we are immediately conscious of it" (7:160*). If mind is thinking substance and thoughts are essentially conscious, perhaps consciousness is the essence of thought?

Perhaps not. Descartes did hold that all thoughts are, in some way, conscious (7:226), but this did not mean that we have reflective awareness of, or notice, every thought that we have ( Corr. 5:220). In the Second Meditation, he describes himself as a thinking thing by enumerating all the modes of thoughts of which he is conscious: understanding (or intellection), willing, imagining, and (at this point, at least seeming to have) sense perceptions (7:28). He thus sets up consciousness as a mark of thought. But is it the essence? Not necessarily. If perception (intellection, representation) is the essence of thought, then all thoughts might have basic consciousness because the character of the intellectual substance is to represent, and any representation present in an intellectual substance is, thereby, present in mind. An intellectual substance (a mind) is a perceiving substance, which intrinsically perceives its own states. Further, he held that any act of will present in an intellectual substance is thereby available to consciousness ( Pass. I.19). (On consciousness and the essence of mind, see Jorgensen 2020, 2.1).

In distinguishing between conscious thoughts and thoughts of which we are reflectively aware, Descartes allowed for conscious thoughts that we don't notice or remember. Indeed, his theory of the senses (Sec. 5) allows for unnoticed sensations and mental operations.

3.4 Mind-body relation

In the Discourse, Descartes presented the following argument to establish that mind and body are distinct substances:

Next I examined attentively what I was. I saw that while I could pretend that I had no body and that there was no world and no place for me to be in, I could not for all that pretend that I did not exist. I saw on the contrary that from the mere fact that I thought of doubting the truth of other things, it followed quite evidently and certainly that I existed; whereas if I had merely ceased thinking, even if everything else I had ever imagined had been true, I should have had no reason to believe that I existed. From this I knew I was a substance whose whole essence or nature is simply to think, and which does not require any place, or depend on any material thing, in order to exist. (6:32-3)

This argument moves from the fact that he can doubt the existence of the material world, but cannot doubt the existence of himself as a thinking thing, to the conclusion that his thoughts belong to a nonspatial substance that is distinct from matter.

While this argument might secure the existence of a thinker, it does not show that the thinker is not a material thing. That conclusion fallaciously rests on conceivability based in ignorance. There is nothing to preclude that the thinking thing is in fact a complex material system. Descartes has merely relied on the fact that he can doubt the existence of matter to conclude that matter is distinct from mind. That argument doesn't work. From the fact that the Joker cannot, at a certain moment, doubt the existence of Batman (because he is with him), but he can doubt the existence of Bruce Wayne (who might, for all the Joker knows, have been killed by the Joker's henchmen), it does not follow that Bruce Wayne is not Batman. In fact, he is Batman. The Joker is merely ignorant of that fact.

In the Meditations, Descartes altered the argument. In the Second Meditation, he again asserted that he could doubt the existence of matter but not the existence of himself as a thinking thing. But he explicitly refrained from concluding that his mind was distinct from body, on the grounds that he remained ignorant of his nature (7:27). Then, in the Sixth Meditation, having established, to his satisfaction, the mark of truth, he used it to support a positive argument that the essence of mind is thought and that a thinking thing is unextended; and that the essence of matter is extension and that extended things cannot think (7:78). He based this argument on clear and distinct intellectual perceptions of the essences of mind and matter, not on the fact that he could doubt the existence of one or the other.

This conclusion asserts the well-known substance dualism of Descartes. That dualism has problems. As Princess Elisabeth of Bohemia, among others, asked: if mind is unextended and matter is extended, how do they interact? This problem vexed not only Descartes, who admitted to Elisabeth that he didn't have a good answer (3:694), but it also vexed Descartes' followers and other metaphysicians. It seems that, somehow, mind and body must be brought into relation, because when we decide (a mental act) to pick up a pencil, our arm (a physical thing) actually moves, and when light hits our eyes, we mentally experience the visible world. But how do mind and body relate? Some of Descartes' followers adopted the position of occasionalism, according to which God mediates the causal relations between mind and body; mind does not affect body and body does not affect mind, but God gives the mind appropriate sensations given the state of the body, and he makes the body move by putting it into the correct brain state when we decide (mentally) to pick up the pencil (see Lee 2020). Other philosophers adopted other solutions, including the monism of Spinoza and the pre-established harmony of Leibniz.

In the Meditations and Principles, Descartes did not focus on the metaphysical question of how mind and body interact. Rather, he discussed the functional role of mind-body union in the economy of life. Our sensations serve us well in avoiding harms and pursuing benefits. Pain-sensations induce us toward behaviors that usually protect our bodily integrity. Pleasure leads us toward things that are usually good for us. Our sense perceptions are reliable enough for distinguishing objects that need distinguishing and for navigating as we move about. As Descartes saw it, "God or nature" set up these relations for our benefit. They are not perfect. Sometimes our senses present things differently than they are, and sometimes we make judgments about sensory things that extend beyond the appropriate use of the senses.

3.5 God and sensory error

In discussing the mark of truth, Descartes maintained that the human intellect is generally reliable because God created it. In discussing the functioning of the senses to preserve the body, he explained that God has arranged mind-body interaction so that our sensations generally are conducive to the good of the body. Nonetheless, errors occur. Our judgments about sensory things may be false, just as, more broadly, human beings may make poor moral choices even though God has given them a will that is intrinsically drawn to the good ( Corr. 1:366, 5:159, Princ. I.42). For example, we may form the childhood habit of judging, with respect to our color sensations, that the color we experience "resembles" something in objects (the Aristotelian theory). Subsequent philosophical reflection (perhaps instigated by reading Descartes) may lead us to conclude that these judgments are unwarranted, because color sensations are not clear and distinct but obscure and confused ( Princ. I.66-70). In other cases, our sense perceptions may represent things as being a certain way when they are not. Sometimes we feel pain because a nerve has been damaged somewhere along its length, and yet there is no tissue damage at the place the pain is felt. Amputees may feel pain as located in their fingers when they have no fingers ( Princ. IV.196).

Descartes responded to these errors differently. He explained cognitive (judgmental) and moral errors as resulting from human freedom ( Med. IV). God provides human beings with a will, and wills are intrinsically free. But we have finite intellects. Because we are free, we can choose to judge in cognitive or moral situations in which we do not have clear and distinct perceptions of the true or the good. If human beings restricted their acts of will to cases of maximum clear and distinct perception, they would never err. If we go wrong, Descartes contends, we are responsible, because we might have restricted our judgments to matters about which we have the certainty of clear and distinct perception. The applicability of this solution to the case of color sensations and resemblance is, however, not apparent, as we may not have made a reflective choice when, as children, we accepted the resemblance account of color experience. In that case, we might become culpable only after reading and accepting Descartes' arguments that color sensations are obscure and confused and applying this finding to our habitual assertion of resemblance.

Matters are different for specific errors of sensory representation, such as the mislocation of pain. The senses depend on media and sense organs and on nerves that must run from the exterior of the body into the brain. God sets up the mind-body relation so that our sensations are good guides in most circumstances. But the media may be poor (the light may not be good), circumstances may be unusual (as when a partially submerged stick appears bent), or the nerves may be damaged (as with the amputee). In these cases, the reports of the senses are misleading. Since God has set up the system of mind-body union, shouldn't God be held accountable for the fact that the senses can misrepresent how things are? Here Descartes does not appeal to our freedom of judgment, for in fact we must often use the senses in suboptimal cognitive circumstances when navigating through life, with no opportunity for reflection and reasoned choice. Rather, he points out that God was working with the finite mechanisms of the human body (7:88), and he suggests that God did the best that could be done given the type of parts needed to constitute such a machine (extended parts that might break or be unusually perturbed). It's not God's fault that a well-made bodily machine can occasionally create sensations that misrepresent.

Some cases of sensory misrepresentation are clear-cut cases of error. With the amputee, the pain seems to be in fingers that are not there. The representational content (that the fingers are damaged) does not match the world. Similarly, a partially submerged stick may look bent. Even if we use our intellects to interpret the illusions or sensory misrepresentations so as to avoid error by withholding judgment or even by judging correctly (7:438), it remains clear that sensory misrepresentation has occurred.

In other cases, Descartes describes the senses as providing material for error, but it remains uncertain whether such error arises from making unwarranted judgments on obscure and confused sensations or is due to straightforward sensory misrepresentation. In the Third Meditation, he describes sensations that were later called "secondary qualities" (e.g., colors, sounds, hot and cold) as "materially false." Scholars have difficulty interpreting this notion. Descartes initially defines material falsity as something that "occurs in ideas, when they represent non-things as things" (7:43). He offers the idea of cold as an example: our senses represent cold as a positive quality of objects, but Descartes considers that cold may instead simply be the absence of heat and so isn't a quality of its own. Accordingly, this case should be assimilated to sensory misrepresentation: representing things as they are not (representing cold as a quality when it is the absence of a quality).

Descartes also offers a different gloss on the obscurity of sensory ideas. He allows that such ideas may be "true" in the sense of representing something positive in things, but that they may do so in such a way that "the reality which they represent is so extremely slight that I cannot even distinguish it from a non-thing" (7:44). Accordingly, sensory ideas are not misrepresentations, they are simply so obscure and confused that we cannot tell what their representational content might be by considering their experienced character, such as the phenomenal character of cold or of color. We then precipitately make an erroneous judgment that color sensations resemble their causes in objects. (Metaphysics and natural philosophy are needed to tell us what our color sensations obscurely represent: properties of object-surfaces that reflect light a certain way-see Sec. 5.) "Material falsity" arises from representations so obscure that they allow room for mistaken judgments (e.g., about resemblance).

The issues surrounding the notion of material falsity in Descartes are intricate but telling of his theory of mind and sensory representation. Entrance to the literature can be gained through Wee (2006), Brown (2006, Ch. 4), and Hatfield (2013).

4. The New Science

During Descartes' school days, there were signs that the scientific conception of the universe was changing. Recall that Galileo's discovery of the four moons of Jupiter was celebrated at La Flèche in 1610 (see Galileo 1610). More generally, Copernicus had, in the previous century, argued forcefully that the sun, not the earth, is at (or near) the center of the solar system (Copernicus 1543). Early in the seventeenth century, Johannes Kepler (1604) announced new results in optics, concerning the formation of images, the theory of lenses, and the fact that the retinal image plays a central role in vision. By the early 1630s, Descartes was aware of William Harvey's claim that the blood circulates in the body ( Corr. 1:263).

Descartes contributed new results to the mathematical description of nature, as developer of an accurate model of the rainbow ( Met. VIII) and as a discoverer of the sine law of refraction. (Descartes was the first to publish the sine law, in Diop. II. Previously, the law had been discovered by the Islamic investigator Ibn Saul [10th c.], the English astronomer Thomas Harriot [ca. 1600], and the Dutch mathematician and astronomer Willebrord Snell [ca. 1620]. See Sabra 1967, 99-103, and Rashed 1990.) As significant as these results are, his primary contribution to the "new science," or the "new mechanical philosophy," lay in his general vision of a mechanistic approach to nature, together with many details of that vision for specific natural phenomena (such as magnetism, the formation of the solar system, and sensory physiology), so as to provide a comprehensive alternative to the dominant Aristotelian physics. The process of forming a new science continued after Descartes, reaching a high point in Newton's mechanics, including a consolidation of the Copernican hypothesis.

In the textbooks of Aristotelian physics of Descartes' day, it was common to divide physics into "general" and "special." General physics pertained to the basic Aristotelian principles for analyzing natural substances: form, matter, privation, cause, place, time, motion. Special physics concerned actually existing natural entities, divided into inanimate and animate. Inanimate physics further divided into terrestrial and celestial, in accordance with the Aristotelian belief that the earth is at the center of the universe and differs in nature from the heavens (including the moon, and everything beyond it). Inanimate terrestrial physics first covered the four elements (earth, air, fire, and water), then the "mixed" bodies composed from them, including the various mineral kinds. Animate terrestrial physics concerned the various powers that Aristotelians ascribed to ensouled beings, where the soul is considered as a principle of life (possessing vital as well as mental or cognitive powers). In the simplest textbooks, the powers of the soul were divided into three groups: vegetative (including nutrition, growth, and reproduction), which pertained to both plants and animals; sensitive (including external senses, internal senses such as memory and imagination, appetite, and motion), which pertains only to animals, both nonhuman and human; and rational powers, pertaining to human beings alone. All the bodies in both inanimate and animate terrestrial physics were governed by a "form" or active principle, as described in Section 1.3.

Descartes' ambition was to provide replacements for all the main parts of Aristotelian physics. In his physics, there is only one matter and it has no active forms. He dissolved the boundary that made celestial and terrestrial differ in kind. His one matter possesses the properties of size, shape, position, and motion; it is infinitely divisible and constitutes space (see Sec. 3.3). This matter is governed by three laws of motion, including a precursor to Newton's law of inertia (but without the notion of vector forces) and a law of impact. Descartes' matter possessed no "force" or active agency; the laws of motion as decreed by God were sustained by his activity. Earth, air, fire, and water were simply four among many natural kinds, all distinguished simply by the characteristic sizes, shapes, positions, and motions of their parts.

Descartes also wanted to provide an account of the formation of plants and animals by mechanical causes ( Disc. V, 6:44-45), but he did not succeed in framing an account that he was willing to publish (so that only portions of his physiology were revealed in the Discourse, Dioptrics, Meditations, Principles, and Passions). In writings that were published only posthumously (in 1664), including the Treatise on Man, he developed an extensive physiological description of animal bodies, in which he explained the functions of life in a purely mechanical manner, without appeal to a soul or vital principle. (The Treatise on Man was circulated in manuscript form during Descartes' lifetime [5:112].) The Description of the Human Body (1664) included his speculations on embryogenesis.

In mechanizing the concept of living thing, Descartes did not deny the distinction between living and nonliving, but he redrew the line between ensouled and unensouled beings. In his view, among earthly beings, only humans have souls. He thus equated soul with mind: souls account for intellection and volition, including conscious sensory experiences, conscious experience of images, and consciously experienced memories. He regarded nonhuman animals as machines, devoid of mind and consciousness, and hence lacking in sentience. (Although Descartes' followers understood him to have denied all feeling to animals, some recent scholars question this interpretation; on this controversy, see Cottingham 1998 and Hatfield 2008.) Consequently, Descartes was required to explain all of the powers that Aristotelians had ascribed to the vegetative and sensitive soul by means of purely material and mechanistic processes ( Man 11:202). These mechanistic explanations extended not merely to nutrition, growth, and reproduction, but also to the functions of the external and internal senses, including the ability of nonhuman animals to respond via their sense organs in a situationally appropriate manner: to approach things that are beneficial to their body (including food) and to avoid danger (as the sheep avoids the wolf).

In the Treatise on Man and Passions, Descartes described purely mechanical processes in the sense organs, brain, and muscles, that were to account for the functions of the sensitive soul. These processes involved "animal spirits," or fine fluid matter as distilled from the blood at the base of the brain and distributed down the nerves to cause muscle motions in accordance with current sensory stimulation. The brain structures that mediate behavior may be innate or acquired. Descartes ascribed some things that animals do to instinct; he explained other aspects of their behavior through a kind of mechanistic associative memory. He regarded human physiology as similar to nonhuman animal physiology, anent both vegetative and (some) sensitive functions-those sensitive functions that do not involve consciousness or intelligence:

Now a very large number of the motions occurring inside us do not depend in any way on the mind. These include heartbeat, digestion, nutrition, respiration when we are asleep, and also such waking actions as walking, singing, and the like, when these occur without the mind attending to them. When people take a fall, and stick out their hands so as to protect their head, it is not reason that instructs them to do this; it is simply that the sight of the impending fall reaches the brain and sends the animal spirits into the nerves in the manner necessary to produce this movement even without any mental volition, just as it would be produced in a machine. (Fourth Replies to Objections, Med. 7:229-30)

Much human behavior occurs without intervention from the mind.

The fact that Descartes offered mechanistic explanations for many features of nature does not mean that his explanations were successful. His followers and detractors debated the success of his proposals for nearly a century after his death. His accounts of magnetism and gravity were challenged. Leibniz questioned the coherence of Descartes' laws of motion and impact. Newton offered his own laws of motion and an inverse square law of gravitational attraction. His force-based account of orbital planetary motions replaced Descartes' vortexes. Others struggled to make Descartes' physiology work. There were also deeper challenges. Some wondered whether Descartes could explain how his infinitely divisible matter coalesces into solid bodies. Why shouldn't collections of particles act like puffs of smoke, that separate upon contact with larger particles? Indeed, how do particles themselves cohere?

