Millet and Courbet saw themselves as Realists,  the term used by Courbet to define his work in the introduction to an 1855 exhibition in Paris.
As landscape artists ("naturalists") they would often put nature - cliffs, stags, waterfalls -  at the heart of their works.  But there is no idealising of nature. It is real, and sometimes dark and menacing.
  After all, at a time when the large majority of the population of France still lived a rural life and tilled the land, nature, the outdoors, was primarily a workplace, not a place for leisure. Life was outdoors, so were death and life.
Many artist found Courbet's realism disgusting but a young up and coming realist who greatly admired Courbet, was Edouard Manet. Best known today as one of the Impressionists, Manet was more exactly a realist who was greatly admired by the younger Impressionists, but never actually exhibited in the Impressionist exhibitionsThrough this lineage, Courbet and Manet have come to be recognised as the founding fathers of modern art.