Naturalists saw evolution as proof that the world is deterministic and that humans do not have free will. Since the evolutionary world is based on a series of links (each of which causes the next), any action humans make is not, as we might otherwise believe, a "first" step. Rather, the action has been caused by prior environmental, social, and biological factors beyond our individual control.

This deterministic view influenced the naturalists in a number of philosophical areas. Since humans do not have free will, the naturalists refrained from making moral judgments on the actions of their characters; after all, the environment, and not the human, has determined these actions. The naturalists also viewed the deterministic environment as indifferent and harsh to its inhabitants; accordingly, keen instinct rather than civilized intellect is necessary for survival (in "To Build a Fire," the man is lacking this instinct).

Naturalists also changed their subjects and language to reflect their ideas. The lower classes were more conducive to depicting the harsh, indifferent environment, and in "To Build a Fire," the lower-class man is trying to strike it rich in the Klondike Gold Rush. Moreover, since the naturalists believed that the deterministic world could be understood only through scientific facts, their prose style was usually more journalistic and spare.

London journeyed to the Yukon Territory in 1897 along with countless others hoping to make a score in the gold rush. In November 1897, he staked a claim in Henderson Creek, the destination of the man in "To Build a Fire." Though he left Alaska the following summer without much gold, he would draw from his rich experiences in the northern wilds for many of his lasting works, including Call of the Wild and White Fang and, of course, 1908's "To Build a Fire," usually considered his most lasting work.

The man continues on and, in a seemingly safe spot, falls through the snow and wets himself up to his shins. He curses his luck; starting a fire and drying his foot-gear will delay him at least an hour. His feet and fingers are numb, but he starts the fire. He remembers the old-timer from Sulphur Creek who had warned him that no man should travel in the Klondike alone when the temperature was fifty degrees below zero.

The man unties his icy moccasins, but before he can cut the frozen strings on them, clumps of snow from the spruce tree above fall down and snuff out the fire. Though building a fire in the open would have been wiser, it had been easier for the man to take twigs from the spruce tree and drop them directly below on to the fire. Each time he pulled a twig, he had slightly agitated the tree until, at this point, a bough high up had capsized its load of snow. It capsized lower boughs in turn until a small avalanche had blotted out the fire.

The man is scared, and sets himself to building a new fire, aware that he is already going to lose a few toes from frostbite. He gathers twigs and grasses. His fingers numb and nearly lifeless, he unsuccessfully attempts to light a match. He grabs all his matches--seventy--and lights them simultaneously, then sets fire to a piece of bark. He starts the fire, but in trying to protect it from pieces of moss, it soon goes out.

The man decides to kill the dog and puts his hands inside its warm body to restore his circulation. He calls out to the dog, but something fearful and strange in his voice frightens the dog. The dog finally comes forward and the man grabs it in his arms. But he cannot kill the dog, since he is unable to pull out his knife or even throttle the animal. He lets it go.

The man realizes that frostbite is now a less worrisome prospect than death. He panics and runs along the creek trail, trying to restore circulation, the dog at his heels. But his endurance gives out, and finally he falls and cannot rise. He fights against the thought of his body freezing, but it is too powerful a vision, and he runs again. He falls again, and makes one last panicked run and falls once more. He decides he should meet death in a more dignified manner. He imagines his friends finding his body tomorrow.

The man falls off into a comfortable sleep. The dog does not understand why the man is sitting in the snow like that without making a fire. As the night comes, it comes closer and detects death in the man's scent. It runs away in the direction of the camp, "where were the other food-providers and fire-providers."