Such problems were real, and Descartes' physics was abandoned over the course of the eighteenth century. Nonetheless, it provided a conception for a comprehensive replacement of Aristotelian physics that persisted in the Newtonian vision of a unified physics of the celestial and terrestrial realms, and that continued in the mechanistic vision of life as revived in the nineteenth century. (On the rise of the new science, or what is sometimes called the "scientific revolution," see Henry 2008 and Shapin 1996; for a survey of Descartes' natural philosophy, see Gaukroger 2002.)

5. Theory of Sense Perception

As the "mechanical philosophy" of Descartes and others replaced the Aristotelian physics, the theory of sensory qualities required revision. This was especially true for what came to be known as the "secondary qualities" (in the terminology of Robert Boyle and John Locke). The secondary qualities include colors, sounds, odors, tastes, and tactile qualities such as hot and cold. The Aristotelians maintained that these qualities exist in objects as "real qualities" that are like instances or samples of the quality as experienced. A red thing possesses the quality red in just the same way it possesses a shape: it simply is red, and we experience that very redness when we see a red object (the "resemblance thesis" from Sec. 3.5).

Descartes sought to replace "real qualities" with a mechanistic account of qualities in objects. The sensory qualities of objects would be explained by what were later called "primary qualities" (such as size, shape, position, and motion). Accordingly, Descartes rendered light as a property of particles and their motions: it is a "tendency to move" as found in a continuous medium and radiating out from a luminous body. When light strikes an object, the particles that constitute light alter their rotation about their axis. "Spin" is what makes light have one color rather than another. When particles with one or another degree of spin interact with retinal nerves, those nerves jiggle in a certain way. In the brain, this jiggling affects the animal spirits, which in turn affect the mind, causing it to experience one or another color. Color in objects is thus that property of their surface that causes light particles to spin in one way or another, and hence to cause one sensation or another ( Diop. I, 6:85, 91-2). Sound arises from vibrations in sounding things, and odor from shaped particles that differentially affect the olfactory nerves.

Descartes introduced this new theory of sensory qualities in the first six chapters of Light. He defended it by arguing that his explanation of qualities in bodies in terms of size, shape, and motion are clearly understood by comparison with the Aristotelian qualities (11:33). Subsequently, in the Meditations and Principles, he supported this account by appeal to the metaphysical result that bodies possess only geometrical modes of extension ( Med. V, Princ. I.69, II.4-5, IV.199). Real qualities are ruled out because they are not themselves instances of size, shape, or motion (even if patches of color have a size and a shape, and can be moved about).

In addition to a new theory of sensory qualities, Descartes offered theories of the way in which the spatial properties-size, shape, distance, and position-are perceived in vision. In Descartes' day and before, "optics" was defined as the theory of vision, including physical, physiological, and psychological aspects. In antiquity, Euclid and Ptolemy wrote on optical problems. During the Middle Ages, the Arabic natural philosopher Ibn al-Haytham produced an important new theoretical work on vision that included an extensive account of the perception of spatial properties.

The theoretical terrain in optics changed with Kepler's doctrine that vision is mediated by the retinal image and that the retina is the sensitive body in the eye. Previous theorists generally believed that the "crystalline humor," now known as the lens, was the sensitive body. Descartes accepted Kepler's result and framed a new theory of spatial perception. Some of his theorizing simply adapted Ibn al-Haytham's theories to the newly discovered retinal image. Ibn al-Haytham held that size is perceived by combining the visual angle that a body subtends with perception of its distance, to arrive at a perception of the true size of the object. (Visual angle is formed by the directions from a vantage point to a seen-object for a given fixation, e.g., the angle formed by the direction to the feet and to the crown of the head of a person standing at moderate distance to us.) In Ibn al-Haytham's scheme, visual angle is registered at the surface of the crystalline humor. Descartes held that size is perceived by combining visual angle with perceived distance, but he now treated visual angle as the extent of an object's projection onto the retina. (On spatial perception from Euclid to Descartes, see Hatfield 2020.)

In Ibn al-Haytham's account, if the size of an object is known, distance may be perceived through an inference; for a given size, an object's distance is inversely proportional to its visual angle. Descartes recognized this traditional account ( Diop. VI, 6:138-40), depending as it does on past experience of an object's size and on an inference or rapid judgment that combines perceived visual angle with this known or remembered size. Descartes held that these judgments are habitual and happen so quickly that they go unnoticed, as do the sensations that present visual angle, being rapidly replaced by visual experiences of objects at a distance (Sixth Replies, Med. 7:437-38).

Ibn al-Haytham further maintained that distance can be perceived by an observer's being sensitive to the number of equal portions of ground space that lie between the observer and a distant object. Descartes did not adopt this explanation. However, Descartes used his mechanistic physiology to frame a new account of how distance might be perceived that drew on Kepler's results.

In the Keplerian theory of how the eye works, an image is formed on the retina as a result of refraction by the cornea and lens. For objects at different distances, the focal properties of the system must be changed, just as the focal length of a camera is changed (Kepler 1611, prop. 64). There were several theories of how this might occur, and Descartes sometimes said merely that the shape of the eye changes to accommodate for near and far vision ( Diop. VI, 6:137), sometimes that specifically the shape of the lens changes ( Man 11:155-56, 159). He then theorized that this change in the shape of the lens must be controlled by muscles, which themselves are controlled by nerve processes in the brain.

When we correctly perceive the distance and combine it with visual angle (by an unnoticed mental act), the result is a veridical perception of a size-at-a-distance. Descartes described the resulting perception as possessing the attributes that were labelled "size constancy" in the twentieth century:

Concerning the manner in which we see the size and shape of objects, I need not say anything in particular since it is included in the way we see the distance and position of their parts. That is, we judge their size by the knowledge or opinion that we have of their distance, compared with the size of the images they imprint on the back of the eye-and not simply by the size of these images. This is sufficiently obvious from the fact that the images imprinted by objects very close to us are a hundred times bigger than those imprinted by objects ten times farther away, and yet they do not make us see the objects a hundred times larger; instead they make the objects look almost the same size, at least if their distance does not deceive us. (Diop. VI, 6:140)

When Descartes speaks of taking into account the "size of the images" on the retina, he need only be speaking of visual angle, which is equivalent to retinal-image size. In saying that a nearby object is a hundred times larger on the retina than one ten times farther away, he is speaking of area; the nearby object would be ten times larger in linear height. If visual angle and object distance are correctly perceived and combined, the object's constant true size is perceived. If we get the distance wrong, as in underperceiving the distance to the sun and moon, we misperceive the size, in this case, as too small ( Diop. VI, 6:144-45).

Descartes' work on visual perception is but one instance of his adopting a naturalistic stance toward conscious mental experience. The Passions constitute another (Sec. 6). It is sometimes said that Descartes' dualism placed the mind outside nature by rendering it as an immaterial substance. That is a retrospective judgment from a perspective in which immaterial substances are automatically deemed "unnatural." For Descartes and his followers, mind-body interaction and its laws were included within the domain of natural philosophy or physics (in the general meaning of the latter term, as the theory of nature). Descartes spoke of regular relations between brain states and the resulting sensory experiences, which his followers, such as Regis, subsequently deemed "laws" of mind-body relation (see Hatfield 2000). In this way, Descartes and his followers posited the existence of psychophysical or psychophysiological laws, long before Gustav Fechner (1801-1887) formulated a science of psychophysics in the nineteenth century.

6. Passions and Emotions

The passions and emotions had been a topic of philosophical interest since antiquity and were much discussed in Descartes' day (James 1997). His own interest intensified during the 1640s, as a result of his correspondence with Princess Elisabeth (trans. Shapiro, 2007). Descartes recognized two separate types of mental states that can be classified as emotions (excitations) of the soul: purely intellectual feelings, such as the intellectual joy of loving God, and body-dependent affects on the mind, called "passions" (in the etymological sense of the mind being passive in receiving them). We will focus on the latter, which are the subject of Descartes' final publication, the Passions of the Soul (1649). In the preferatory letters to this work, Descartes acknowledged that he approaches the passions "as a physicist" or natural philosopher (11:326); but he in fact engaged to some extent all the main genres of writings on the passions: medical, natural philosophical, and moral.

Recall that Descartes attributed only two powers to the mind: intellect and will. The intellect is the seat of ideas or perceptions, which are representations. The will is an active power that responds to the represented content, by affirming its truth, denying its truth, doubting it, or desiring it to be the case. The will performs acts such as choosing to think about mathematics or to raise one's arm ( Princ. I.32, Pass. I.17-19).

In the Passions (I.22-29), Descartes divided the perceptions of the intellect that are caused by the body into three kinds: perceptions of external objects (such as a candle or a bell); perceptions of internal bodily states (such as hunger or thirst); and perceptions that we refer to the soul itself, such as bodily love, hate, and desire. These passions are "bodily" because it is part of their definition that they have a bodily cause (as opposed to purely intellectual emotions, which have the soul as cause, Pass. I.91, 147). This third type of bodily passions can be called the "passions proper."

Descartes defined the passions proper as "perceptions or sensations or emotions of the soul that we refer particularly to it and that are caused, maintained, and strengthened by some movements of the spirits" ( Pass. I.27). Like external sensations, the passions are immediately caused by brain activity (animal spirits). But they are referred to the soul, not to an external object, even if they are in many cases triggered by an external object. Suppose that we see a ferocious animal. The sense of vision gives us a perception of the shape, size, and color of the animal, according to the ways in which the nerves are affected and cause animal spirits to flow (see Sec. 5). But the flow of animal spirits also causes us to experience the passion of fear. Interestingly, Descartes did not hold that we then run from the animal because we experience the fear. Rather, the same flow of spirits that causes us to feel fear also, at the same time, by purely mechanical processes (see Sec. 4), affects the muscles of the legs so as to make us run. This occurs "witout any contribution from the soul" ( Pass. I.38). The function of the experienced passion (the feeling of fear) is to "move and dispose the soul to want the things for which they prepare the body" (I.40; see also II.52), in this case, to make us want to continue running.

The passions do not respond to all the properties that affect the external senses. That is, "the objects which stimulate the senses do not excite different passions in us because of all the differences in them, but only because of the various ways in which they may harm or benefit us, or in general have importance for us" ( Pass. II.52). Felt fear responds to the ferocious animal under the aspect of potential harm. Descartes recognized six simple or primitive passions: wonder, love, hatred, desire, joy, and sadness (II.69), from which all the other passions are generated. Wonder arises when we perceive something that is novel. Rare objects affect the brain differently than objects we have seen before. The resulting brain state causes the body to stay physically focused on the novel object. The experienced passion of wonder then causes us to focus mentally on the same object (II.70-78). The inclusion of wonder as a primitive type of passion was new (or newly made prominent) in the Passions.

The other five passions present objects as good or bad for us. The passion of love makes us want to join ourselves willingly with its object; hatred makes us want to separate ourselves. Desire is a future-oriented feeling of wanting something it represents as good to be the case. Joy arises from the present possession of a good thing; sadness arises when an evil is represented as pertaining to us (II.78-93). Again, these passions are caused by brain states that arise in the machine of the body. States arise in the brain which then prepare the body for behaviors and cause the felt passion. Here, Descartes speaks of brain states as "representations"; joy, for example, "is a pleasant emotion that the soul has when it enjoys a good that impressions in the brain represent to it as its own" (II.91). The Passions relies heavily on the mechanistic physiology in Descartes' earlier works.

Descartes' theory of the passions is complex and covers many aspects, including the role of the brain, the role of judgment in triggering passions, and the passions in morality. Entrance to the literature can be found in Brown (2006), Rutherford (2021), Shapiro (2003), and Hatfield (2007).

7. Reception and Legacy

The things that readers find valuable in Descartes' work have changed across the centuries. His natural philosophy had an immediate impact that lasted into the eighteenth century. His theory of vision was part of that heritage, as were his results in mathematics. His mechanistic account of the psychology of the sensitive soul and his view that animals are like machines were revived in the nineteenth century.

Descartes was strongly invoked in some works on the equality of the sexes and the education of women. The seventeenth-century French Cartesian François Poulain de la Barre used substance dualism in arguing to the conclusion that "the mind has no sex" (trans. Clarke, 157), although he was not above suggesting that female physiology yields cognitive advantages (Schmitter 2018, 5). The early eighteenth-century English philosopher Mary Astell used Cartesian philosophy in her advocacy of education for women (Atherton 2001). In contrast, Nicolas Malebranche was less favorable toward attributing cognitive equality to women, showing that there was no single position shared among the Cartesians concerning equality (Hamerton 2008). (For entry into recent work on feminism and Descartes, including historical studies, see Bordo 1999 and Schmitter 2018.)

The fortune of Descartes' metaphysics and epistemology is complex. In his own time, he inspired a raft of followers, who sought to develop his metaphysics, epistemology, and natural philosophy, and to add a worked-out ethics. These authors included Géraud de Cordemoy, Arnold Geulincx, Poulin de la Barre, Antoine Le Grand, Malebranche, Regis, and Rohault. The British philosopher Henry More had a long history of engaging with Descartes' mechanical philosophy and metaphysics, toward which he at first expressed qualified admiration, which later turned to deep hostility (More 1662, 1671; see Leech 2013). Pierre-Daniel Huet (1689) offered a scathing critique of Descartes' philosophy. Other major philosophers, including Benedict de Spinoza and G. W. Leibniz, were influenced by Descartes' thought but developed their own, distinct systems.

Descartes' project to examine the knower as a means to determine the scope and possibilities of human knowledge had a profound effect on early modern epistemology and metaphysics. Among his immediate followers, Malebranche most fully developed this aspect of Descartes' philosophy. Subsequent philosophers who were not followers of Descartes also adopted the strategy of investigating the knower. The epistemological works of Locke, George Berkeley, David Hume, Thomas Reid, and Immanuel Kant pursued this investigation. These authors came to different conclusions than had Descartes concerning the ability of the human mind to know things as they are in themselves. Hume and Kant especially-and each in his own way-rejected the very notion of a metaphysics that reveals reality as it is in itself. They did not merely deny Descartes' particular metaphysical theories; they rejected his sort of metaphysical project altogether. But they did so through the type of investigation that Descartes himself had made prominent: the investigation of the cognitive capacities of the knower.

During the twentieth century and into the twenty-first, various aspects of Descartes' philosophy were widely invoked and perhaps just as widely misinterpreted. The first is Descartes' skepticism. In the early twentieth century, one response to the threat of skepticism about our knowledge of the external world was to retreat to the position that we can only know our own sense data, where "sense data" are equated with the supposed contents of immediate sensory experience: for vision, color patches having a shape (e.g., Russell 1914). Some authors then treated Descartes' project in the Meditations as that of reducing human knowledge to immediate sense data, from which knowledge of the external world was to be constructed. (A version of this reading is Williams 1986.)

As a reading of Descartes, this position has little to offer. As we have seen, in the Second and Third Meditations Descartes argues from the indubitability of the cogito reasoning to the trustworthiness of intellectual perception to the existence of a perfect being (God). This argument does indeed seek to infer the reality of a being external to the mind. But the inference does not invoke sensory experience. It proceeds from a non-sensory and innate idea of God to the existence of that God. Whatever one may think of the quality of this argument, it has nothing to do with sense data. Descartes used skeptical arguments as a tool to temporarily disengage the reader from the sensory world in order to undertake metaphysical investigations that rely on the pure intellect. But again, sense data are not in the mix. The use of skepticism to "withdraw the mind from the senses" ( Med. pref., 7:9) may be read as an instance of Descartes re-purposing, for his own philosophical ends, a type of writing from his time: the spiritual exercise, now reconfigured as cognitive exercises. Accordingly, Descartes' meditator is guided by the text of Meditations in purging our everyday reliance on the senses in order to uncover the pure intellect (see Sec. 3.1). Recall that the metaphysical doubt is to be used once in one's life in order to find the true metaphysics. The search for absolute certainty supports this project and does not extend to ordinary life ( Med. I, VI, 7:22, 89-90). (On Descartes and meditative writing, see Underkuffler 2019.)

Another line of twentieth- and twenty-first century interpretation also focused on the isolation of the subject in the Second Meditation. In the course of that Meditation, Descartes accepts that he knows the contents of his mind, including putative sensory experiences, even though he doubts the existence of his body. Some philosophers have concluded from this that Descartes believed that human beings actually can, in their natural state, have sensory experiences while lacking a body. But Descartes rejected that possibility. In his metaphysics, sense perception and imagination depend for their existence on mind-body union. Purely intellectual perceptions do not depend on the brain. But acts of imagination and sense perception require the brain ( Pass. I.19-20, 43). Descartes did allow that God conceivably could produce sense experiences in us independent of the brain; but because God's perfection is inconsistent with deceit, he would never do this. In any event, in the natural case, human sensations require a brain.

