Existentialism is humanism: that is, it could never have come into being in an age of religious faith. It is a post-Christian philosophy. It is a commonplace to say that this had happened because of the rise of science; but this is only a half-truth. The real reason was that science suddenly became a way of thought, a way of life. William Blake and Seren Kierkegaard pointed out that even if religion is ninety-nine per cent nonsense, it is the one per cent of truth that counts, and wasted their energies on diatribes against science and reason. There is no space to try to trace in detail the complicated history of existentialism since Kierkegaard. For purposes of exposition, Jean Paul Sartre remains the best representative of the modern existentialist tradition. All its problems and its faults can be seen most clearly in his work. His work makes it plain why existentialism has advanced so little after Kierkegaard.