French philosopher, novelist, playwright, and public intellectual — the most visible embodiment of mid-20th-century Existentialism and, for three decades, a figure who seemed to stand for French intellectual life itself. With Simone de Beauvoir, his lifelong partner, he ran a remarkable joint project of philosophy, fiction, and political engagement that made them the most-photographed thinkers of the postwar era.
Being and Nothingness (1943), written under Nazi occupation, is his philosophical masterwork. Adapting and quarreling with Heidegger's phenomenology, Sartre distinguished being-in-itself (the solid thereness of objects) from being-for-itself (consciousness, which is always consciousness of something and therefore defined by a kind of interior lack — a nothingness). Consciousness is nothing in itself; it is always surpassing itself toward what is not. From this follows the famous thesis that "existence precedes essence": there is no human nature that determines what we must be. We are thrown into the world, condemned to freedom, obliged to choose, and responsible for what we make of ourselves — with nothing (no God, no nature, no fixed values) to take the responsibility off our hands.
His novels (Nausea, 1938; The Roads to Freedom trilogy, 1945–49) and plays (No Exit, 1944 — "hell is other people"; The Flies, 1943; Dirty Hands, 1948) are not illustrations but extensions of the philosophy. His postwar political trajectory moved him steadily toward Marxism and anti-colonial engagement. Critique of Dialectical Reason (1960) is the great unfinished attempt to synthesize his existentialism with Marxist historical materialism. He declined the Nobel Prize for Literature in 1964 on principle.