At the core of Kant's ethics lies the claim that if there is a supreme principle of morality, then it is not a utilitarian or Aristotelian perfectionist principle, or even a principle resembling the Ten Commandments. The only viable candidate for the supreme principle of morality is the Categorial Imperative. This book is the most detailed investigation of this claim. It constructs a new, criterial reading of Kant's derivation of one version of the Categorial Imperative: The Formula of Universal Law. This reading shows this derivation to be far more compelling than contemporary philosophers tend to believe. It also reveals a novel approach to deriving another version of the Categorial Imperative, the Formula of Humanity, a principle widely considered to be the most attractive Kantian candidate for the supreme principle of morality. Lucidly written and dealing with a foundational topic in the history of ethics, this book will be important not just for Kant scholars but for a broad swath of students of philosophy. Samuel Kerstein is Associate Professor of Philosophy at the University of Maryland, College Park