Jean-Paul Charles Aymard Sartre (21 June 1905 – 15 April 1980), commonly known simply as Jean-Paul Sartre, was a French existentialist philosopher, playwright, novelist, screenwriter, political activist, biographer, and literary critic. He was one of the leading figures in 20th century French philosophy.

Jean-Paul Sartre was born and raised in Paris to Jean-Baptiste Sartre, an officer of the French Navy, and Anne-Marie Schweitzer. His mother was of Alsatian origin, and was a cousin of German Nobel prize laureate Albert Schweitzer. When Sartre was 15 months old, his father died of a fever. Anne-Marie raised him with help from her father, Charles Schweitzer, a high school professor of German, who taught Sartre mathematics and introduced him to classical literature at a very early age.

As a teenager in the 1920s, Sartre became attracted to philosophy upon reading Henri Bergson's Essay on the Immediate Data of Consciousness. He studied and earned a doctorate in philosophy in Paris at the elite École Normale Supérieure, an institution of higher education which was the alma mater for several prominent French thinkers and intellectuals. Sartre was influenced by many aspects of Western philosophy, absorbing ideas from Immanuel Kant, Georg Wilhelm Friedrich Hegel, Edmund Husserl and Martin Heidegger among others. In 1929 at the École Normale, he met Simone de Beauvoir, who studied at the Sorbonne and later went on to become a noted philosopher, writer, and feminist. The two, it is documented, became inseparable and lifelong companions, initiating a romantic relationship, though they were not monogamous. Sartre served as a conscript in the French Army from 1929 to 1931 and he later argued in 1959 that each French person was responsible for the collective crimes during the Algerian War of Independence.

Together, Sartre and de Beauvoir challenged the cultural and social assumptions and expectations of their upbringings,which they considered bourgeois, in both lifestyle and thought. The conflict between oppressive, spiritually-destructive conformity (mauvaise foi, literally, "bad faith") and an "authentic" way of "being" became the dominant theme of Sartre's early work, a theme embodied in his principal philosophical work L'Être et le Néant (Being and Nothingness) (1943). Sartre's introduction to his philosophy is his work Existentialism is a Humanism (1946), originally presented as a lecture.

The basis of Sartre's existentialism is found in The Transcendence of the Ego. To begin with, the thing-in-itself is infinite and overflowing. Sartre refers to any direct consciousness of the thing-in-itself as a "pre-reflective consciousness." Any attempt to describe, understand, historicize etc. the thing-in-itself, Sartre calls "reflective consciousness." There is no way for the reflective consciousness to subsume the pre-reflective, and so reflection is fated to a form of anxiety, i.e. the human condition. The reflective consciousness in all its forms, (scientific, artistic or otherwise) can only limit the thing-in-itself by virtue of its attempt to understand or describe it. It follows, therefore, that any attempt at self-knowledge (self-consciousness - a reflective consciousness of an overflowing infinite) is a construct that fails no matter how often it is attempted. Consciousness is consciousness of itself insofar as it is consciousness of a transcendent object.

The same holds true about knowledge of the "Other." The "Other" (meaning simply beings or objects that are not the self) is a construct of reflective consciousness. One must be careful to understand this more as a form of warning than as an ontological statement. However, there is an implication of solipsism here that Sartre considers fundamental to any coherent description of the human condition.[20] Sartre overcomes this solipsism by a kind of ritual. Self consciousness needs "the Other" to prove (display) its own existence. It has a "masochistic desire" to be limited, i.e. limited by the reflective consciousness of another subject. This is expressed metaphorically in the famous line of dialogue from No Exit, "Hell is other people."

The main idea of Jean-Paul Sartre is that we are, as men, "condemned to be free." This theory relies upon his atheism, and is formed using the example of the paper-knife. Sartre says that if one considered a paper-knife, one would assume that the creator would have had a plan for it: an essence. Sartre said that human beings have no essence before their existence because, there is no Creator. Thus: "existence precedes essence". So, and just for that, the sartrian man with his freedom will became a god, but he remain always only a bankrupt god.

As a junior lecturer at the Lycée du Havre in 1938, Sartre wrote the novel La Nausée (Nausea) which serves in some ways as a manifesto of existentialism and remains one of his most famous books. Taking a page from the German phenomenological movement, he believed that our ideas are the product of experiences of real-life situations, and that novels and plays can well describe such fundamental experiences, have as much value as do discursive essays for the elaboration of philosophical theories. With such purpose, this novel concerns a dejected researcher (Roquentin) in a town similar to Le Havre who becomes starkly conscious of the fact that inanimate objects and situations remain absolutely indifferent to his existence. As such, they show themselves to be resistant to whatever significance human consciousness might perceive in them.

This indifference of "things in themselves" (closely linked with the later notion of "being-in-itself" in his Being and Nothingness) has the effect of highlighting all the more the freedom Roquentin has to perceive and act in the world; everywhere he looks, he finds situations imbued with meanings which bear the stamp of his existence. Hence the "nausea" referred to in the title of the book; all that he encounters in his everyday life is suffused with a pervasive, even horrible, taste — specifically, his freedom. The book takes the term from Friedrich Nietzsche's Thus Spoke Zarathustra, where it is used in the context of the often nauseating quality of existence. No matter how much Roquentin longs for something else or something different, he cannot get away from this harrowing evidence of his engagement with the world. The novel also acts as a terrifying realization of some of Kant's fundamental ideas; Sartre uses the idea of the autonomy of the will (that morality is derived from our ability to choose in reality; the ability to choose being derived from human freedom; embodied in the famous saying "Condemned to be free") as a way to show the world's indifference to the individual. The freedom that Kant exposed is here a strong burden, for the freedom to act towards objects is ultimately useless, and the practical application of Kant's ideas prove to be bitterly rejected.

The stories in Le Mur (The Wall) emphasize the arbitrary aspects of the situations people find themselves in and the absurdity of their attempts to deal rationally with them. A whole school of absurd literature subsequently developed.