While a prisoner of war in 1940/1941 Sartre read Martin Heidegger's Being and Time, an ontological investigation through the lens and method of Husserlian phenomenology (Husserl was Heidegger's teacher). Reading Being and Time initiated Sartre's own enquiry leading to the publication in 1943 of Being and Nothingness whose subtitle is 'A Phenomenological Essay on Ontology'. Sartre's essay is clearly influenced by Heidegger though Sartre was profoundly skeptical of any measure by which humanity could achieve a kind of personal state of fulfillment comparable to the hypothetical Heideggerian re-encounter with Being. In his much gloomier account in Being and Nothingness, man is a creature haunted by a vision of "completion", what Sartre calls the ens causa sui, and which religions identify as God. Born into the material reality of one's body, in an all-too-material universe, one finds oneself inserted into being (with a lower case "b"). Consciousness is in a state of cohabitation with its material body, but has no objective reality; it is nothing ("no thing"). Consciousness has the ability to conceptualize possibilities, and to make them appear, or to annihilate them.

In the introduction, Sartre sketches his own theory of consciousness, being and phenomena through criticism of both earlier phenomenologists (most notably Husserl and Heidegger) as well as idealists, rationalists and empiricists. According to him one of the major achievements of modern philosophy has been to free us of the kinds of dualism that set the existent up as having a "hidden" nature as with Kant's noumenon; Phenomenology has removed "the illusion of worlds behind the scene."

Based on an examination of the nature of phenomena, he describes the nature of two aspects of being, being-in-itself and being-for-itself. While being-in-itself is something that can only be approximated, a sort of being that can only be imagined as itself if it is imagined without a witnessing consciousness, being-for-itself is the being of consciousness.

When we go about the world, we have expectations which are often not fulfilled. For example, Pierre is not at the café where we thought we would meet him, so there is a negation, a void, a nothingness, in the place of Pierre. When looking for Pierre his lack of being there becomes a negation; everything he sees as he searches the people and objects about him are "not Pierre." So Sartre claims "It is evident that non-being always appears within the limits of a human expectation."

Bad faith or "Self-Deception", as translations vary, can be understood as the guise of existing as a character, individual or person who defines himself through the social categorization of his formal identity. This essentially means that in being a waiter, grocer, etc. one must believe that his or her social role is equivalent to his or her human existence. Living a life defined by one's occupation, social, racial or economic class, is the very faith of "bad faith", the condition in which people cannot transcend their situations in order to realize what they must be (human) and what they are not (a grocer, etc). It is also essential for an existent to understand that negation allows the self to enter what Sartre calls the "great human stream". The great human stream arises from a singular realization that nothingness is a state of mind in which we can become anything, in reference to our situation, that we desire.

The possibility of playing is afforded by time and situation. It isn't difficult to see how Sartre's ideas are linked to post-modernist/structural claims offered by Michel Foucault. However, the theories differ vastly with regard to human identity.

In any light, the difference in existence and identity projection remains at the heart of human subjects who are swept up by their own condition, their "bad faith." One of the most widely discussed examples of projection (via Freud's conception of the human mind) that Sartre uses is the café waiter who performs the duties, traditions, functions and expectations of a cafe waiter.

Sartre consistently mentions that in order to get out of bad faith, one must realize that his or her existence and his formal projection of a self are distinctly separate and within the means of human control. This separation is a form of nothingness. Nothingness, in terms of bad faith, is characterized by Sartre as the internal negation which separates pure existence and identity, and thus we are subject to playing our lives out in a similar manner. An example is something that is what it is (existence) and something that is what it is not (a waiter defined by his occupation).

Yet, Sartre takes a stance against characterizing bad faith in terms of "mere social positions"; I am never any one of my attitudes, any one of my actions. The good speaker is the one who plays at speaking because he cannot be speaking. This literally means that, like the cafe waiter, the speaker is not his condition or social categorization, but is a speaker consumed by bad faith. Thus, we must realize what we are (beings who exist) and what we are not (a social, historical, preoccupation) in order to step out of bad faith. Yet, existents (human beings) must maintain a balance between existence, their roles and nothingness to become authentic beings.

Additionally, an important tenet of bad faith is that we must enact a bit of good faith in order to take advantage of our role to reach an authentic existence. The authentic domain of bad faith, is realizing that the role we are playing is the lie. The goal of authenticity can be traced back to Dostoevsky's Notes from Underground. Dostoevsky has been called "The Grandfather of existentialism." To live and project into the future as a project of a self, while keeping out of bad faith and living by the will of the self is living life authentically. This is perhaps one of the main goals of Sartre's opus.