A third conception is little more than the use (or abuse) of Descartes as a straw-man representative of a kind of over-arching "Western rationality" that over-rationalized the human being and denied the body and emotions. One recent version of this caricature suggests that Descartes affirmed a "sense-represent-plan-move" cycle in all explanations of human behavior (Wheeler 2005, Chap. 3). As mentioned above, Descartes explained many human behaviors through the machine of the body, without mental involvement. As he said in the Fourth Replies, "When people take a fall, and stick out their hands so as to protect their head, it is not reason that instructs them to do this" (7:230); rather, the machine of the body (material processes in the sense organs, brain, and muscles) produces this behavior. Descartes envisioned similar purely mechanistic explanations for many of the behaviors that arise with the passions or emotions, in which the body acts first and the felt experience of the passion serves to get the mind to want to do what the body is already doing ( Pass. I.37-40). Descartes by no means held that all human behavior does or should arise from rational deliberation. Which is not to say that he devalued rational deliberation when there is time and need to undertake it. But he was under no illusion that all effective human behavior stems from reason.

How could interpreters get Descartes so wrong? One recent explanation suggests that many post-modern theorists have absorbed their Descartes at second hand, and the same explanation might be extended to others who invoke Descartes after only cursory engagement with his writings. As the literary historian Michael Moriarty explains, leading French theorists such as Jacques Lacan and Michel Foucault would have, in the course of their French educations, "received a solid grounding in philosophy, and in Descartes' works in particular" (Moriarty 2003, 52). They then use Descartes as a stalking horse. Moriarty suggests that many readers of Lacan and Foucault have not received the same education in philosophy or in Descartes. Such individuals, "who read Lacan or Foucault without, or before, reading Descartes, thus imbibe a certain perception of Descartes, more negative, perhaps, than the authors themselves, writing against the grain of their own culture, may have intended to convey" (2003, 53). The implication is that Lacan and Foucault engaged Descartes from a knowledge of his writings, whereas others who lack such knowledge misunderstand the value of such genuine engagement and take away misunderstood implications.

What is Descartes' legacy now? His influence on the seventeenth century is historically permanent, including his specific contributions in mathematics and optics, his vision for a mechanistic physiology, and the model he offered to Newton of a unified celestial and terrestrial physics that assigns a few basic properties to a ubiquitous matter the motions of which are governed by a few simple laws. In this regard, Descartes' work offers an example of culturally engaged philosophy. Descartes had a sense for the fundamental philosophical issues of his time, many of which concerned the theory of nature and the attempt to found a new natural science. He not only offered a systematic reformulation of the extant natural philosophy, but he did so in a way that could be heard and understood.

Beyond past historical influences, Descartes' philosophy continues to speak to us now and to offer new insights to new generations of philosophers who are in position to hear what he said. This can be seen in the revival of body-first theories of the emotions. (Ironically, some of Descartes' most vocal detractors among scientists who study the emotions, including Damasio 1994, espouse theories similar in many respects to Descartes' own, on which, see Hatfield 2007.) Further, his theories of sensory qualities have inspired new reflections (Simmons 2003), as has his account of distance perception (see Wolf-Devine 1993 and the entries on optics and perception in Nolan 2014). More generally, his Meditations is one of the most finely crafted examples of philosophical prose ever written. That in itself ensures its ongoing value.

In the end, Descartes' legacy partly consists of problems he raised, or brought into prominence, but did not solve. The mind-body problem is a case in point. Descartes himself argued from his ability clearly and distinctly to conceive mind and body as distinct beings to the conclusion that they really are separate substances. Most philosophers today accept neither the methodological basis for his claim nor the claim itself. All the same, the mind-body problem persists. In distinguishing the domain of the mental from that of the physical, Descartes struck a chord. Many philosophers accept the conceptual distinction, but remain uncertain of the underlying metaphysics: whether mind is identical with brain; or the mental emerges from complex processes in the brain; or constitutes a property that is different from any purely physical property, even while being instantiated by the brain. In this case, a problem that Descartes made prominent has lived far beyond his proposed solution.

Bibliography

Note on references and abbreviations: References to Descartes' works as found herein use the pagination of the Adam and Tannery volumes (AT), Oeuvres de Descartes, 11 vols. The citations give volume and page numbers only (dropping the abbreviation "AT"). Where possible, the Cottingham, Stoothoff, Murdoch, and Kenny translation, The Philosophical Writings of Descartes, 3 vols., has been used; it shows the AT pagination in the margins. The AT volume numbers provide a guide to which work is being cited in translation: vols. 1-5, 10, correspondence; vol. 6, Discourse and essays (including the Dioptrics and Meteorology); vol. 7, Meditations; vol. 10, Rules; vol. 11:1-118, World, or Treatise on Light; vol. 11:119-222, Treatise on Man; vol. 11:301-488, Passions. Where there is no accessible translation for a citation from AT, the citation is shown in italics. Works that are broken into parts and/or articles are cited by abbreviated title, part, and article: Diop. for Dioptrics, Disc. for Discourse on the Method, Med. for the Meditations, Met. for the Meteorology, Pass. for the Passions, and Princ. for the Principles. When context doesn't show that Descartes' letters are being referred to, Correspondence, abbreviated as Corr., is added.

Primary Literature: Works by Descartes

Original editions and early translations

See Otegem 2002 for a full bibliography.

1637, Discours de la methode pour bien conduire sa raison, & chercher la verité dans les sciences. Plus La dioptrique, Les meteores, et La geometrie. Qui sont des essais de cete Methode, Leiden: Jan Maire. Digitized photographic reproduction (DPR)online (pdf).

1641, Meditationes de prima philosophia, in qua Dei existentia et animae immortalitas demonstrantur, Paris: Michel Soly. DPRonline (pdf).

1642, Meditationes de prima philosophia, in quibus Dei existentia & animae humanae à corpore distinctio demonstrantur: his adjunctae sunt variae objectiones doctorum virorum in istas de Deo & anima demonstrationes, cum responsionibus authoris, Amsterdam: Elzevir, 2nd edn. The main title was changed from the first edition, which had promised to demonstrate "the immortality of the soul"; this edition promises to demonstrate "the distinction of the human soul from the body." The seventh set of Objections and Replies, by Pierre Bourdin, first appeared in the second edition, along with Descartes' letter to Father Dinet, Bourdin's superior in the Jesuit order.

1643, Epistola ad celeberrimum virum D. Gisbertum Voetium, Amsterdam: Elzivir. DPRonline (pdf). Published in Dutch the same year. A related polemical work appeared posthumously (1656; see Verbeek 1988).

1644, Principia philosophiae, Amsterdam: Elzevir. DPRonline (pdf and tiff).

1644, Specimina philosophiae, seu Dissertatio de methodo recte regendae rationis & veritatis in scientiis investigandae: Dioptrice et Meteora, trans. Etienne de Courcelles, Amsterdam: Elzevir. DPRonline (pdf).

1647, Les meditations metaphysiques, touchant la premiere philosophie, dans lesquelles l'existence de Dieu, & la distinction réele entre l'ame & le corps de l'homme, sont demonstrées: et les Objections faites contre ces Meditations par diverses personnes tres-doctes, avec les réponses de l'Auteur, trans. Louis-Charles d'Albert, duc de Luynes (Meds.) and Claude Clerselier (Objections and Replies). Paris: Jean Camusat and Pierre Le Petit. DPRonline (pdf). The Seventh Objections and Replies appeared first in the 2nd French edn. (1661).

1647, Les principes de la philosophie, trans. Claude Picot, Paris: Henry Le Gras. DPRonline (pdf). Descartes added an "Author's letter" to the translation, as a preface.

1649, A discourse of a method for the well guiding of reason, and the discovery of truth in the sciences, London: Thomas Newcombe. Available through Early Modern Books (EMB), accessible through many College and University libraries.

1649, Geometria, trans. Frans van Schooten, Leiden: Jan Maire. DPRonline (pdf).

1649, Les passions de l'ame, Paris: Henry Le Gras. DPRonline (pdf).

1650, Compendium musicae, Utrecht: Gijsbert van Zijll and Dirck van Ackersdijck. DPRonline (pdf).

1650, Meditationes de prima philosophia, in quibus Dei existentia, & animæ humanæ à corpore distinctio, demonstrantur: his adjunctæ sunt variæ objectiones doctorum virorum in istas de Deo & anima demonstrationes; cum responsionibus authoris, Amsterdam: Elzevir, 3rd edn. DPRonline (pdf).

1650, Passiones animae, Henry Desmarets (trans.), Amsterdam: Elzevir. DPRonline (pdf).

1650, The passions of the soule, London: John Martin and John Ridley. Available through EMB.

1653, Compendium of musick: With necessary and judicious animadversions thereupon, William Brouncker (trans.), London: Thomas Harper. DPRonline (pdf).

1656, Querela apologetica, Groningen: Misopodem. A response to Voetius, written in 1648 in French and translated into Dutch and Latin (Otegem 2:472-75).

1657-67, Lettres, où sont traittées les plus belles questions de la morale, physique, medecine, et des mathematiques, 3 vols., Claude Clerseliers (ed.), Paris: Charles Angot. DPRs online,Vol. 1,Vol. 2,Vol. 3 (pdf).

1662, De homine figuris, trans. Florent Schuyl, Leiden: Leffen and Moyardum. DPRonline (pdf). A Latin translation of the original French, which was published asL'Homme in 1664.

1664, Le monde, ou, Le traite de la lumiere, et des autres principaux objects des sens, Paris: Girard. DPRonline (pdf).

1664, L'homme, et un Traitté de la formation du foetus, Claude Clerselier (ed.), Paris: Charles Angot. DPRonline (pdf). This is the first edition of Descartes' original French. It includes Remarks by Louis de la Forge and a translation of Florentius Schuyl's Preface to the Latin translation.

1668, Abbregé de la musique, Nicolas Poisson (trans.), Paris: Charles Angot. DPRonline (pdf).

1677, L' Homme, et La formation du foetus, avec Les remarques de Louis de La Forge; à quoy l'on a ajouté Le monde ou Traité de la lumière, Claude Clerselier (ed.), Paris: Theodore Girard, 2nd edition.L'Homme and Le Monde are brought together in this second edition. Clerselier had the firgures forLe Monde redrawn, and used a putatively more accurate manuscript for that work. DPRonline (pdf).

1680, Six metaphysical meditations wherein it is proved that there is a God and that mans mind is really distinct from his body: hereunto are added the objections made against these meditations by Thomas Hobbes, with the authors answers, William Molyneux (trans.), London: Benjamin Tooke. This translation of the six Meditations is reprinted in Gaukroger (2006). The entire book (including the Third Objections and Replies) is available through EMB.

1701, Opuscula posthuma, physica et mathematica, Amsterdam: Blaeu. DPRonline (pdf). The first publication of theRules in Latin (a Dutch translation had appeared in 1684), together with other writings.

Modern editions of Descartes' works: French and Latin

1936-1963, Correspondance, Charles Adam and Gaston Milhaud (eds.), 8 vols., Paris: Presses Universitaires de France. A complete edition of the known correspondence at date, with French translations accompanying letters in Latin.

1996, Oeuvres de Descartes,Charles Adam and Paul Tannery (eds.), 11 vols., Paris: Vrin/CNRS, new edition. Remains the standard edition, presenting works in the original French or Latin along with early translations of major works from Latin into French (Meditations, Principles) or from French into Latin (Discourse, Dioptrics, Meteorology), and also presenting manuscript material and posthumously published works. Cited by volume and page number.

2010, Oeuvres philosophiques, Fernand Alquié, Denis Moreau (eds.), 3 vols., Paris: Garnier, new edition. Contains major works in French, including correspondence (Latin letters are translated into French), with extensive notes.

2009-, Oeuvres complètes, Jean-Marie Beyssade and Denis Kambouchner (eds.), 8 vols. Paris: Gallimard. A new edition in French, including correspondence, with Latin facing page for theRules, Meditations, and Principles. Extensive notes and bibliography, with an overview of both French and English scholarship and some notice of material in German and Italian. (4 volumes have appeared to date.)

Recent English translations (selected)

1965, Discourse on Method, Optics, Geometry, and Meteorology, Paul J. Olscamp (trans.), Indianapolis: Bobbs-Merrill.

1972, Treatise of Man, Thomas S. Hall (trans.), Cambridge: Harvard University Press. With an introduction and many explanatory notes.

1979, Le Monde, ou Traité de la lumiere = The World, or Treatise on Light, Michael S. Mahoney (trans.), New York: Abaris Books. Facing page translation, with an introduction and explanatory notes.

1983, Principles of Philosophy, V. R. Miller and R. P. Miller (trans.), Dordrecht: Reidel.

1984-91, Philosophical Writings of Descartes, 3 vols., John Cottingham, Robert Stoothoff, Dugald Murdoch, and Anthony Kenny (ed. and trans.), Cambridge: Cambridge University Press.

1989, Passions of the Soul, Stephen H. Voss (trans.), Indianapolis: Hackett.

1990, Meditations on First Philosophy = Meditationes de prima philosophia, George Heffernan (trans.), Notre Dame: University of Notre Dame Press. A literal translation of the six Meditations proper, with facing-page Latin.

1994, Discours de la méthode: pour bein conduire sa raison et chercher la verité dans les sciences = Discourse on the Method: of Conducting One's Reason Well and of Seeking the Truth in the Sciences, George Heffernan (trans.), Notre Dame: University of Notre Dame Press. A bilingual edition with an interpretive essay.

1998, Meditations and Other Metaphysical Writings, Desmond M. Clarke (trans.), London: Penguin.

1998, Regulae ad directionem ingenii = Rules for the Direction of the Natural Intelligence: A Bilingual Edition of the Cartesian Treatise on Method, George Heffernan (trans.), Amsterdam: Editions Rodopi.

1998, The World and Other Writings, Stephen Gaukroger (trans.), Cambridge: Cambridge University Press.

1999, Discourse on Method and Related Writings, Desmond M. Clarke (trans.), London: Penguin.

2007, The Correspondence between Princess Elisabeth of Bohemia and René Descartes, Lisa Shapiro (trans.), Chicago: University of Chicago Press.

2006, A Discourse on the Method of Correctly Conducting One's Reason and Seeking Truth in the Sciences, Ian Maclean (trans.), Oxford: Oxford University Press.

2008, Meditations on First Philosophy: With Selections from the Objections and Replies, Michael Moriarty (trans.), Oxford: Oxford University Press. A fresh translation with detailed explanatory notes.

2015, The Passions of the Soul and Other Late Philosophical Writings, Michael Moriarty (trans.), Oxford: Oxford University Press. Also includes the correspondence with Elisabeth and other letters.

Other Primary Literature

Some works known to Descartes (selected)

Ordered alphabetically, translations shown where available.

Alhazen, or Ibn al-Haytham, and Witelo. Opticae thesaurus: Alhazeni Arabis libri septem nunc primum editi; ejusdem Liber de crepusculis et nubium ascensionibus. Item Vitellonis, libri X, Friedrich Risner (ed.), Basel: Episcopius. DPRonline (pdf). Translation of Alhazen's theory of vision:Alhacen's Theory of Visual Perception: A Critical Edition, with English Translation and Commentary, of the first three books of Alhacen's De aspectibus, the medieval Latin version of Ibn al-Haytham's Kitab al-Manazir, A. Mark Smith (trans.), 2 vols., Philadelphia: American Philosophical Society, 2001. Descartes cites Witelo (10:8).

Coimbran Commentators, 1598, Commentarium in tres libros De anima Aristotelis, Coimbra: Antonio de Maris.

Copernicus, Nicholas, 1543, De revolutionibus orbium caelestium, Nuremberg: Johannes Petreius; On the Revolutions of the Heavenly Spheres, Edward Rosen (trans.), Baltimore: Johns Hopkins University Press, 1978.

Eustachius a Sancto Paulo, 1638, Summa philosophiae quadripartita: de rebus dialecticis, ethicis, physicis, & metaphysicis, rev. ed., Cologne: Philip Albert. Available through EMB. Translation (selections):A Compendium of Philosophy in Four Parts, in Descartes' Meditations: Background Source Materials, Roger Ariew, John Cottingham, and Tom Sorell (eds.), Cambridge: Cambridge University Press,1998, 68-96.

