Sartre's primary idea is that people, as humans, are "condemned to be free". He explained, "This may seem paradoxical because condemnation is normally an external judgment which constitutes the conclusion of a judgment. Here, it is not the human who has chosen to be like this. There is a contingency of human existence. It is a condemnation of their being. Their being is not determined, so it is up to everyone to create their own existence, for which they are then responsible. They cannot not be free, there is a form of necessity for freedom, which can never be given up."

This theory relies upon his position that there is no creator, and is illustrated using the example of the paper cutter. Sartre says that if one considered a paper cutter, 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". This forms the basis for his assertion that because one cannot explain one's own actions and behavior by referring to any specific human nature, they are necessarily fully responsible for those actions. "We are left alone, without excuse." "We can act without being determined by our past which is always separated from us."

Sartre maintained that the concepts of authenticity and individuality have to be earned but not learned. We need to experience "death consciousness" so as to wake up ourselves as to what is really important; the authentic in our lives which is life experience, not knowledge. Death draws the final point when we as beings cease to live for ourselves and permanently become objects that exist only for the outside world. In this way death emphasizes the burden of our free, individual existence. "We can oppose authenticity to an inauthentic way of being. Authenticity consists in experiencing the indeterminate character of existence in anguish. It is also to know how to face it by giving meaning to our actions and by recognizing ourselves as the author of this meaning. On the other hand, an inauthentic way of being consists in running away, in lying to oneself in order to escape this anguish and the responsibility for one's own existence."

While Sartre had been influenced by Heidegger, the publication of Being and Nothingness did mark a split in their perspectives, with Heidegger remarking in Letter on Humanism:

Existentialism says existence precedes essence. In this statement he is taking existentia and essentia according to their metaphysical meaning, which, from Plato's time on, has said that essentia precedes existentia. Sartre reverses this statement. But the reversal of a metaphysical statement remains a metaphysical statement. With it, he stays with metaphysics, in oblivion of the truth of Being.

Herbert Marcuse also had issues with Sartre's metaphysical interpretation of human existence in Being and Nothingness and suggested the work projected anxiety and meaninglessness onto the nature of existence itself:

Insofar as Existentialism is a philosophical doctrine, it remains an idealistic doctrine: it hypostatizes specific historical conditions of human existence into …

Read more on Wikipedia

While the broad focus of Sartre's life revolved around the notion of human freedom, he began a sustained intellectual participation in more public matters towards the end of the Second World War, around 1944–1945. Before World War II, he was content with the role of an apolitical liberal intellectual: "Now teaching at a lycée in Laon ... Sartre made his headquarters the Dome café at the crossing of Montparnasse and Raspail boulevards. He attended plays, read novels, and dined [with] women. He wrote. And he was published." Sartre and his lifelong companion, de Beauvoir, existed, in her words, where "the world about us was a mere backdrop against which our private lives were played out".

The war opened Sartre's eyes to a political reality he had not yet understood until forced into continual engagement with it: "the world itself destroyed Sartre's illusions about isolated self-determining individuals and made clear his own personal stake in the events of the time." Returning to Paris in 1941, he formed the "Socialisme et Liberté" resistance group. In 1943, after the group disbanded, Sartre joined a writers' Resistance group, in which he remained an active participant until the end of the war. He continued to write ferociously, and it was due to this "crucial experience of war and captivity that Sartre began to try to build up a positive moral system and to express it through literature".

The symbolic initiation of this new phase in Sartre's work is packaged in the introduction he wrote for a new journal, Les Temps modernes, in October 1945. Here he aligned the journal, and thus himself, with the Left and called for writers to express their political commitment. Yet, this alignment was indefinite, directed more to the concept of the Left than a specific party of the Left.

During the trial of Robert Brasillach Sartre was among a small number of prominent intellectuals advocating for his execution for 'intellectual crimes'.

Sartre's philosophy lent itself to his being a public intellectual. He envisaged culture as a very fluid concept; neither pre-determined, nor definitely finished; instead, in true existential fashion, "culture was always conceived as a process of continual invention and re-invention." This marks Sartre, the intellectual, as a pragmatist, willing to move and shift stance along with events. He did not dogmatically follow a cause other than the belief in human freedom, preferring to retain a pacifist's objectivity. It is this overarching theme of freedom that means his work "subverts the bases for distinctions among the disciplines". Therefore, he was able to hold knowledge across a vast array of subjects: "the international world order, the political and economic organisation of contemporary society, …

Read more on Wikipedia

Sartre wrote successfully in a number of literary modes and made major contributions to literary criticism and literary biography. His plays are richly symbolic and serve as a means of conveying his philosophy. The best-known, Huis-clos (No Exit), contains the famous line "L'enfer, c'est les autres", usually translated as "Hell is other people." Aside from the impact of Nausea, Sartre's major work of fiction was The Roads to Freedom trilogy which charts the progression of how World War II affected Sartre's ideas. In this way, Roads to Freedom presents a less theoretical and more practical approach to existentialism.

John Huston got Sartre to script his film Freud: The Secret Passion. However it was too long and Sartre withdrew his name from the film's credits. Nevertheless, many key elements from Sartre's script survive in the finished film.

Despite their similarities as polemicists, novelists, adapters, and playwrights, Sartre's literary work has been counterposed, often pejoratively, to that of Camus in the popular imagination. In 1948 the Roman Catholic Church placed Sartre's œuvre on the Index Librorum Prohibitorum (List of Prohibited Books).