Much of the disagreement between Sartre and the Marxists, to which “Existentialism is a Humanism” is a response, stems from Sartre’s focus on the individual. Sartre takes Descartes’s cogito—the individual’s awareness of himself or herself as a thinking being–as a piece of foundational knowledge. He also emphasizes the radical freedom of human beings; for him, each person is wholly free. A person can make any choice he likes, and in doing so he determines who and what he really is. Of course, according to Sartre, choosing for oneself implies choosing for others, as well, so Sartre disagreed with the Marxists’ criticism that his philosophy was too individualistic and bourgeois.

Marxists objected to this picture of human beings because they conceived of the individual not as a radically-free unit but rather as a socially- and historically-conditioned member of a collective. A person cannot make any choice he likes; he can only make the choices that are available to a person of his class, at his time in history, in his location. Moreover, even if a person were free to make any choice whatsoever, it is not individual choices that construct history and lead to meaningful social change.