Galilei, Galileo, 1610, Siderius nuncius, Venice: Tommaso Baglioni;Starry Messenger, Albert van Helden (trans.), Chicago: University of Chicago Press, 1989.

---, 1632, Dialogo sopra i due massimi sistemi del mondo, Tolemaico e Copernicano, Florence: Batista Landini;Dialogue Concerning the Two Chief World Systems, Ptolemaic & Copernican, Stillman Drake (trans.), Berkeley: University of California Press, 2nd edition, 1967.

---, 1638, Discorsi e dimostrazioni matematiche: intorno à due nuoue scienze, attenenti alla mecanica & i movimenti locali, Leiden: Elzevir; Two New Sciences, Including Centers of Gravity & Force of Percussion, Stillman Drake (ed. and trans.), Madison: University of Wisconsin Press.

Kepler, Johannes, 1604, Ad Vitellionem paralipomena, quibus astronomiae pars optica traditur, Frankfurt: Claudius Marnius and Ioannes Aubrius. Available through EMB. Translation:Optics: Paralipomena to Witelo and Optical Part of Astronomy,William H. Donahue (ed. and trans.), Santa Fe, N.M.: Green Lion Press, 2000.

---, 1611, Dioptrice, Augsburg: David Franke; reprint, Cambridge, UK: Heffer, 1962.

Rubio, Antonio, 1611, Commentarium in libros Aristotelis De anima, Alcalá de Henares: Andre Sanchez.

Toledo, Francisco de, 1575, Commentaria una cum quaestionibus in tres libros Aristotelis De anima, Venice: Iuntas.

For additional background sources, see Descartes' Meditations: Background Source Materials, Roger Ariew, John Cottingham, and Tom Sorell (eds.), Cambridge: Cambridge University Press, 1998.

Early reactions to Descartes' philosophy, favorable and unfavorable.

Ordered chronologically. Not exhaustive.

Heereboord, Adrianus, 1643, Parallelismus Aristotelicae et Cartesianae philosophiae naturalis, Leiden. Reported to have been read at Harvard College in the seventeenth century (Morison 1936, 1:233).

Schoock, Martin, 1643, Admiranda methodvs novae philosophiae Renati Des Cartes, Utrecht: Joannis van VVaesberge. An attack on Descartes; according to Schoock, Gisbertus Voetius also contributed. See Verbeek (1988, 1992).

Hogelande, Cornelius van, 1646, Cogitationes, quibus Dei existentia: item animae spiritalitas, et possibilis cum corpore unio, demonstrantur: nec non, brevis historia oeconomiae corporis animalis, proponitur, atque mechanice explicator, Amsterdam: Elzevir. Book dedicated to Descartes. Available online tlhrough Early Modern Books (EMB) in many university libraries.

Regius, Henricus, 1646, Fundamenta physices, Amsterdam: Elzevir. DPRonline (pdf). Influenced by Descartes' physics and physiology.

---, 1648, Brevis explicatio mentis humanae, sive animae rationalis: ubi explicatur, quid sit et quid esse possit, Utrecht: Dirck van Ackersdijck. Descartes replied with hisComments on a Certain Broadsheet.

Raei, Johannes de, 1654, Clavis philosophiae naturalis, seu Introductio ad naturae contemplationem Aristotelico-Cartesiana, Leiden: Elzevir. DPRonline (pdf). A student of Regius, favorable toward Descartes (see Verbeek 1992).

More, Henry, 1662, A Collection of Several Philosophical Writings, 2 vols., London: Flesher, 2nd edition; reprint, New York, Garland, 1978.

Heereboord, Adrianus, 1663, Philosophia naturalis: cum commentariis peripateticis antehac edita, Leiden: Cornelius Driehuysen. Discusses Descartes' mechanization of the vegetative and sensitive souls. Later editions available through EMB.

Spinoza, Benedictus de, 1663, Renati Des Cartes Principiorum philosophiae pars I et II, more geometrico demonstratae, Amsterdam: Joannes Riewerts. DPRonline (pdf). Spinoza's reconstruction of the arguments of the first two parts of Descartes'Principles of Philosophy.

La Forge, Louis de, 1666, Traitté de l'esprit de l'homme, de ses facultez et fonctions, et de son union avec le corps, suivant les Principes de René Descartes, Paris: Michel Bobin and Nicolas Le Gras. DPRonline (pdf). A treatise on the human mind following Descartes' principles.

More, Henry, 1671, Enchiridion metaphysicum, London: Flesher;Manual of Metaphysics, Alexander Jacob (trans.), 2 vols., Hildesheim: Georg Olms, 1995.

Rohault, Jacques, 1671, Traité de physique, Paris: Charles Savreux. DPRonline (pdf).

Poulain de la Barre, François, 1673, De l'égalité des deux sexes, discours physique et moral où l'on voit l'importance de se défaire des préjugez, Paris: Jean du Puis. DPRonline (pdf). Translation:A Physical and Moral Descourse concerning the Equality of Both Sexes, Desmond M. Clarke (trans.), in The Equality of the Sexes: Three Feminist Texts of the Seventeenth Century, Desmond M. Clarke (ed.), Oxford: Oxford University Press, 2013.

Huet, Pierre-Daniel, 1689, Censura philosophiae cartesianae, Paris: D. Horthemels; Against Cartesian Philosophy, Thomas M. Lennon (ed. and trans.), New York: Humanity Books, 2003.

Regis, Pierre Sylvain, 1690, Système de philosophie, contenant la logique, la métaphysique, la physique et la morale, Paris: Denys Thierry. DPR:Tome 1,Tome 2,Tome 3.

Le Grande, Antoine, 1694, An Entire Body of Philosophy, According to the Principles of the Famous Renate des Cartes, Richard Blome (trans.), 2 vols., London: Samuel Roycroft. Available through EMB (under EEBO). Originally published in Latin, 1672-1675.

Secondary Literature

References

Atherton, Margaret, 2001, "Cartesian Reason and Gendered Reason", in A Mind of One's Own: Feminist Essays on Reason and Objectivity, Louise M. Antony and Charlotte E. Witt (eds.), Boulder, CO: Westview Press, 2nd edition, 21-37.

Ben-Yami, Hanoch, 2015, Descartes' Philosophical Revolution: A Reassessment, Houndmills: Palgrave Macmillan.

Bordo, Susan (ed.), 1999, Feminist Interpretations of René Descartes, University Park, PA: Pennsylvania State University Press.

Boyer, Carl B., 1885, A History of Mathematics, Princeton: Princeton University Press, Ch. 17.

Brown, Deborah J., 2006, Descartes and the Passionate Mind, Cambridge: Cambridge University Press.

Brown, Deborah J., and Calvin G. Normore, 2019, Descartes and the Ontology of Everyday Life, Oxford: Oxford University Press.

Carriero, John, 2008, "Cartesian Circle and the Foundations of Knowledge", in Janet Broughton and John Carriero (eds.), A Companion to Descartes, Malden, MA: Blackwell, 302-18.

Clark, Desmond M., 2006, Descartes: A Biography. Cambridge: Cambridge University Press.

Cottingham, John, 1998, "Descartes' Treatment of Animals", in Descartes, ed. John Cottingham. Oxford: Oxford University Press, 225-33.

Curley, Edwin, 1978, Descartes against the Skeptics, Cambridge, MA: Harvard University Press.

Damasio, Antonio, 1994, Descartes' Error: Emotion, Reason, and the Human Brain, New York: Putnam.

Des Chene, Dennis, 2001, Spirits and Clocks: Machine and Organism in Descartes, Ithaca, NY: Cornell University Press.

Domski, Mary, 2022, "Descartes' Mathematics", The Stanford Encyclopedia of Philosophy (Fall 2022 Edition), Edward N. Zalta & Uri Nodelman (eds.), URL = <https://plato.stanford.edu/archives/fall2022/entries/descartes-mathematics/>.

Doney, Willis (ed.), 1987, Eternal Truth and the Cartesian Circle, New York: Garland Publishing.

Frankfurt, Harry G., 1962, "Memory and the Cartesian Circle", Philosophical Review, 71: 504-11.

---, 1965. "Descartes' Validation of Reason", American Philosophical Quarterly, 2: 149-56.

Garber, Daniel, 1992, Descartes' Metaphysical Physics, Chicago: University of Chicago Press.

Gaukroger, Stephen, 1995, Descartes: An Intellectual Biography, Oxford: Oxford University Press.

---, 2002, Descartes' System of Natural Philosophy, Cambridge: Cambridge University Press.

Hamerton, Katharine J., 2008, "Malebranche, Taste, and Sensibility: The Origins of Sensitive Taste and a Reconsideration of Cartesianism's Feminist Potential", Journal of the History of Ideas, 69(4): 533-58.

Hatfield, Gary, 1993, "Reason, Nature, and God in Descartes", in Stephen Voss (ed.), Essays on the Philosophy and Science of Rene Descartes, New York: Oxford University Press, 259-87.

---, 2000, "Descartes' Naturalism about the Mental," in Stephen Gaukroger, John Schuster, and John Sutton (eds.), Descartes' Natural Philosophy, London: Routledge, 630-58.

---, 2006, "Cartesian Circle," in Stephen Gaukroger (ed.), Blackwell Guide to Descartes' Meditations, Oxford: Blackwell, 122-41.

---, 2007, "The Passions of the Soul and Descartes's Machine Psychology", Studies in History and Philosophy of Science, 38: 1-35.

---, 2008, "Animals," in John Carriero and Janet Broughton (eds.), Companion to Descartes, Oxford: Blackwell, 404-25.

---, 2013, "Descartes on Sensory Representation, Objective Reality, and Material Falsity", in Karen Detlefsen (ed.), Descartes' Meditations: A Critical Guide, Cambridge: Cambridge University Press, 127-50.

---, 2020, "Geometry and Visual Space from Antiquity to the Early Moderns", in Andrew Janiak (ed.), Space, New York: Oxford University Press, 184-222.

Henry, John, 2008, The Scientific Revolution and the Origins of Modern Science, New York: Palgrave Macmillan.

Jacquette, Dale, 1996, "Descartes' Lumen Naturale and the Cartesian Circle", Philosophy and Theology: Marquette University Quarterly, 9: 273-320.

James, Susan, 1997, Passion and Action: The Emotions in Seventheetnth-Century Philosophy, Oxford: Clarendon Press.

Jorgensen, Larry M., 2020, "Seventeenth-Century Theories of Consciousness", The Stanford Encyclopedia of Philosophy (Spring 2020 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/spr2020/entries/consciousness-17th/>.

Lee, Sukjae, 2020, "Occasionalism", The Stanford Encyclopedia of Philosophy (Fall 2020 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/fall2020/entries/occasionalism/>.

Leech, David, 2013, The Hammer of the Cartesians: Henry More's Philosophy of Spirit and the Origins of Modern Atheism, Leuven: Peeters.

Loeb, Louis, 1992, "Cartesian Circle", in John Cottingham (ed.), Cambridge Companion to Descartes, Cambridge: Cambridge University Press, 200-35.

Machamer, Peter, and J. E. McGuire, 2006, "Descartes's Changing Mind", Studies in History and Philosophy of Science, 37: 398-419.

Manning, Gideon, 2014, "Descartes and the Bologna Affair", British Journal for the History of Science, 47: 1-13.

Mantovani, Mattia, 2022, "The Institution of Nature: Descartes on Human and Animal Perception", in Donald Rutherford (ed.), Oxford Studies in Early Modern Philosophy Volume 11, Oxford: Oxford University Press, 1-30.

McMullin, Ernan, 2008, "Explanation as Confirmation in Descartes's Natural Philosophy," in Janet Broughton and John Carriero (eds.), A Companion to Descartes, Malden, Mass.: Blackwell, 84-102.

Moriarty, Michael, 2003, Early Modern French Thought: The Age of Suspicion, Oxford: Oxford University Press.

Morison, Samuel Eliot, 1936, Harvard College in the Seventeenth Century, 2 vols., Cambridge, Mass.: Harvard University Press.

Newman, Lex, 2019, "Descartes' Epistemology", The Stanford Encyclopedia of Philosophy (Spring 2019 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/spr2019/entries/descartes-epistemology/>.

Nolan, Lawrence (ed.), 2014, The Cambridge Descartes Lexicon, Cambridge: Cambridge University Press.

---, 2021, "Descartes' Ontological Argument", The Stanford Encyclopedia of Philosophy (Spring 2021 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/spr2021/entries/descartes-ontological/>.

Otegem, Matthijs van, 2002, A Bibliography of the Works of Descartes (1637-1704) = Een bibliografie van de werken van Descartes (1637-1704), 2 vols., Utrecht: Zeno.

Popkin, Richard H., 1979, History of Scepticism from Erasmus to Spinoza, Berkeley: University of California Press.

Rashed, Roshdi, 1990, "A Pioneer in Anaclastics: Ibn Sahl on Burning Mirrors and Lenses", Isis, 81(3): 464-91.

Rodis-Lewis, Geneviève, 1998, Descartes: His Life and Thought, J. M. Todd (trans.), Ithaca: Cornell University Press.

Rozemond, Marleen, 2006, "The Nature of the Mind", in Stephen Gaukroger (ed.), Blackwell Guide to Descartes' Meditations, Oxford: Blackwell, 48-66.

Russell, Bertrand, 1914, Our Knowledge of the External World as a Field for Scientific Method in Philosophy, Chicago: Open Court.

Rutherford, Donald, 2021, "Descartes' Ethics", The Stanford Encyclopedia of Philosophy (Winter 2021 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/win2021/entries/descartes-ethics/>.

Sabra, A. I., 1967, Theories of Light from Descartes to Newton, London: Oldbourne Press.

Schmitter, Amy M., 2018, "Cartesian Prejudice: Gender, Education, and Authority in Poulain de la Barre", Philosophy Compass, 13(12): 1-12.

Schuster, John, 1980, "Descartes' Mathesis Universalis, 1619-28", in Stephen Gaukroger (ed.),Descartes: Philosophy, Mathematics and Physics, New Jersey: Barnes and Noble, 41-96.

Sebba, Gregor, 1987, The Dream of Descartes, Carbondale, IL: Southern Illinois University Press.

Shapin, Steven, 1996, The Scientific Revolution, Chicago: University of Chicago Press.

Shapiro, Lisa, 2003, "Descartes' Passions of the Soul and the Union of Mind and Body", Archiv für Geschichte der Philosophie, 85: 211-48.

Shea, William R., 1991, The Magic of Numbers and Motion: The Scientific Career of René Descartes, Canton, MA: Science History Publications.

Simmons, Alison, 2003, "Descartes on the Cognitive Structure of Sensory Experience", Philosophy and Phenomenological Research, 67: 549-79.

Slowik, Edward, 2021, "Descartes' Physics", The Stanford Encyclopedia of Philosophy (Winter 2021 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/win2021/entries/descartes-physics/>.

Underkuffler, Wilson, 2019, "Teresa, Descartes, and de Sales: The Art of Augustinian Meditation", Intellectual History Review, 30(4): 561-84.

Verbeek, Theo (ed.), 1988, La querelle d'Utrecht, Paris: Les impressions nouvelles. Includes writings by Martin Schoock and Descartes.

---, 1992, Descartes and the Dutch: Early Reactions to Cartesian Philosophy, 1637-1650, Carbondale, IL: Southern Illinois University Press.

Watson, Richard, 2007, Cogito, Ergo Sum, Boston: Godine, rev. edition.

Wee, Cecilia, 2006, Material Falsity and Error in Descartes' Meditations, London: Routledge.

Wells, Norman J., 1982, "Descartes' Uncreated Eternal Truths", The New Scholasticism, 56: 185-99.

Wheeler, Michael, 2005, Reconstructing the Cognitive World: The Next Step, Cambridge, MA: MIT Press.

Williams, Michael, 1986, "Descartes and the Metaphysics of Doubt", in Amélie Rorty (ed.), Essays on Descartes' Meditations, Berkeley: University of California Press, 117-139.

Wolf-Devine, Celia, 1993, Descartes on Seeing: Epistemology and Visual Perception, Carbondale, IL: Southern Illinois University Press.

Other Readings

Alanen, Lilli, 1994, "Sensory Ideas, Objective Reality, and Material Falsity", in John Cottingham (ed.), Reason, Will, and Sensation: Studies in Descartes's Metaphysics, Oxford: Clarendon Press, 229-50.

Ariew, Roger, 2011, Descartes among the Scholastics, Leiden: Brill.

Broughton, Janet, 2002, Descartes's Method of Doubt, Princeton: Princeton University Press.

Broughton, Janet, and John Carriero (eds.), 2008, Companion to Descartes, Malden, Mass.: Blackwell.

