Naturalism is the view that reality is coextensive with nature and that, hence, human knowledge has no object beyond the natural realm. A trend towards naturalism has been a pervasive characteristic of European thought for two and a half millennia, a slow drift away from an original dualist mode in European thought towards an ever more stringent naturalistic monism. In Plato's philosophy, the original dualism was put forth with exceptional starkness. It was primarily defined in ontological terms, postulating a special, privileged realm of being composed of eternal and immutable essences; from this followed an epistemological dualism. A retreat from this extreme position was made already by Aristotle and in subsequent Greek and Mediaeval philosophy; but the most significant impetus to the naturalizing trend came with the scientific revolutions in the 16th and 17th centuries. From now on, naturalization would be driven primarily by epistemic considerations. The science-driven naturalizing trend assumed a particularly aggressive form in logical positivism in the early 20th century. But logical positivism embodied a fundamental tension: Its conception of science was itself non-naturalistic and non-empirical. Some of these problems were addressed by a post-positivist thinker in the analytic tradition, namely Willard Van Orman Quine, who urged a naturalization of epistemology that heralded the rise of an empirical investigation of man's cognitive powers under the name of cognitive science.