This chapter focuses on Section I of the general introduction to The Metaphysics of Morals in which Kant explains what a metaphysics of morals is, and why there must be one. To properly understand Kant’s views on these matters requires explaining Kant’s conception of philosophy and the place of metaphysics as a branch of philosophy. In spelling this out, the chapter discusses key distinctions between theoretical and practical cognition, empirical (a posteriori) versus rational (a priori) sources of cognition, and the analytic/synthetic distinction as Kant understood it. For Kant, a metaphysics of morals is that branch of philosophy concerned with those synthetic a priori propositions and principles fundamental to morality. The chapter also explains the role of anthropology in a metaphysics of morals.