Brown, Deborah J., 2012, "Cartesian Functional Analysis", Australasian Journal of Philosophy, 90(1): 75-92.

Carriero, John, 2009, Between Two Worlds: A Reading of Descartes's Meditations, Princeton: Princeton University Press.

Clarke, Desmond M., 1982, Descartes' Philosophy of Science, University Park: Pennsylvania State University Press.

Cottingham, John (ed.), 1992, Cambridge Companion to Descartes, Cambridge: Cambridge University Press.

--- (ed.), 1994, Reason, Will and Sensation: Studies in Descartes's Metaphysics, Oxford: Clarendon Press.

--- (ed.), 1998, Descartes, Oxford: Oxford University Press.

Cunning, David, 2010, Argument and Persuasion in Descartes' Meditations, Oxford: Oxford University Press.

Des Chene, Dennis, 1996, Physiologia: Natural Philosophy in Late Aristotelian and Cartesian Thought, Ithaca: Cornell University Press.

Detlefsen, Karen (ed.), 2013, Descartes' Meditations: A Critical Guide, Cambridge: Cambridge University Press.

Dicker, Georges, 2013, Descartes: An Analytical and Historical Introduction, 2nd edn., New York: Oxford University Press.

Dobre, Mihnea, and Tammy Nyden (eds.), 2013, Cartesian Empiricisms, Dordrecht: Springer.

Flage, Daniel E., and Clarence A. Bonnen, 1999, Descartes and Method: A Search for a Method in Meditations, London: Routledge.

Frankfurt, Harry G., 1970, Demons, Dreamers, and Madmen, Indianapolis: Bobbs-Merrill.

Gaukroger, Stephen (ed.), 2006, The Blackwell Guide to Descartes' Meditations, Malden, Mass.: Blackwell.

Gaukroger, Stephen, John Schuster, and John Sutton (eds.), 2000, Descartes' Natural Philosophy, London: Routledge.

Gaukroger, Stephen, and Catherine Wilson (eds.), 2017, Descartes and Cartesianism: Essays in Honour of Desmond Clarke, Oxford: Oxford University Press.

Grene, Marjorie, 1985, Descartes, Minneapolis: University of Minnesota Press.

Guèroult, Martial, 1984-85, Descartes' Philosophy Interpreted According to the Order of Reasons, trans. R. Ariew, 2 vols., Minneapolis: University of Minnesota Press.

Hatfield, Gary, 1986, "The Senses and the Fleshless Eye: The Meditations as Cognitive Exercises", in Amèlie Rorty (ed.),Essays on Descartes' Meditations, Berkeley: University of California Press, 45-79.

---, 2014. Descartes' Meditations, London: Routledge.

Kenny, Anthony, 1968, Descartes: A Study of His Philosophy, New York: Random House.

Laudens, Laurens, 1966, "The Clock Metaphor and Probabilism: The Impact of Descartes on English Methodological Thought, 1650-65", Annals of Science, 22: 74-104.

Manning, Gideon, 2012, "Descartes' Healthy Machines and the Human Exception", in Sophie Roux and Dan Garber (eds.), The Mechanization of Natural Philosophy, New York: Kluwer, 237-62.

Menn, Stephen, 1998, Descartes and Augustine, Cambridge: Cambridge University Press.

Nelson, Alan (ed.), 2005, Blackwell Companion to Rationalism, Oxford: Blackwell.

Rorty, Amélie (ed.), 1986, Essays on Descartes' Meditations, Berkeley: University of California Press.

Rozemond, Marleen, 1998, Descartes's Dualism, Cambridge: Harvard University Press.

Secada, Jorge, 2000, Cartesian Metaphysics: The Late Scholastic Origins of Modern Philosophy, Cambridge: Cambridge University Press.

Shapiro, Lisa, 2003, "The Health of the Body-Machine? 17th Century Mechanism and the Concept of Health", Perspectives on Science, 11: 421-42.

Shea, William R., 1991, Magic of Numbers and Motion, Canton, Mass.: Science History Publications.

Simmons, Alison, 1999, "Are Cartesian Sensations Representational?" Noûs, 33: 347-69.

---, 2001, "Sensible Ends: Latent Teleology in Descartes' Account of Sensation", Journal of the History of Philosophy, 39: 49-75.

Smith, Norman Kemp, 1953, New Studies in the Philosophy of Descartes: Descartes as Pioneer, London: Macmillan.

Voss, Stephen (ed.), 1993, Essays on the Philosophy and Science of René Descartes, New York: Oxford University Press.

Williams, Bernard, 1978, Descartes, The Project of Pure Inquiry, London: Penguin.

Wilson, Margaret D., 1978, Descartes, London: Routledge & Kegan Paul.

Other Internet Resources

Links to digitized photographic reproductions of early editions of Descartes' works may be found under Original editions and early translations of major works. The following links are to other online editions:

French Meditations and Discourse, online French text of the six Meditations and theDiscourse (from the Association de Bibliophiles Universels,http://abu.cnam.fr/)

French Dioptrics, online French text ofLa dioptrique (from Les classiques des sciences sociales, University of Quebec at Chicoutimi)

French Discourse and Meditations, online French text of theDiscourse and six Meditations plus the first three Objections and Replies, from the edition of Descartes' works by Victor Cousin (Project Gutenburg)

Latin Meditations, online Latin text of the six Meditations (from The Latin Library, Classics Page, Neo-Latin, Descartes)

Latin Meditations, online Latin text of the six Meditations, ed. Artur Buchenau. Leipzig: Felix Meiner, 1913 (from Project Gutenburg)

Latin, French, and English Meditations, Latin text of the six Meditations, plus the 1647 French translation and the 1901 John Veitch translation into English (Wright State University)

French and English Passions (1649/1650), side by side (Descartes Web Project, Claremont Graduate University)

The following provide access to digital photographic reproductions of many additional texts:

Acknowledgments

The author thanks Elle Kirsch, who served as research assistant and offered many useful suggestions, and Tiina Rosenqvist, who provided many helpful comments.

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First published Wed Dec 3, 2008; substantive revision Mon Oct 23, 2023 René Descartes (1596-1650) was a creative mathematician of the first order, an important scientific thinker, and an original metaphysician. During the course of his life, he was a mathematician first, a natural scientist or "natural philosopher" second, and a metaphysician third.

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17th century pioneer of chemistry | Robert Boyle | A Short History

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Johannes Kepler, who is regarded as a pioneer in astronomy, is best known for his three laws of planetary motion. The first and most well-known law claims that, contrary to what was previously believed, planets revolve in ellipses rather than circles. Among Kepler's many other accomplishments, he offered a new and accurate explanation of how vision works, came up with a creative explanation for how light behaves in the newly invented telescope, discovered a number of new semiregular polyhedra, and provided a new theoretical foundation for astrology while also limiting the range of situations in which its predictions could be trusted. His influence is still important to learn today because he has made some of the most important discoveries in scientific history.

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Sir Isaac Newton contributed significantly to the field of science over his lifetime. He invented and provided a clear understanding of optics. But his most significant work had to do with forces, and specifically with the development of a universal law of gravitation and his laws of motion.

Isaac Newton's early life

Isaac Newton was born on Christmas Day to a poor farming family in Woolsthorpe, England, in 1642. At the time of Newton's birth England used the Julian calendar, however, when England adopted the Gregorian calendar in 1752, his birthday became 4th January 1643.

Isaac Newton arrived in the world only a few months after his father, Isaac Newton Sr, had died. "The boy expected to live managing the farm in the place of the father he had never known," wrote James Gleick in "Isaac Newton" ( Vintage, 2004).

However, when it became clear a farming life was not for him, Newton attended Trinity College in Cambridge, England. "He did not know what he wanted to be or do, but it was not tend sheep or follow the plough and the dung cart," wrote Gleick. While there, he took an interest in mathematics, optics, physics, and .

After his graduation, he began to teach at the college and was appointed as the second Lucasian Chair there. Today, the chair is considered the most renowned academic chair in the world, held by the likes of Charles Babbage and Stephen Hawking.

In 1689, Newton was elected as a member of parliament for the university. In 1703, he was elected as president of the Royal Society, a fellowship of scientists that still exists today. He was knighted by Queen Anne in 1705. He never married.

What are Isaac Newton's laws of motion?

Newton's most famous work came with the publication of his "Philosophiae Naturalis Principia Mathematica" ("Mathematical Principles of Natural Philosophy"), generally called Principia. In it, he determined the three laws of motion for the universe.

Newton's first law describes how objects move at the same velocity unless an outside force acts upon them. (A force is something that causes or changes motion.) Thus, an object sitting on a table remains on the table until a force - the push of a hand, or - acts upon it. Similarly, an object travels at the same speed unless it interacts with another force, such as friction.

His second law of motion provided a calculation for how forces interact. The law states that a force is equal to the change in the momentum (mass multiplied by velocity) per change in time. Therefore, when more force is applied to an object, its acceleration also increases, but when the mass of the object increases and the force remains constant, its acceleration decreases.

Newton's third law states that for every action in nature, there is an equal and opposite reaction. If one body applies a force on a second, then the second body exerts a force of the same strength on the first, in the opposite direction.

From all of this, Newton calculated the universal law of gravitation. He found that as two bodies move farther away from one another, the gravitational attraction between them decreases by the inverse of the square of the distance. Thus, if the objects are twice as far apart, the gravitational force is only a fourth as strong; if they are three times as far apart, it is only a ninth of its previous power.

These laws helped scientists understand more about the motions of planets in the solar system, and of around Earth.

Related: What makes Newton's laws work? Here's the simple trick.

Isaac Newton's apple

(Image credit: Photos.com via Getty Images)

A popular myth tells of an apple falling from a tree in Newton's garden, which brought Newton to an understanding of forces, particularly gravity. Whether the incident actually happened is unknown, but historians doubt the event - if it occurred - was the driving force in Newton's thought process.

The myth tells of Isaac Newton having returned to his family farm in Woolsthorpe, escaping Cambridge for a short time as it was dealing with a plague outbreak. As he sat in the farm's orchard, an apple fell from one of the trees (in some tellings it hit Newton on the head). Watching this happen, Newton began to consider the forces that meant the apple always fell directly towards the ground, beginning his examination of gravity.

One of the reasons that this story gained a foothold in popular understanding is that it is an anecdote Newton himself seems to have shared. "Toward the end of his life, Newton told the apple anecdote around four times, although it only became well known in the nineteenth century," wrote Patricia Fara, a historian of science at the University of Cambridge, in a chapter of "Newton's Apple and Other Myths about Science" (Harvard University Press, 2020).

However, it would be at least 20 years before Newton published his theories on gravity. It seems more likely that Newton used the story as a means of connecting the concept of gravity's impact on objects on with its impact on objects in space for his contemporary audience.

The apple tree in question - known as the "Flower of Kent" - still blooms in the orchard of Woolsthorpe Manor, and is now a popular tourist attraction.

Isaac Newton's inventions and discoveries

(Image credit: Apic / Contributor via Getty Images)

While a student, Newton was forced to take a two-year hiatus when plague closed Trinity College. At home, he continued to work with optics, using a prism to separate white light, and became the first person to argue that white light was a mixture of many types of rays, rather than a single entity. He continued working with light and color over the next few years and published his findings in "" in 1704.

Disturbed by the problems with telescopes at the time, he invented the reflecting telescope, grinding the mirror and building the tube himself. Relying on a mirror rather than lenses, the telescope presented a sharper image than refracting telescopes at the time. Modern techniques have reduced the problems caused by lenses, but large telescopes such as the James Webb Space Telescope use mirrors.

As a student, Newton studied the most advanced mathematical texts of his time. While on hiatus, he continued to study mathematics, laying the ground for differential and integral calculus. He united many techniques that had previously been considered separately, such as finding areas, tangents, and the lengths of curves. He wrote De Methodis Serierum et Fluxionum in 1671 but was unable to find a publisher.

Newton also established a cohesive scientific method, to be used across disciplines. Previous explorations of science varied depending on the field. Newton established a set format for experimentation still used today.

However, not all of Newton's ideas were quite as revolutionary. In P rincipia, Newton describes how rarefied vapor from comet tails is pulled into Earth's gravitational grasp and enables the movements of the planet's fluids along with the "most subtle and useful part of our air, and so much required to sustain the life of all things with us."

Isaac Newton quotes

"Amicus Plato amicus Aristoteles magis amica verita."

(Plato is my friend, Aristotle is my friend, but my greatest friend is truth.)

-Written in the margin of a notebook while a student at Cambridge. In Richard S. Westfall, Never at Rest (1980), 89.

"Genius is patience."

-The Homiletic Review, Vol. 83-84 (1922), Vol. 84, 290.

"If I have seen further it is by standing on the shoulders of giants."

-Letter to Robert Hooke (5 Feb 1675-6).In H. W. Turnbull (ed.), The Correspondence of Isaac Newton, 1, 1661-1675 (1959), Vol. 1, 416.

"I see I have made my self a slave to Philosophy."

-Letter to Henry Oldenburg (18 Nov 1676). In H. W. Turnbull (ed.), The Correspondence of Isaac Newton, 1676-1687 (1960), Vol. 2, 182.

"I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."

-First reported in Joseph Spence, Anecdotes, Observations and Characters, of Books and Men (1820), Vol. 1 of 1966 edn, sect. 1259, p. 462

"To any action there is always an opposite and equal reaction; in other words, the actions of two bodies upon each other are always equal and always opposite in direction."

- The Principia: Mathematical Principles of Natural Philosophy (1687)

"Truth is ever to be found in simplicity, and not in the multiplicity and confusion of things."

-'Fragments from a Treatise on Revelation". In Frank E. Manuel, The Religion of Isaac Newton (1974), 120.

How did Sir Isaac Newton die?

Newton died in 1727 during his sleep at the age of 84. Although the cause of death is unknown, a 1979 study published by Newton's own Royal Society suggests mercury poisoning may have contributed to the decline of his physical and mental health. During the exhumation of his body, large amounts of mercury were found in the scientist's system, likely due to his work with alchemy. Newton conducted several experiments to convert base metals, such as mercury and copper into precious metals, such as gold and silver.

"In 1693 Newton suffered from insomnia and poor digestion; and he also wrote irrational letters to friends. Although most scholars have attributed Newton's breakdown to psychological factors, it is possible that mercury poisoning may have been the principal cause," wrote L. W. Johnson and M. L. Wolbarsht "Mercury Poisoning: A probable cause of Isaac Newton's physical and mental ills: Notes and Records of the Royal Society of London Vol. 34. No. 1." .

After his death, his body was moved to a more prominent place in Westminster Abbey. His white and grey marble monument stands in the nave of the Abbey's choir screen and boasts sculptures of Newton lounging surrounded by children using the many instruments, such as telescopes, associated with Newton's work. The inscription on the monument - originally written in Latin - reads:

" Here is buried Isaac Newton, Knight, who by a strength of mind almost divine, and mathematical principles peculiarly his own, explored the course and figures of the planets, the paths of comets, the tides of the sea, the dissimilarities in rays of light, and, what no other scholar has previously imagined, the properties of the colours thus produced. Diligent, sagacious and faithful, in his expositions of nature, antiquity and the holy Scriptures, he vindicated by his philosophy the majesty of God mighty and good, and expressed the simplicity of the Gospel in his manners. Mortals rejoice that there has existed such and so great an ornament of the human race! He was born on 25th December 1642, and died on 20th March 1726." The date of his death on his monument is given in the Julian calendar.

Additional resources

If you want to learn more about the impact of this celebrated scientist, then you should read about how Isaac Newton Changed the World. If you're wondering whether Newton's second law of motion works in space then an Astronaut has tested the theory out.

Bibliography

"Isaac Newton" by James Gleick (Vintage, 2004)

"Mercury Poisoning: A probable cause of Isaac Newton's physical and mental ills: Notes and Records of the Royal Society of London Vol. 34. No. 1." by L. W. Johnson and M. L. Wolbarsht (July 1979)

"The Mathematical Principles of Natural Philosophy" by Isaac Newton (Flame Tree Collections, 2020)

"Newton's Apple and Other Myths about Science" edited by Ronald L. Numbers and Kostas Kampourakis (Harvard University Press, 2020)

"Life After Gravity: Isaac Newton's London Career" by Patricia Fara (Oxford University Press, 2021)

"Isaac Newton" Stanford Encyclopedia of Philosophy (2007)

"Isaac Newton" University of St Andrews (2000)

"Sir Isaac Newton" Westminster Abbey (2023)

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Sir Isaac Newton: Quotes, Facts & Biography

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A short history of Sir Isaac Newton, the mathematician and physicist that helped invent and explain some of the most fundamental laws of science.

