W. V. Quine was one of the most important analytic philosophers of the twentieth century, with contributions rivaling those of Bertrand Russell, Ludwig Wittgenstein, and Rudolf Carnap. His early work was strongly influenced by Russell and Carnap, but by 1950 he had begun to develop a distinct philosophy based on his long-term commitments to naturalism and extensionalism. As a naturalist, Quine believed that science is common sense become self-conscious, and philosophy is science become self-conscious. Philosophy, therefore, has no special knowledge with which to justify science; if science needs a justification it will have to come from within. As an extensionalist, Quine held that to state a theory or explanation in intensional idiom is to block the road to inquiry; to state it in extensional idiom is necessary, though not sufficient for understanding. Quine's naturalism is the view that it is up to natural science to determine what exists...