As a junior lecturer at the Lycée du Havre in 1938, Sartre wrote the novel i) 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 describing such fundamental experiences have as much value as do discursive essays for the elaboration of philosophical theories. With this mandate, the 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 i) 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. No matter how much he longs for something other or something different, he cannot get away from this harrowing evidence of his engagement with the world.

The stories in i) 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.

In 1939 Sartre was drafted into the French army, where he served as a meteorologist. German troops captured him in 1940 in Padoux, and he spent nine months in prison — later in Nancy and finally in Stalag 12D, Treves, where he wrote his first theater piece: "Barionà, fils du tonnerre", a drama concerning Christmas. Due to poor health (he claimed that his poor eyesight affected his balance) Sartre was released in April 1941. Given civilian status, he then escaped to Paris where he became involved in the French Resistance, and participated in the founding of the resistance group Socialisme et Liberté. It was while engaged in the resistance that he met Albert Camus, a philosopher and author who held similar beliefs, and remained friends with him until Camus turned away from communism, a schism between them that eventually divided them in 1951, after the publication of Camus' book entitled i), a monthly literary and political review, and started writing full-time as well as continuing his political activism. He would draw on his war experiences for his great trilogy of novels, i (1943), gave way to a second period as a politically engaged activist and intellectual. His 1948 work i) in particular explored the problem of being both an intellectual at the same time as becoming "engaged" politically. He embraced communism, though he never officially joined the Communist party, and took a prominent role in the struggle against French colonialism in Algeria. He became perhaps the most eminent supporter of the Algerian war of liberation. He had an Algerian mistress, Arlette Elkaïm, who became his adopted daughter in 1965. He opposed the Vietnam War and, along with Bertrand Russell and other luminaries, he organized a tribunal intended to expose U.S. war crimes, which became known as the Russell Tribunal.

As a fellow-traveller, Sartre spent much of the rest of his life attempting to reconcile his existentialist ideas about self-determination with communist principles, which taught that socio-economic forces beyond our immediate, individual control play a critical role in shaping our lives. His major defining work of this period, the i) appeared in 1960.

Sartre's emphasis on the humanist values in the early works of Marx led to a dispute with the leading Communist intellectual in France in the 1960s, Louis Althusser, who claimed that the ideas of the young Marx were decisively superseded by the "scientific" system of the later Marx.

During the 1940s and 1950s Sartre's ideas remained ambiguous, and existentialism became a favoured philosophy of the beatnik generation. Sartre's views were counterposed to those of Albert Camus in the popular imagination. In 1948, the Catholic Church placed his complete works on the Index of prohibited books. Most of his plays are richly symbolic and serve as a means of conveying his philosophy. The best-known, i), contains the famous line: "L'enfer, c'est les autres", usually translated as "Hell is other people".

Besides the obvious impact of i presents a less theoretical and more practical approach to existentialism. The first book in the trilogy, i

In 1964, Sartre renounced literature in a witty and sardonic account of the first six years of his life, i). The book is an ironic counterblast to Marcel Proust, whose reputation had unexpectedly eclipsed that of André Gide (who had provided the model of i for Sartre's generation). Literature, Sartre concluded, functioned as a bourgeois substitute for real commitment in the world. In the same year he was awarded the Nobel Prize for Literature, but he resoundingly declined it, stating that he had always refused official honors and didn't wish to align himself with institutions.

Though he was now world-famous and a household word (as was "existentialism" during the tumultuous 1960s), Sartre remained a simple man with few possessions, actively committed to causes until the end of his life, such as the student revolution strikes in Paris during the summer of 1968.

In 1975, when asked how he would like to be remembered, Sartre replied: "I would like [people] to remember i...If these are remembered, that would be quite an achievement, and I don't ask for more. As a man, if a certain Jean-Paul Sartre is remembered, I would like people to remember the milieu or historical situation in which I lived,...how I lived in it, in terms of all the aspirations which I tried to gather up within myself."

Sartre's physical condition deteriorated, partially due to the merciless pace of work he put himself through during the writing of the i), both of which remain unfinished. He died April 15, 1980 in Paris from an edema of the lung.

Sartre lies buried in Cimetière du Montparnasse in Paris. His funeral was attended by some 50,000 people.

Sartre was an atheist most of his adult life, atheism being foundational for his Existentialist Philosophy, although towards the end of his life he reportedly converted to Judaism.

When eleven Israeli Olympians were killed by the Palestinian organization Black September in Munich 1972, Sartre referred to terrorism as a "terrible weapon but the oppressed poor have no others." He also found it "perfectly scandalous that the Munich attack should be judged by the French press and a section of public opinion as an intolerable scandal." (Sartre: The Philosopher of the Twentieth Century, Bernard-Henri Lévy, p.343).

Although this has been understood by many as an apology to terrorism, these comments must be read together with others where he indicated that no means should be used which dehumanize its targets and disfigure its goal. He in fact identified as one of those "who affirm the sovereignty of the Israeli state and also believe the Palestinians have a right to sovereignty for the same reason..." He was also known for his strong opposition to anti-semitism.