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Zacharias Janssen (born between 1580 and 1588, died in 1638) was a famous Dutch inventor and spectacle-maker that is today best known for his associations in the creation of early models of telescope and compound microscope. However, today's historians cannot find conclusive evidence of Janssen truly being first man that created those instruments, with several clams of discovery that were made even after his death.

Zacharias Janssen was born in Hague, as a son of Hans Martens and Maeyken Meertens who were most probably from Antwerp, Belgium. After birth the family moved to Middelburg, Netherlands, and young Zacharias Janssen grow up on the street, constantly breaking the law and being chased by the authorities. Between age 30 and 35 he was appointed as the guardian of two children of local spectacle maker Loys Lowyssen. During that time, he started to focus his work on creating spectacles, which was very lucrative work that often required spectacle-makers to work in secret and hide their findings from general population. During that time, Zacharias Janssen lived door to door to the spectacle maker Hans Lippershey who is today credited as the creator of the first telescope.

The reason why so many historians cannot be sure about validity of Zacharias Janssen's claims is because his entire life he had run-ins with the law, and none of his claims received thorough verification. He was accused several times for counterfeiting coins, (as a lens-grinder he had tools and expertise to do that), he was arrested several times, and escaped from law until he managed to get all his charges dropped. Reports of his discoveries were often very conflicting, and they came only from their family or friends. Additionally, almost all records of his life were destroyed in the bombing of Netherlands by Nazis in 1940, but many investigation files managed to be saved (original documents were lost).

Janssen's claim on Telescope arose at the same time as Hans Lippershey applied for the patent for his telescope. In addition to them two, Jacob Metius of Alkmaar also tried to secure patent claim. None of them received patent, but only Lippershey received award from government for being the first who officially applied for patent. Some reports clearly described that Jacob Metius created telescope first, while Jenssen claimed that his father created first design in 1590. Jassen's son testified in court that this was true. In the end, none of them were awarded the patent.

Few years after his unsuccessful attempt to patent microscope and telescope, Zacharias Janssen started counterfeiting coins, and from that point he did not work anymore on any new discoveries in the world of the optics.

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Zacharias Janssen - Inventor of Optical Telescope

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Zacharias Janssen (born between 1580 and 1588, died in 1638) was a famous Dutch inventor and spectacle-maker that is today best known for his associations in the creation of early models of telescope and compound microscope.

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Copernicus had different achievements but his biggest and most well known is his proposal of the heliocentric solar system. This changed the entire idea of space and the Earth's place in the solar system. This and the round earth discovery were and are the basis of education and experimentation related to space.

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Anders Celsius Biography - Swedish Astronomer, Physicist and Mathematician

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Tycho Brahe was a Danish astronomer in the pre-telescope era. He created the geo-heliocentric theory of the solar system, he claimed that the Sun and Moon orbit Earth but the rest of the planets orbited the Sun. He also contributed to the lunar theory by discovering the variation of the Moon's longitude. Another discovery he made was observing one of the few recorded supernovas in the Milky Way Galaxy and the new star that came from it. He did all of this with his naked eye and tools to be made himself. Brahe would employ Johannes Kepler to work for him, after Brahe's death Kepler would use Brahe's work to write his own theories about the movement of planets. He also owned a pet elk and employed a dwarf jester named Jepp who Tycho believed was psychic. We should learn about him because he contributed many things to the Scientific Revolution with other well known astronomers like Kepler building upon his work.

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Rediscovering Robert Hooke - Summer Science Exhibition 2006

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Giordano Bruno was an italian philosopher, poet, cosmological theorist and esotericist. The most notable of Bruno's discoveries were his theories of the infinite universe and the multiplicity of words. In this theory he actively rejected the the traditional geocentric astronomy. He also went beyond the Copernican Heliocentric theory, which maintained a universe with a sphere of fixed stars. We study Bruno today becasue what he preached went agianst the Catholic Church's beliefs and he was burned alive on a stake for this.

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Joseph Priestley was a chemist in 18th-century England. He was vocal in politics and sciences and also spoke in support of the American and French revolutions. Joseph's most notable discovery was when discovered oxygen by heating up mercury and molten glass into water. With the discovery of oxygen, he also discovered why things burn and went on to discover many other chemical compounds.

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Evangelista Torricelli was an Italian physicist and mathematician who invented the mercury barometer and discovered that it revealed the height of our planet's atmosphere through experimentation. In 1644, he came to the conclusion that we live at the bottom of "a sea of air."

Source:
https://www.famousscientists.org/evangelista-torricelli/

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Marie Lavoisier is known as the Mother of Chemistry and was a french chemist. She was the first female chemist in history. She was married to the chemist Antoine Lavoisier and is known for assisting him in experiments and taking part in much of his work (she was critical to his success). She is known for using her linguistic skills to write up her husband's work and spread it to international audiences.  She is the most known accomplishment of translating Richard Kirwan's "Essay on Phlogiston" from English to French. She also assisted her husband in his discoveries of oxygen and the chemical composition of water (they discovered oxygen gas) and established the law of conservation of mass.

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Rene Descartes - “I think, therefore I am”

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Robert Boyle also known as the father of chemistry created Boyle's law which states that as pressure incresses the volume of gas decresses. This is important today because any invention that uses pressurized gas like scuba tanks and air compressers. It is important to learn about Robert Boyle because he is known as the father of chemistry and his reaserch lead to the discovery of many scientific achevments.

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Francis Bacon was a scientist, philosopher, statesman, jurist and was the Attorney General and Lord Chancellor of England.  Francis is most known for his creation of the Scientific Method. Another contribution he made was the plate tectonic theory / solid Earth geophysics theory. Empiricism

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The History of Barometer | Know More About Evangelista Torricelli

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Galileo was an Italian philosopher, astronomer, and mathematician. He invented the thermometer, and discovered the four biggest moons of Jupiter, and the rings of Saturn. We should still learn about Galileo because invented and discovered many major aspects of science, and his improvements to the telescope later helped Isacc Newton to expand on his work and come up with his own theories. His curiousity to science helped other scientists to expand their knowledge and influence todays world.

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William Harvey was a renowned English physician who tended to many English kings in the 17th century. Through his work with kings and other patients, and animals, he discovered how the heart and the circulatory system worked. Prior to Harvey, people didn't understand the effect that oxygen had on blood and how the blood circulated. He also investigated the nature of the heart beat and what caused it to slow and speed up. Later in his life he lost many of his notes and works in the Great Fire of London and when troops ransacked his house. This led him to unsuccessfully attempt to take his own life, eventually dying a few years later by stroke.

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Edward Jenner is well known all over the world for his innovative contribution to vaccination and the eventual elimination of smallpox. Jenner's work is usually regarded as the founding work of immunology, even though he was neither the first to hypothesize that cowpox infection gave a specific immunity against smallpox nor the first to carry out cowpox injection for this purpose.

He relocated to St. George's Hospital in London in 1770 to finish his medical education there under the tutelage of the eminent surgeon and experimenter John Hunter. Edward's skills in dissection and research, as well as his knowledge of plant and animal anatomy, were rapidly recognized by Hunter. The two guys would continue to contact and be friends for life.

When Jenner, then 23 years old, returned to Berkeley in 1772, he quickly established himself as the town's physician and surgeon. Though he eventually opened up medical offices in London and Cheltenham, for the most part, Jenner spent the remainder of his life at Berkeley.

A dairymaid named Sarah Nelmes sought Jenner's advice in May 1796 over a rash on her hand. Instead of smallpox, he identified cowpox, and Sarah confirmed that one of her cows, a Gloucester cow named Blossom, recently had the disease. He decided on James Phipps, his gardener's eight-year-old son. On May 14, he scratched James' arm a little and smeared some material from one of Sarah's handpocks into those scratches. James suffered from cowpox for a few days before recovering fully a week later.

We study Edward Jenner because of his large innovative contributions to immunization and the ultimate eradication of smallpox.

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Danish astronomer Tycho Brahe made the most accurate celestial observations of his time and challenged the prevailing belief in how the universe was organized. And while most people may think of scientists as stodgy academic types, Brahe's flamboyant lifestyle and ignominious death would have made some of today's wild celebrities look like choirboys.

Born in Denmark in 1546, Brahe's parents were members of the nobility. He was raised by his wealthy uncle and attended universities in Copenhagen and Leipzig. Though he studied arts subjects with the intention of becoming a lawyer, Brahe chose instead to pursue astronomy.

In 1566, 20-year-old Brahe fought a fellow student in a duel over who was the better mathematician. As a result, he lost a large chunk of his nose. "It's the least desirable dueling scar. A dueling scar is a mark of aristocratic courage, but that's if you have a little line on your cheek or a little scratch on your forehead," John Robert Christianson, a Professor Emeritus of History at Luther College and the author of On Tycho's Island (Cambridge University Press, 2000) and Tycho Brahe and the Measure of the Heavens (Reaktion Books, 2020), told our sister publication All About History magazine. "Having half of your nose whacked off is not very appealing so he had to wear a prosthesis his whole life long. That was a source of ridicule on the part of his astronomical opponents."
Related:

Famous astronomers: How these scientists shaped astronomy
Tycho Brahe pronunciation

When Brahe was born, his Danish name was Tyge Otteson Brahe. However, he adopted a Latinized form, Tycho Brahe, when he was about 15 years old. There is not much consensus about how the name is pronounced in English. Some say his first name is "tee-ko"; others say "tie-ko." His last name is pronounced either "brah," "bra-hay" or "bra-hee." He is sometimes referred to only by his first name, as in Tycho Crater on , the Tycho Deep Space capsule and Tycho's Supernova Remnant.

Contribution to astronomy

Despite his colorful life, Brahe contributed a great deal to the field of . At the time, astronomers held to the idea that the heavens were composed of separate individual spheres, with everything revolving around Earth.

In 1572, Brahe observed a supernova in the constellation of Cassiopeia. Brighter than , the new star remained visible for a year and a half. In 1577, he observed a . Current theory taught that both were disturbances in the atmosphere. However, Brahe's precise measurements revealed differently. He proved that the never changed with regard to the surrounding and that the comet orbited beyond the path of the moon, contradicting the idea that the heavens never changed.

"Upon learning that Tycho intended to leave Denmark [in 1575] in order to pursue his interests in studying the universe, King Frederick granted him the island of Hven, and helped finance the building of his observatory there," Associate Professor of History at Swansea University and author of Bearing the Heavens: Tycho Brahe and the Astronomical Community of the Late Sixteenth Century (Cambridge University Press, 2007) Dr Adam Mosley told All About History magazine in an interview. "Tycho served as a kind of consultant to the royal court, commenting on and explaining unusual natural phenomena, such as the great comet of 1577, and he produced horoscopes for the royal family."

At his observatory, Brahe kept meticulous observations of the heavens. While most astronomers only focused on observing heavenly bodies at specific, unusual points in their orbits, Brahe intently tracked them in their entire visible orbit across the sky, creating the most precise observations made at the time. Some of his measurements were accurate to half an arc minute, which is especially admirable given that they were all made before the advent of the telescope.

Although Brahe's observations revealed the flaws of the current system, he did not embrace Nicolaus Copernicus' newly proposed sun-centered model. Instead, he offered a model that combined the two, setting the moon and sun in orbit around even as the other five known planets circled . The model became popular among those who wanted to leave the older view behind but weren't ready to embrace the idea of the sun at the center of the solar system.

(Image credit: Fototeca Gilardi/Getty Images)

Related: Geocentric model: The Earth-centered view of the universe

"In Tycho Brahe's geo heliocentric system, the planets moved around the sun, and the stars, sun, and moon moved around Earth, with Earth at the center of the universe," explained Mosley. "This geo heliocentric system was thought by Tycho Brahe, and by some astronomers and philosophers in the seventeenth century, to overcome some of the problems with the two alternative systems."

According to Rice University's Galileo Project, "Tycho Brahe's contributions to astronomy were enormous." In addition to his observations, he also designed and built instruments, periodically calibrating them and checking their accuracy. "He thus revolutionized astronomical instrumentation."

Another of Tycho's most impressive accomplishments was his use of statistical information. "This was his great contribution - he basically invented the concept of data. He was the first person to repeat an observation or experiment in order to verify it," stated Christianson. The vast body of data collected by Tycho not only helped him in his work but also served the astronomers who followed him.

(Image credit: myLoupe/Universal Images Group via Getty Images)

Tycho's Observatory

On the island of Hven, Tycho's scientific sanctuary was built and named Uraniborg. "You can still look out the window of the king's apartment at Kronborg Castle and you're looking right at that island," Christianson told All About History. "That was how the king chose that island. He gave it to Tycho Brahe and said: "I want you to continue to pursue your studies because you will bring fame to the Kingdom of Denmark. I want you to do it on that island within sight of the new castle that I'm building here. That will be the showplace of the realm and I want your little island to be part of that showplace." When monarchs built palaces at that time, you had to have a chamber of wonders - a display of marvelous things that are exemplars of your royal power and King Frederick II wanted Tycho Brahe to build that little island of wonders."

The elaborate Renaissance villa on Hven had towers on the outer sides which were part of his observatory and housed his large scientific instruments. Panels in the roofs of these towers could be removed to reveal the sky to those inside. At Uraniborg, Tycho carried out his observations for around 20 years. He made a variety of new instruments to help him in his work, ran his own printing press and educated and trained upcoming astronomers from around Europe. The island of Hven now has a Tycho Brahe museum where visitors can also see the ruins of his famous observatory.

How Kepler got his start

Brahe's precise measurements laid the foundation for a new understanding of the motion of the planets. German astronomer Johannes Kepler contacted him at the end of the 16th century in an effort to obtain copies of the Danish astronomer's research. Brahe countered with a suggestion that Kepler could work as his assistant, helping him to compile his data.

By this time, Brahe was based in Prague at the court of Holy Roman Emperor Rudolf II and worked as the Imperial Mathematician assisted by Kepler. Their relationship was often a fraught one, with Kepler wanting to use Brahe's data to support Copernicus' theory of heliocentrism, rather than help Brahe refine his own model of the universe. However, the two men continued to collaborate despite their differences.

Brahe suggested that Kepler work on solving the dilemma that plagued astronomers. Because of its orbit, Mars appears to occasionally move backward across the sky, causing many astronomers to suggest epicycles, tiny circles within their orbit. Even Copernicus' suggestion that the planets orbited the sun in circles could not account for the red planet's strange motion.

After his death, the body of data that Brahe had accumulated during his lifetime proved to be particularly valuable, both intellectually, and importantly for his children, financially. "The Emperor offered to buy all of Tycho's instruments and his data but he was short on cash and so he made a down payment. For the rest of the lives of Tycho Brahe's children he continued, annually, to pay them the amount of the purchase," said Christianson. As the succeeding Imperial Mathematician, Kepler used the data purchased by the Emperor to further his own career and make a name for himself.

Kepler, using Brahe's detailed observations, realized that the planets moved around the sun not in circles but in stretched-out circles known as ellipses. However, the problem took him almost a decade to solve, and Kepler didn't publish it until well after Brahe's death.

A mysterious death

Brahe died in 1601 at the age of 54. While attending a banquet, societal customs did not allow him to excuse himself before his host. Brahe had drunk excessively but refused to leave to use the bathroom. It is thought that this exacerbated some kind of bladder infection the astronomer was suffering from and as a result, he developed uremia. According to Kepler in his account of his mentor's death, Brahe died five days after the banquet.

However, scientists who opened Brahe's grave in 1901 to mark the 300th anniversary of his death claimed to find mercury in his remains, fueling rumors that the astronomer was poisoned. Some even accused a jealous Johannes Kepler of the crime.

Brahe's body was exhumed again in 2010. Tests on his bones and beard hairs showed that mercury concentrations in his body were not high enough to have killed him.

"In fact, chemical analyses of the bones indicate that Tycho Brahe was not exposed to an abnormally high mercury load in the last five to ten years of his life," researcher Kaare Lund Rasmussen, an associate professor of chemistry at the University of Southern Denmark who analyzed mercury levels in Brahe's beard and bones, said in a statement.

However, further research revealed that the astronomer was exposed to high levels of gold in his lifetime. "We found traces of gold in Tycho Brahe's hair, and we can establish that he was exposed to gold while these hairs were still on his body," Rasmussen said in a 2016 statement.

"It may have been the cutlery and plates of gold, or maybe the wine he drank contained gold leaf. It's also possible that he concocted and consumed elixirs containing gold, or that he worked with alchemy,"

Researchers also found that greenish stains around the nasal areas of Brahe's corpse contained traces of copper and zinc, indicating that his fake nose was made of brass and not silver or gold, as many had believed.

