Existence Precedes Essence

The foundational thesis of Sartre's lecture Existentialism Is a Humanism (1945, published 1946). Where a craftsman conceives a tool's purpose before making it — essence precedes existence — Sartre argues that humans arrive in the world first, without a pre-given nature or purpose, and then define themselves through their choices and actions. This reversal demolishes the idea of a fixed human nature and places the full weight of self-creation on the individual. The principle underpins virtually all of Sartre's ethics, and he acknowledged it as the single most controversial and central claim of his existentialism.