In the book, Sartre develops a philosophical account in support of his existentialism, dealing with topics such as consciousness, perception, social philosophy, self-deception, the existence of "nothingness", psychoanalysis, and the question of free will.

While a prisoner of war in 1940 and 1941, Sartre read Martin Heidegger's Being and Time (1927), which uses the method of Husserlian phenomenology as a lens for examining ontology.

Though influenced by Heidegger, 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".

On these grounds, Sartre goes on to offer a philosophical critique of Sigmund Freud's theories, based on the claim that consciousness is essentially self-conscious.

Being and Nothingness is regarded as both the most important non-fiction expression of Sartre's existentialism and his most influential philosophical work, original despite its debt to Heidegger.

Many have praised the book's central notion that "existence precedes essence", its introduction of the concept of bad faith, and its exploration of "nothingness", as well as its novel contributions to the philosophy of sex.

Sartre's existentialism shares its philosophical starting point with René Descartes: The first thing we can be aware of is our existence, even when doubting everything else (Cogito ergo sum).

[2] 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.

From Sartre's phenomenological point of view, nothingness is an experienced reality and cannot be a merely subjective mistake.

Though "it is evident that non-being always appears within the limits of a human expectation",[5] the concrete nothingness differs from mere abstract inexistence, such as the square circle.

Living a life defined by one's occupation, social, racial, or economic class, is the very essence 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 (waiter, grocer, etc.).

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 difference between existence and identity projection remains at the heart of human subjects who are swept up by their own condition, their "bad faith".

[8] Sartre consistently mentions that in order to get out of bad faith, one must realize that one's existence and one's formal projection of a self are distinctly separate and within the means of human control.

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.

Sartre believes that it is often created as a means of making the unbearable anguish of a person's relationship to their "facticity" (all of the concrete details against the background of which human freedom exists and is limited, such as birthplace and time) bearable.

Instead, "double reciprocal incarnation" is a form of mutual awareness which Sartre takes to be at the heart of the sexual experience.

Subsequently, humans seek to flee our anguish through action-oriented constructs such as escapes, visualizations, or visions (such as dreams) designed to lead us toward some meaningful end, such as necessity, destiny, determinism (God), etc.

This leads to failed dreams of completion, as Sartre described them, because inevitably we are unable to bridge the void between the purity and spontaneity of thought and all-too constraining action; between the being and the nothingness that inherently coincide in our self.

Though Sartre's conclusion seems to be that being diminishes before nothingness since consciousness is probably based more on spontaneity than on stable seriousness, he contends that any person of a serious nature is obliged to continuous struggle between two things: In Sartre's opinion, consciousness does not make sense by itself: it arises only as an awareness of objects.

Sartre offers a critique of the psychoanalyst Sigmund Freud's theory of the unconscious, based on the claim that consciousness is essentially self-conscious.

Further: [T]he resistance of the patient implies on the level of the censor an awareness of the thing repressed as such, a comprehension of the end toward which the questions of the psychoanalyst are leading .

Psychoanalysis thus does not yield any special insight, since hiding something from oneself occurs at the level of consciousness as a unified phenomenon, not as part of some intra-psychic mechanism.

Marcel saw one of the most important merits of the work to be to show "that a form of metaphysics which denies or refuses grace inevitably ends by setting up in front of us the image of an atrophied and contradictory world where the better part of ourselves is finally unable to recognise itself".

[19] The philosopher A. J. Ayer wrote that, apart from some psychological insights, the book was "a pretentious metaphysical thesis" and "principally an exercise in misusing the verb 'to be'".

[24] The philosopher David Pears criticized Sartre's critique of Freud, describing it as complex but imprecisely formulated and open to potential objections.

[11] The director Richard Eyre recalled that Being and Nothingness was popular among British students in the 1960s, but suggests that among them the work usually went unread.

[26] Several authors, including the sociologist Murray S, Davis,[27] the philosophers Roger Scruton and Maxine Sheets-Johnstone,[28][29] and the physician Frank Gonzalez-Crussi,[30] have praised Sartre for his contributions to the philosophy of sex.

[28] He has also credited Sartre with providing a "stunning apology for sado-masochism",[31] and characterized Being and Nothingness as a "great work of post-Christian theology".

[29] Naomi Greene, arguing that there is a "distaste for sexuality" in Sartre's work, identifies a clear "anti-sexual bias" present in Being and Nothingness.