"When we exhumed the body in 2010, we took a small bone sample from the nose so that we could examine its chemical composition," project leader Jens Vellev, an archaeologist at Aarhus University in Denmark, said the same statement. "Surprisingly, our analyses revealed that the prosthesis was not made of precious metals, as was previously supposed ... So Tycho Brahe's famous 'silver nose' wasn't made of silver after all."

Additional resources

To learn more about Tycho Brahe's supernova, NASA has some useful resources. you Or, if you want to take a trip to Brahe's observatory on the island of Hven, information can be found about the Tycho Brahe museum there on their official website.

Bibliography

Christianson, J. R. (2000). On Tycho's Island. Cambridge University Press.

Christianson, J. R. (2020). Tycho Brahe and the Measure of the Heavens. Reaktion Books.

Mosley, A. (2007). Bearing the Heavens: Tycho Brahe and the Astronomical Community of the Late Sixteenth Century. Cambridge University Press.

O'Connor, J.J. and Robertson, E.F. (2003). Tycho Brahe. Retrieved from https://mathshistory.st-andrews.ac.uk/Biographies/Brahe/

Smith, Y. (2019, Oct 23). The Tycho Supernova: Death of a Star. NASA. Retrieved from https://www.nasa.gov/image-feature/the-tycho-supernova-death-of-a-star

Tycho Brahe (1546-1601) (1995). Retrieved from http://galileo.rice.edu/sci/brahe.html

The Astronomers Tycho Brahe and Johannes Kepler. Retrieved from https://chandra.harvard.edu/edu/formal/icecore/The_Astronomers_Tycho_Brahe_and_Johannes_Kepler.pdf

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Tycho Brahe was a pioneering Danish astronomer from the 16th century whose data collection and discoveries changed the way scientists looked at the skies.

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Before 1796, the only known way to prevent smallpox infection was to deliberately infect a person with scabs from a person with smallpox. This deliberate infection was called variolation, and it was done under the supervision of a physician or someone who knew how to give just enough infectious materials to elicit an immune response without a full-blown infection. The latter was not always the case, but the severity and risk of death from variolation were lower than the risk of acquiring smallpox "the natural way."

In May 1796, a British physician named Edward Jenner tested his hypothesis that direct inoculation of a person with the much milder and less deadly cowpox would render that person immune to smallpox. Jenner based this theory on his observations of milkmaids. Those milkmaids who had acquired cowpox through their contact with cows were immune to smallpox even when exposed multiple times to the deadly disease. There is also ample evidence that Jenner had studied the findings of other researchers of his time, who reported a similar protective effect from having cowpox. However, it was Edward Jenner's detailed description of his experiments that convinced his colleagues and the authorities that inoculation with cowpox -- which he called vaccination -- was preferable in terms of safety to variolation.

Edward Jenner was born in 1749 in Berkeley, Gloucestershire, England. At the early age of 14, Jenner apprenticed for 7 years with surgeon Daniel Ludlow. Then, at the age of 21, Jenner attended St. George's Hospital in London, where he continued his medical studies. In 1788, he was elected fellow of the Royal Society. His keen observations of the nested cuckoo, and his publication of those findings, earned him that distinction.

It was that same ability to observe and describe events around him that led to the publication of his findings of a series of case studies on people exposed to cowpox and rendered immune to smallpox. Together with his experiment on James Phipps, his observations earned him the recognition of being the first person to develop a successful vaccine against smallpox. By 1803, Jenner's findings were translated to French and Spanish, and the King of Spain launched a vaccination campaign to the Americas and the Far East.

Throughout his life, Edward Jenner received many accolades for his contribution to the understanding of infectious disease. This was at a time when the prevailing theory of infectious disease was miasma theory, a theory stating that it is "bad air" causing disease. It would not be until later in the 1800s that the next and current theory of disease, germ theory, would be proven by Louis Pasteur and others, building on Jenner's work.

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History of Vaccines is an educational resource by the College of Physicians of Philadelphia, one of the oldest professional medical organizations in the US.

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Why are they important and what did they discover? 

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Why are they important and what did they discover? 

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Aristarchus was an inspiration for Nicolaus Copernicus's work. Aristarchus studied astronomy and wrote a book "On the Sizes and Distances of the Sun and Moon." He focuses on the geometric method rather than the numerical measurements. We should still learn about him today because he was the first to propose a sun-centered universe.

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Background and why he's important today.

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Background and why he's important today.

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Brief background.

During Andreas's lifetime, a physician named "Galen" was the most popular read for those learning about Anatomy, through his various studies about the human body and how it functioned in the 1100's. His studies were believed by the general public for centuries. (Galen was prevented from studying human corpses by Law)

However, Andreas Vesalius, a man who pursued a career in Anatomy as well, had to constantly look back to Galen's studies for reference. But as he continued reading back, he discovered that there were many discrepancies in Galen's work, especially as Andreas grew more literate in the subject.

Galen came to the conclusion that Galen had never actually dissected a human being, only various animals, which he used for his sources. At the age of 25, Andreas began challenging this "Galenism" through various amounts of lectures and teachings, as well as cooperating with other people to try and publish a new book that would provide more effective ideas and studies about Anatomy.

Why is he important today? Why should we keep reading about them?

At the age of 28, he published what is known today as the first scientifically accurate study of anatomical structure called "de Humanis corporis fabrica" (The structure of the Human body). A feat that Scientists would build off of for years to come.

We should keep reading about Andreas because he's an extremely important figure who, without his contribution to anatomy and science as a whole, may have left humans with incorrect and false depictions of how human organs functioned and how the human anatomical structure appears for centuries if he'd not disclosed his research.

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Aristarchus of Samos (born c. 310 bce-died c. 230 bce) was a Greek astronomer who maintained that Earth rotates on its axis and revolves around the Sun. On this ground, the Greek philosopher Cleanthes the Stoic declared in his Against Aristarchus that Aristarchus ought to be indicted for impiety "for putting into motion the hearth of the universe."

Aristarchus's work on the motion of Earth has not survived, but his ideas are known from references by the Greek mathematician Archimedes, the Greek biographer Plutarch, and the Greek philosopher Sextus Empiricus. Archimedes said in his Sand-Reckoner that Aristarchus had proposed a new theory which, if true, would make the universe vastly larger than was then believed. (This is because a moving Earth should produce a parallax, or annual shift, in the apparent positions of the fixed stars, unless the stars are very far away indeed.)

In the 16th century Aristarchus was an inspiration for Polish astronomer Nicolaus Copernicus's work. In his manuscript of Six Books Concerning the Revolutions of the Heavenly Orbs (1543), Copernicus cited Aristarchus as an ancient authority who had espoused the motion of Earth. However, Copernicus later crossed out this reference, and Aristarchus's theory was not mentioned in the published book.

Aristarchus's only extant work is On the Sizes and Distances of the Sun and Moon, the oldest surviving geometric treatment of this problem. Aristarchus takes as premises that

(1) as observed during a lunar eclipse, the diameter of Earth's shadow is twice the diameter of the Moon;

(2) the Moon and Sun are each 2 degrees in angular diameter; and

(3) at the time of quarter Moon, the angular distance between the Moon and the Sun is 87 degrees.

Using premise 3, Aristarchus showed that the Sun is between 18 and 20 times farther away from Earth than the Moon is. (The actual ratio is about 390.) Using this result and premises 1 and 2 in a clever geometric construction based on lunar eclipses, he obtained values for the sizes of the Sun and Moon. He found the Moon's diameter to be between 0.32 and 0.40 times the diameter of Earth and the Sun's diameter to be between 6.3 and 7.2 times the diameter of Earth. (The diameters of the Moon and the Sun compared with that of Earth are actually 0.27 and 109, respectively.)

In Aristarchus's day the geometric method was considered more important than numerical measurements. His premise 1 is reasonably accurate. Premise 2 overestimates the Moon's angular diameter by a factor of four, which is puzzling, since this is an easy measurement to make. (In a later publication, Aristarchus gave the angular size of the Moon as half a degree, which is about right, but he apparently did not modify his earlier work.) Premise 3 was probably not based on measurement but rather on an estimate; it is equivalent to assuming that the time from first quarter Moon to third quarter Moon is one day longer than the time from third quarter to first quarter. The true angle between Sun and Moon at the time of quarter Moon is less than 90 degrees by only 9 minutes of arc-a quantity impossible to measure in antiquity.

Later Greek astronomers, especially Hipparchus and Ptolemy, refined Aristarchus's methods and arrived at very accurate values for the size and distance of the Moon. However, because of the influence of premise 3, all ancient results grossly underestimated the size and distance of the Sun. Aristarchus's 19:1 ratio nevertheless remained more or less standard until the 17th century.

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Aristarchus of Samos | Ancient Astronomer & Heliocentrist

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Aristarchus of Samos was a Greek astronomer who maintained that Earth rotates on its axis and revolves around the Sun. On this ground, the Greek philosopher Cleanthes the Stoic declared in his Against Aristarchus that Aristarchus ought to be indicted for impiety "for putting into motion the hearth

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Edward Jenner was born in Berkeley, Gloucestershire on 17th May 1749. He was the eighth of the nine children born to the vicar of Berkeley, the Reverend Stephen Jenner, and his wife Sarah.

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the reason we should still learn about janssen is because of his invention it allowed us to reaserch and see things on such a small level and that helped out so much when it came to science as we use microscopes even today in classes and labs

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In full:

Galileo Galilei

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Galileo (born February 15, 1564, Pisa [Italy]-died January 8, 1642, Arcetri, near Florence) Italian natural philosopher, astronomer, and mathematician who made fundamental contributions to the sciences of motion, astronomy, and strength of materials and to the development of the scientific method. His formulation of (circular) inertia, the law of falling bodies, and parabolic trajectories marked the beginning of a fundamental change in the study of motion. His insistence that the book of nature was written in the language of mathematics changed natural philosophy from a verbal, qualitative account to a mathematical one in which experimentation became a recognized method for discovering the facts of nature. Finally, his discoveries with the telescope revolutionized astronomy and paved the way for the acceptance of the Copernican heliocentric system, but his advocacy of that system eventually resulted in an Inquisition process against him.

Early life and career

Galileo was born in Pisa, Tuscany, on February 15, 1564, the oldest son of Vincenzo Galilei, a musician who made important contributions to the theory and practice of music and who may have performed some experiments with Galileo in 1588-89 on the relationship between pitch and the tension of strings. The family moved to Florence in the early 1570s, where the Galilei family had lived for generations. In his middle teens Galileo attended the monastery school at Vallombrosa, near Florence, and then in 1581 matriculated at the University of Pisa, where he was to study medicine. However, he became enamoured with mathematics and decided to make the mathematical subjects and philosophy his profession, against the protests of his father. Galileo then began to prepare himself to teach Aristotelian philosophy and mathematics, and several of his lectures have survived. In 1585 Galileo left the university without having obtained a degree, and for several years he gave private lessons in the mathematical subjects in Florence and Siena. During this period he designed a new form of hydrostatic balance for weighing small quantities and wrote a short treatise, La bilancetta ("The Little Balance"), that circulated in manuscript form. He also began his studies on motion, which he pursued steadily for the next two decades.

In 1588 Galileo applied for the chair of mathematics at the University of Bologna but was unsuccessful. His reputation was, however, increasing, and later that year he was asked to deliver two lectures to the Florentine Academy, a prestigious literary group, on the arrangement of the world in Dante's Inferno. He also found some ingenious theorems on centres of gravity (again, circulated in manuscript) that brought him recognition among mathematicians and the patronage of Guidobaldo del Monte (1545-1607), a nobleman and author of several important works on mechanics. As a result, he obtained the chair of mathematics at the University of Pisa in 1589. There, according to his first biographer, Vincenzo Viviani (1622-1703), Galileo demonstrated, by dropping bodies of different weights from the top of the famous Leaning Tower, that the speed of fall of a heavy object is not proportional to its weight, as Aristotle had claimed. The manuscript tract De motu ( On Motion), finished during this period, shows that Galileo was abandoning Aristotelian notions about motion and was instead taking an Archimedean approach to the problem. But his attacks on Aristotle made him unpopular with his colleagues, and in 1592 his contract was not renewed. His patrons, however, secured him the chair of mathematics at the University of Padua, where he taught from 1592 until 1610.

Although Galileo's salary was considerably higher there, his responsibilities as the head of the family (his father had died in 1591) meant that he was chronically pressed for money. His university salary could not cover all his expenses, and he therefore took in well-to-do boarding students whom he tutored privately in such subjects as fortification. He also sold a proportional compass, or sector, of his own devising, made by an artisan whom he employed in his house. Perhaps because of these financial problems, he did not marry, but he did have an arrangement with a Venetian woman, Marina Gamba, who bore him two daughters and a son. In the midst of his busy life he continued his research on motion, and by 1609 he had determined that the distance fallen by a body is proportional to the square of the elapsed time ( the law of falling bodies) and that the trajectory of a projectile is a parabola, both conclusions that contradicted Aristotelian physics.

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Galileo | Biography, Discoveries, Inventions, & Facts

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Galileo, the brilliant Italian polymath, revolutionized our understanding of the universe through his groundbreaking discoveries in astronomy and contributions to scientific methodology.

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Without Harvey's discovery about the circulatory system, people would have continued to believe that there were two types of blood in the body; purple "nutritive" blood carrying nutrients to the liver and red "vital" blood distributing life-giving principle to the lungs. His findings about heartbeats help athletes, doctors, etc. to be able to control their heartbeat and their breathing when they need to.

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We should still study Joseph Priestly because he had a huge impact on not only chemistry but philosophy and sciences in general. He wrote papers on electricity and many gases. He discovered the most important gas in history that all living things need which was air and he also observed a plant releasing oxygen into the air known as photosynthesis by keeping a plant and a mouse in a sealed jar. Priestly advocated for the American and French revolution and with his controversial view of theology led him to flee to America and founded the American Chemical Society which today is the worlds largest scientific society in 1876.

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Robert Hooke worked with Robert Boyle to construct the Boylean air pump. Hooke discovered the law of elasticity or Hooke's law. The law states that the stretching of a solid body is proportional to the force applied to it, and it makes a base for the studies of stress and strain and the understanding of elastic materials. He was an early proponent of the theory of evolution. Hooke was interested in timekeeping, making balance springs for watches, and improvements in the pendulum for clock regulation. He also used the pendulum motion to measure the force of gravity and prove the Earth and Moon follow an elliptical path around the sun. Using the Gregorian-reflecting telescope he built, Robert Hooke discovered the fifth star in the Trapezium. In 1662, Hooke was appointed curator of experiments to the Royal Society of London and was elected again the following year. In 1665, Hooke published Micrographia ("Small Drawings"). He first used the word "cell" in this work.

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https://engines.egr.uh.edu/episode/1673#:~:text=Born%20Marie%20Anne%20Pierrette%20Paulze,collected%20taxes%20for%20the%20crown.

https://link.springer.com/article/10.1007/s00897980249a#:~:text=Marie%20Anne%20Lavoisier%20translated%20Richard,by%20Lavoisier%20and%20his%20colleagues.

https://www.metmuseum.org/press/news/2021/lavoisier-portrait-discoveries#:~:text=Lavoisier%20was%20a%20prominent%20chemist,which%20his%20wife%20actively%20collaborated.

https://sciencehistory.org/education/scientific-biographies/antoine-laurent-lavoisier/#:~:text=Antoine%2DLaurent%20Lavoisier%2C%20a%20meticulous,nomenclature%2C%20among%20many%20other%20accomplishments.

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Harvey's key work was Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Exercise on the Motion of the Heart and Blood in Animals), published in 1628, with an English version in 1653. Harvey's greatest achievement was to recognize that the blood flows rapidly around the human body, being pumped through a single system of arteries and veins, and to support this hypothesis with experiments and arguments. There had been suggestions, both within the European tradition (by 16th-century Spanish physician Servetus) and within the Islamic tradition (by 13th-century Muslim physician Ibn al-Nafīs) of a "lesser circulation," whereby blood circulated from the heart to the lungs and back, without circulating around the whole body.

Prior to Harvey, it was believed there were two separate blood systems in the body. One carried purple, "nutritive" blood and used the veins to distribute nutrition from the liver to the rest of the body. The other carried scarlet, "vivyfying" (or "vital") blood and used the arteries to distribute a life-giving principle from the lungs. Today these blood systems are understood as deoxygenated blood and oxygenated blood. However, at the time, the influence of oxygen on blood was not understood. Furthermore, blood was not thought to circulate around the body-it was believed to be consumed by the body at the same rate that it was produced. The capillaries, small vessels linking the arteries and veins, were unknown at the time, and their existence was not confirmed until later in the 17th century, after Harvey, when the microscope had been invented.

