Jean-Paul Sartre was a French philosopher, left-wing political activist, playwright, and novelist. One of the leading French public intellectuals of the twentieth century, he was a key figure in the intellectual field in the decade following the Second World War. Influenced by Edmund Husserl, Martin Heidegger, and Marxism, he was a major theorist of existentialism. For Sartre, human nature is not a given but derives from personal choice. Based on ontological reflection, Sartre’s existentialism thus leads to the practical realm of ethics and politics by laying emphasis on the moral responsibility of the individual cut off from the thing-in-itself and condemned to be free.