Sartre uses Zola's naturalistic novels, which show how deterministic forces grind down the characters' lives, to explain how individuals are reassured when they can hold something outside themselves responsible. Although the plots are grim in Zola's novels, the reader understands that the unhappy fate of characters is inevitable, given the circumstances of their lives. In Sartre's own novel Nausea the main character seems merely unpleasant and perverse because there is no ostensible reason for him to behave as he does, with no external force acting on him.
In "Existentialism Is a Humanism," Sartre embraces the Cartesian cogito, "I think; therefore, I am," describing it as "the absolute truth of consciousness confronting itself" and argues that "outside of this Cartesian cogito, all objects are merely probable." He believes that this cogito is "the only theory that endows man with human dignity."
Sartre was strongly influenced by the writings of Heidegger and views the German philosopher as part of his existential tradition. Heidegger, for his own part, took issue with Sartre's inclusion of him in "Existentialism Is a Humanism." His "Letter on Humanism" (1946), a critique of Sartre's lecture, argues that Sartre's privileging of the Cartesian cogito ("I think; therefore, I am") means that Sartre has unwittingly embraced essence before existence—if a person can think, there is already an essential being.
Kant and Sartre approach ethics from diametrically opposed positions. While Kant believes in an absolute moral law, Sartre believes that each person must invent his or her own values and that there is no essential morality. Although he agrees with Kant's statement that "freedom wills itself and the freedom of others," ultimately he views Kant as grounded too much in notions of universality.
In "Existentialism Is a Humanism" Sartre discusses several concepts introduced by Kierkegaard in works such as Purity of Heart Is to Will One Thing, Either/Or, and Fear and Trembling. Foremost among these concepts is the idea of bad faith.