Harvey claimed he was led to his discovery of the circulation by consideration of the venous valves. It was known that there were small flaps inside the veins that allowed free passage of blood in one direction but strongly inhibited the flow of blood in the opposite direction. It was thought that these flaps prevented pooling of the blood under the influence of gravity, but Harvey was able to show that all these flaps are cardiocentrically oriented. For example, he showed that in the jugular vein of the neck they face downward, inhibiting blood flow away from the heart, instead of upward, inhibiting pooling due to gravity.

Harvey's main experiment concerned the amount of blood flowing through the heart. He made estimates of the volume of the ventricles, how efficient they were in expelling blood, and the number of beats per minute made by the heart. He was able to show, even with conservative estimates, that more blood passed through the heart than could possibly be accounted for based on the then current understanding of blood flow. Harvey's values indicated the heart pumped 0.5-1 litre of blood per minute (modern values are about 4 litres per minute at rest and 25 litres per minute during exercise). The human body contains about 5 litres of blood. The body simply could not produce or consume that amount of blood so rapidly; therefore, the blood had to circulate.

It is also important that Harvey investigated the nature of the heartbeat. Prior to Harvey, it was thought that the active phase of the heartbeat, when the muscles contract, was when the heart increased its internal volume. So the active motion of the heart was to draw blood into itself. Harvey observed the heart beating in many animals-particularly in cold-blooded animals and in animals near death, because their heartbeats were slow. He concluded that the active phase of the heartbeat, when the muscles contract, is when the heart decreases its internal volume and that blood is expelled with considerable force from the heart.

It is tempting to view Harvey, with his quantitative experiment and his model of the heart as a pump, as someone who supported or was inspired by the new mathematical and mechanical ideas of the 17th century, which played significant roles in the scientific revolution of the time. However, there is a need for considerable caution here. Harvey did quantify blood flow, but his quantification is very approximate, and he deliberately used underestimates to further his case. This is very different from the precise quantification leading to the mathematical laws of someone like Galileo. It was important that Harvey saw the heart as a pump, but he saw it as an organic pump, rather than as a mechanical pump. He also interpreted the blood as having an irreducible life force of its own. Harvey was deeply and bitterly opposed to the mechanical philosophy of French mathematician and philosopher René Descartes as well as to any purely mechanical conception of the human body.

Harvey's theory of circulation was opposed by conservative physicians, but it was well established by the time of his death. It is likely that Harvey actually made his discovery of the circulation about 1618-19. Such a major shift in thinking about the body needed to be very well supported by experiment and argument to avoid immediate ridicule and dismissal; hence the delay before the publication of his central work. In 1649 Harvey published Exercitationes Duae Anatomicae de Circulatione Sanguinis, ad Joannem Riolanem, Filium, Parisiensem ( Two Anatomical Exercises on the Circulation of the Blood) in response to criticism of the circulation theory by French anatomist Jean Riolan.

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William Harvey - Circulation, Anatomy, Physiology

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William Harvey - Circulation, Anatomy, Physiology: Harvey's key work was Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Exercise on the Motion of the Heart and Blood in Animals), published in 1628, with an English version in 1653.

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Oxygen and Other Discoveries in England

Joseph Priestley was born in Yorkshire, the eldest son of a maker of wool cloth. His mother died after bearing six children in six years. Young Joseph was sent to live with his aunt, Sarah Priestley Keighley, until the age of 19. She often entertained Presbyterian clergy at her home, and Joseph gradually came to prefer their doctrines to the grimmer Calvinism of his father. Before long, he was encouraged to study for the ministry. And study, as it turned out, was something Joseph Priestley did very well.

Aside from what he learned in the local schools, he taught himself Latin, Greek, French, Italian, German and a smattering of Middle Eastern languages, along with mathematics and philosophy. This preparation would have been ideal for study at Oxford or Cambridge, but as a Dissenter (someone who was not a member of the Church of England) Priestley was barred from England's great universities. So he enrolled at Daventry Academy, a celebrated school for Dissenters, and was exempted from a year of classes because of his achievements.

After graduation, he supported himself, as he would for the rest of his life, by teaching, tutoring and preaching. His first full-time teaching position was at the Dissenting Academy in Warrington. (Although obviously brilliant, original, outspoken and, by one report, of "a gay and airy disposition," Priestley had an unpleasant voice and a sort of stammer. That he made a living through lectures and sermons is further evidence of his extraordinary nature.)

In 1762, he was ordained and married Mary Wilkinson, the daughter of a prominent iron-works owner. She was, he noted, "of an excellent understanding, much improved by reading, of great fortitude and strength of mind, and of a temper in the highest degree affectionate and generous; feeling strongly for others and little for herself."

Priestley traveled regularly to London, and became acquainted with numerous men of science and independent thought, including an ingenious American named Benjamin Franklin, who became a lifelong friend. Franklin encouraged Priestley in his research, one result of which was The History and Present State of Electricity. For that work, and his growing reputation as an experimenter, Priestley was made a Fellow of the Royal Society in 1766.

The History book was too tough for a popular audience, and Priestley determined to write a more accessible one. But he could find no one to create the necessary illustrations. So, in typical fashion, he taught himself perspective drawing. Along the way, he made many mistakes, and discovered that India rubber would erase lead pencil lines - a fact he mentioned in the preface.

By the age of 34, Priestley was a well-established and respected member of Britain's scientific community. He was still paying a price for his religious nonconformity, however. When the explorer Captain James Cook was preparing for his second voyage, Priestley was offered the position of science adviser. But the offer was rescinded under pressure from Anglican authorities who protested his theology, which was evolving into a strongly Unitarian position that denied the doctrine of the trinity.

In retrospect, the Cook affair may have been all for the best. In 1773, the Earl of Shelburne asked Priestley to serve as a sort of intellectual companion, tutor for the earl's offspring, and librarian for his estate, Bowood House. The position provided access to social and political circles Priestley could never have gained on his own, while leaving ample free time for the research that would earn him a permanent place in scientific history.

He systematically analyzed the properties of different "airs" using the favored apparatus of the day: an inverted container on a raised platform that could capture the gases produced by various experiments below it. The container could also be placed in a pool of water or mercury, effectively sealing it, and a gas tested to see if it would sustain a flame or support life.

In the course of these experiments, Priestley made an enormously important observation. A flame went out when placed in a jar in which a mouse would die due to lack of air. Putting a green plant in the jar and exposing it to sunlight would "refresh" the air, permitting a flame to burn and a mouse to breathe. Perhaps, Priestley wrote, "the injury which is continually done by such a large number of animals is, in part at least, repaired by the vegetable creation." Thus he observed that plants release oxygen into the air - the process known to us as photosynthesis.

On August 1, 1774, he conducted his most famous experiment. Using a 12-inch-wide glass "burning lens," he focused sunlight on a lump of reddish mercuric oxide in an inverted glass container placed in a pool of mercury. The gas emitted, he found, was "five or six times as good as common air." In succeeding tests, it caused a flame to burn intensely and kept a mouse alive about four times as long as a similar quantity of air.

Priestley called his discovery "dephlogisticated air" on the theory that it supported combustion so well because it had no phlogiston in it, and hence could absorb the maximum amount during burning. (The year before, Swedish apothecary Carl Wilhelm Scheele isolated the same gas and observed a similar reaction. Scheele called his material "fire air." But his findings were not published until 1777.)

Whatever the gas was called, its effects were remarkable. "The feeling of it in my lungs," Priestley wrote, "was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards. Who can tell but that in time, this pure air may become a fashionable article in luxury. Hitherto only two mice and myself have had the privilege of breathing it."

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Joseph Priestley, Discoverer of Oxygen National Historic Chemical Landmark - American Chemical Society

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American Chemical Society: Chemistry for Life.

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Polish:

Mikołaj Kopernik

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Nicolaus Copernicus (born February 19, 1473, Toruń, Royal Prussia, Poland-died May 24, 1543, Frauenburg, East Prussia [now Frombork, Poland]) Polish astronomer who proposed that the planets have the Sun as the fixed point to which their motions are to be referred; that Earth is a planet which, besides orbiting the Sun annually, also turns once daily on its own axis; and that very slow long-term changes in the direction of this axis account for the precession of the equinoxes. This representation of the heavens is usually called the heliocentric, or "Sun-centred," system-derived from the Greek helios, meaning "Sun." Copernicus's theory had important consequences for later thinkers of the Scientific Revolution, including such major figures as Galileo, Kepler, Descartes, and Newton. Copernicus probably hit upon his main idea sometime between 1508 and 1514, and during those years he wrote a manuscript usually called the Commentariolus ("Little Commentary"). However, the book that contains the final version of his theory, De revolutionibus orbium coelestium libri vi ("Six Books Concerning the Revolutions of the Heavenly Orbs"), did not appear in print until 1543, the year of his death.

Early life and education

Certain facts about Copernicus's early life are well established, although a biography written by his ardent disciple Georg Joachim Rheticus (1514-74) is unfortunately lost. According to a later horoscope, Nicolaus Copernicus was born on February 19, 1473, in Toruń, a city in north-central Poland on the Vistula River south of the major Baltic seaport of Gdańsk. His father, Nicolaus, was a well-to-do merchant, and his mother, Barbara Watzenrode, also came from a leading merchant family. Nicolaus was the youngest of four children. After his father's death, sometime between 1483 and 1485, his mother's brother Lucas Watzenrode (1447-1512) took his nephew under his protection. Watzenrode, soon to be bishop of the chapter of Varmia (Warmia), saw to young Nicolaus's education and his future career as a church canon. ( See Researcher's Note for information about Copernicus's nationality.)

Between 1491 and about 1494 Copernicus studied liberal arts-including astronomy and astrology-at the University of Cracow ( Kraków). Like many students of his time, however, he left before completing his degree, resuming his studies in Italy at the University of Bologna, where his uncle had obtained a doctorate in canon law in 1473. The Bologna period (1496-1500) was short but significant. For a time Copernicus lived in the same house as the principal astronomer at the university, Domenico Maria de Novara (Latin: Domenicus Maria Novaria Ferrariensis; 1454-1504). Novara had the responsibility of issuing annual astrological prognostications for the city, forecasts that included all social groups but gave special attention to the fate of the Italian princes and their enemies. Copernicus, as is known from Rheticus, was "assistant and witness" to some of Novara's observations, and his involvement with the production of the annual forecasts means that he was intimately familiar with the practice of astrology. Novara also probably introduced Copernicus to two important books that framed his future problematic as a student of the heavens: Epitoma in Almagestum Ptolemaei ("Epitome of Ptolemy's Almagest") by Johann Müller (also known as Regiomontanus, 1436-76) and Disputationes adversus astrologianm divinatricenm ("Disputations against Divinatory Astrology") by Giovanni Pico della Mirandola (1463-94). The first provided a summary of the foundations of Ptolemy's astronomy, with Regiomontanus's corrections and critical expansions of certain important planetary models that might have been suggestive to Copernicus of directions leading to the heliocentric hypothesis. Pico's Disputationes offered a devastating skeptical attack on the foundations of astrology that reverberated into the 17th century. Among Pico's criticisms was the charge that, because astronomers disagreed about the order of the planets, astrologers could not be certain about the strengths of the powers issuing from the planets.

Only 27 recorded observations are known for Copernicus's entire life (he undoubtedly made more than that), most of them concerning eclipses, alignments, and conjunctions of planets and stars. The first such known observation occurred on March 9, 1497, at Bologna. In De revolutionibus, book 4, chapter 27, Copernicus reported that he had seen the Moon eclipse "the brightest star in the eye of the Bull," Alpha Tauri ( Aldebaran). By the time he published this observation in 1543, he had made it the basis of a theoretical claim: that it confirmed exactly the size of the apparent lunar diameter. But in 1497 he was probably using it to assist in checking the new- and full-moon tables derived from the commonly used Alfonsine Tables and employed in Novara's forecast for the year 1498.

In 1500 Copernicus spoke before an interested audience in Rome on mathematical subjects, but the exact content of his lectures is unknown. In 1501 he stayed briefly in Frauenburg but soon returned to Italy to continue his studies, this time at the University of Padua, where he pursued medical studies between 1501 and 1503. At this time medicine was closely allied with astrology, as the stars were thought to influence the body's dispositions. Thus, Copernicus's astrological experience at Bologna was better training for medicine than one might imagine today. Copernicus later painted a self-portrait; it is likely that he acquired the necessary artistic skills while in Padua, since there was a flourishing community of painters there and in nearby Venice. In May 1503 Copernicus finally received a doctorate-like his uncle, in canon law-but from an Italian university where he had not studied: the University of Ferrara. When he returned to Poland, Bishop Watzenrode arranged a sinecure for him: an in absentia teaching post at Wrocław. Copernicus's actual duties at the bishopric palace, however, were largely administrative and medical. As a church canon, he collected rents from church-owned lands; secured military defenses; oversaw chapter finances; managed the bakery, brewery, and mills; and cared for the medical needs of the other canons and his uncle. (Despite serving as a canon, Copernicus did not become a priest.) Copernicus's astronomical work took place in his spare time, apart from these other obligations. He used the knowledge of Greek that he had acquired during his Italian studies to prepare a Latin translation of the aphorisms of an obscure 7th-century Byzantine historian and poet, Theophylactus Simocattes. The work was published in Cracow in 1509 and dedicated to his uncle. It was during the last years of Watzenrode's life that Copernicus evidently came up with the idea on which his subsequent fame was to rest.

Copernicus's reputation outside local Polish circles as an astronomer of considerable ability is evident from the fact that in 1514 he was invited to offer his opinion at the church's Fifth Lateran Council on the critical problem of the reform of the calendar. The civil calendar then in use was still the one produced under the reign of Julius Caesar, and, over the centuries, it had fallen seriously out of alignment with the actual positions of the Sun. This rendered the dates of crucial feast days, such as Easter, highly problematic. Whether Copernicus ever offered any views on how to reform the calendar is not known; in any event, he never attended any of the council's sessions. The leading calendar reformer was Paul of Middelburg, bishop of Fossombrone. When Copernicus composed his dedication to De revolutionibus in 1542, he remarked that "mathematics is written for mathematicians." Here he distinguished between those, like Paul, whose mathematical abilities were good enough to understand his work and others who had no such ability and for whom his work was not intended.

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Nicolaus Copernicus | Biography, Facts, Nationality, Discoveries, Accomplishments, & Theory

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Nicolaus Copernicus, Polish astronomer who proposed that the Sun is the center of the solar system and that the planets circle the Sun. Copernicus also noted that Earth turns once daily on its own axis and that very slow long-term changes in the direction of this axis account for the precession of the equinoxes.

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He is best known for his discovery of cells, the law of elasticity, and his work as curator of experiments at the Royal Society. Learning about Hooke and his contributions can inspire further scientific inquiry and innovation. Understanding the history of science and the pioneers who made significant contributions to their fields helps to contextualize and appreciate the progress made in modern scientific research.

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Antoine Lavoisier known as the "Father of Modern Chemistry", was a brilliant French chemist. His groundbreaking experiments and discoveries revolutionized the field of chemistry. He was the first to recognize the role of oxygen in combustion and named it accordingly. Lavoisier also played a crucial role in the development of the metric system, which is still widely used today. His work, including the law of conservation of mass, laid the foundation for modern chemistry and continues to inspire scientists around the world. We should still learn Antoine Lavoisier today because his contributions to chemistry have had a lasting impact on our understanding of the subject. His discovered and experiments paved the way for modern chemistry and laid the foundation for many scientific principles that will still rely on today. By studying Lavoisier's work, we can gain a deeper appreciation for the scientific method and the importance of experimentation in advancing our understanding of the world. Plus, it is good to learn about the pioneers who shaped the field of science.

Cited Work:  Antoine Lavoisier | Biography, Discoveries, & Facts | Britannica

Couple words to describe him:
-"Father of modern chemistry"
- the law of conservation of mass
-paved way for modern chemistry
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Antoine Lavoisier and the Origin of Modern Chemistry  | AMS OpenMind

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Britannica, The Editors of Encyclopaedia. "Robert Hooke". Encyclopedia Britannica, 9 Oct. 2023, https://www.britannica.com/biography/Robert-Hooke. Accessed 12 October 2023.

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first to discover an element (phosphorus by mistake)

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first to discover an element (phosphorus by mistake)