Arguably, Willard Van Orman Quine is the most influential philosopher of the second half of the twentieth century. In many ways, his position and role in the second half of the century are comparable to Bertrand Russell's in the first half. Quine is the leading advocate of a thoroughgoing form of naturalism whose central theme is the unity of philosophy and natural science. Philosophy so construed is an activity within nature wherein nature examines itself. This contrasts with views that distinguish philosophy from science and place philosophy in a special transcendent position for gaining special knowledge. The methods of science are empirical; so Quine, who operates within a scientific perspective, is an empiricist, but with a difference. Traditional empiricism, as in Locke, Berkeley, Hume, Mill and some twentieth-century forms, takes impressions, ideas or sense data as the basic unit of empirical thought. Quine's empiricism, by contrast, takes account of the theoretical as well as the observational facets of science. The unit of empirical significance is not simple impressions (ideas) or even isolated individual observation sentences, but whole systems of beliefs. The broad theoretical constraints for choice between theories/systems such as explanatory power, parsimony, precision and so on are foremost in this empiricism. He is a fallibilist, and no belief is held as certain since each individual belief in a system is, in principle, revisable. Quine proposes a new conception of observation sentences, a naturalized account of our knowledge of the external world including a rejection of a priori knowledge, and he extends the same empiricist and fallibilist account to our knowledge of logic and mathematics.