This short chapter introduces the book as a study in the history of knowledge, and specifically of changing conceptions of what kind of knowledge was and wasn’t worth pursuing in the period 1500–1700, with speculative forms of philosophising coming out as the loser. It summarises the contents and argument of the book, and warns against reading early modern intellectual history from the perspective of the eighteenth and nineteenth centuries, with philosophical ‘rationalism’ as a driving force. It also sets out the book’s double approach: first, a longue durée structural account of the changing nature of the early modern system of knowledge; second, in-depth contextualisations of Bayle and Newton showing how they were individual products of that system. Both of them ended up developing elaborate genealogical visions of a ‘Kingdom of Darkness’ no less interesting and sophisticated than that of Thomas Hobbes. As for him, the inheritance of speculative philosophy ended up explaining almost all the evils (both intellectual and social) in the world.