“Bad faith”, as a first approximation, refers to self-deception. While lying to oneself might be the clearest example of what is meant by bad faith, most of the examples that Sartre discusses involve techniques that are subtler than overt lying, and might better be characterized as attempts to evade the truth and to keep it hidden from oneself. Such conduct is widespread and common, according to Sartre, especially when the truth to be evaded concerns one's own freedom and consequent responsibility. Accordingly, Sartre, as a champion of both freedom and truth, devotes a great deal of attention to describing, explaining, and attacking bad faith. Indeed, bad faith emerges as a central concept in his thought, one that is repeatedly taken up both in his philosophical work and (implicitly) in his literary writings, and throughout all phases of his career.

The concept of bad faith serves a number of functions in Sartre's work. For example:

1. • It helps him to explain the widespread acceptance of certain beliefs that he regards as obviously false.

2. • It aids him in his attempt to prove that the being of consciousness must be radically different from that of non-conscious things.

3. • As a central concept in his “existential psychoanalysis”, it serves as an indispensable tool in his attempt to understand human lives, both real (as in his biographical studies of Genet and Flaubert) and fictional (as in his development of characters in the story “The Childhood of a Leader” and the play No Exit, among other works).

4. Implicitly in his early works, and explicitly in his later ones, it functions as an instrument of moral criticism, as it is identified as a vice to be overcome, and is contrasted with a corresponding virtue, that of “authenticity”.