This introduction broaches the question of how naturalism rose to dominance in the modern West. Naturalism in this context is understood as a rejection of belief in the supernatural. This distinctive feature of Western modernity is at odds, not only with its own religious past, but also with what has been true for virtually all other cultures. Whereas it was once impossible not to assume the existence of the supernatural, this has now become one option among others, and one that is typically thought to be lacking in rational support. The book seeks to account for this unique historical development in two related ways. First, it explores the histories of the two key terms in this understanding—‘belief’ and ‘supernatural’—showing how they came to take on their present meanings in the modern period. Second, it shows how advocates of naturalism necessarily subscribe to a progressive view of history that can vindicate the adoption of these two categories in their modern sense.