This paper examines the importance of the theory of intellectual cognition in the development of early modern philosophy. It compares three conceptions of the intellect, held respectively by some scholastic Aristotelians, Descartes, and Locke. Examination of these three cases provides an opportunity to locate early modern discussions of the cognitive faculties in relation to recent understandings of psychology, epistemology, logic, mind, and their relations. The early modern discussions are not easily fit into the modern categories of epistemology and psychology. Reflection on this fact may help us to delimit more precisely and to see some problems in recent concepts of naturalism in relation to philosophy and psychology.