This book attempts to sketch an ethical theory based on the principles found in the writings of Immanuel Kant. It is not primarily a study of those writings but an attempt to develop out of Kant's thought the most defensible theory possible on that basis. Thus I will not refrain from criticizing Kant – at times, quite roundly – when I think his moral opinions or conclusions do not follow from or cohere well with his fundamental principles, or when I think a more defensible approach to some topic involves correcting or revising what Kant thought and wrote.

The idea of writing the present book was suggested to me by the late Terence Moore of Cambridge University Press. The basic reading of Kant represented here was presented in my 1999 book Kant's Ethical Thought. But Moore thought it would be a good thing if I provided a briefer, less scholarly, and more approachable version of Kantian ethics. The present book, however, is only a partial fulfillment of his request. Though shorter than my earlier book, it is probably longer (and no doubt less popular) than he had in mind. Its primary focus is on Kantian ethics rather than on Kant scholarship. This book fulfills a promise of the earlier book by developing Kant's conception of virtue and his theory of duties in greater detail than was done there.