This essay argues that we can only develop a proper grasp of Kant's practical philosophy if we appreciate the central role played in his thought by the notion of the interests of reason. While it is generally acknowledged that Kant does not regard reason as a purely instrumental faculty, but sees it as endowed with its own essential interests, this book is the first to explain how the notion of the interests of reason lies at the heart of his philosophical project - and how it allows us to make sense of some of the most puzzling aspects of his practical philosophy.