If contemporary analytic philosophy can be said to have a philosophical ideology, it undoubtedly is naturalism. Naturalism is often invoked as a motivating ground for many philosophical projects, and “naturalization” programs abound everywhere, in theory of knowledge, philosophy of mind, theory of meaning, metaphysics, and ethics. But what is naturalism, and where does it come from? This paper examines the naturalism debate in midtwentieth-century America as a proximate source of contemporary naturalism. Views of philosophers like Roy Wood Sellars, John Dewey, John Herman Randall, Sydney Hook, and Ernest Nagel are cited, and some of the central tenets of naturalism, such as an adherence to “scientific method” as the sole source of knowledge and the causal/ explanatory closure of the natural world, are explored. The paper ends with a brief discussion of how certain naturalistic constraints lead to some of the problems currently debated in metaethics and philosophy of mind.