Jean-Paul Sartre was a professional phi- losopher who also sought to preach to a mass audience. For a time at least it looked as though he had succeeded. Certainly no philosopher this century has had so direct an impact on the minds and attitudes of so many human beings, especially young peo- ple, all over the world. Existentialism was the popular philosophy of the late 1940s and 1950s. His plays were hits. His books sold in enormous quantities, some of them over two million copies in France alone....