 In one sense, it might seem obvious why Kant insists on an a priori method. A “metaphysics of morals” would be, more or less, an account of the nature and structure of moral requirements — in effect, a categorization of duties and values. Such a project would address such questions as, What is a duty? What kinds of duties are there? What is the good? What kinds of goods are there?, and so on. These appear to be metaphysical questions. Any principle used to provide such categorizations appears to be a principle of metaphysics, in a sense, but Kant did not see them as external moral truths that exist independently of rational agents. Moral requirements, instead, are rational principles that tell us what we have overriding reason to do. Metaphysical principles of this sort are always sought out and established by a priori methods.