 In sharp contrast with Hume, Kant insists on the need for an a priori investigation of morality’s foundation. His detailed treatment of virtue and moral judgment draws heavily on observations and ideas about human nature. But Kant makes explicit that morality must be based on a supreme moral principle, which can only be discovered a priori, through a method of pure moral philosophy (G 4:387–92). By “pure” or a priori moral philosophy, Kant has in mind a philosophy grounded exclusively on principles that are inherent in and revealed through the operations of reason. According to Kant, the a priori approach is required because morality’s commands are unconditional. We could never discover a principle that commands all rational beings with such absolute authority through Hume’s method. An empirical approach, he argues, can tell us how people do act, but it cannot tell us how we ought to act. Once equipped with the supreme principle of morality, however, one still requires an understanding of human beings in order to apply it (MM 6:217). This is important for Kant. One can say little about what the supreme moral principle requires in concreto without knowing such things as the sorts of ends people may be inclined to adopt and the conditions under which human agency will characteristically thrive or wither. As noted below and throughout, this is one area of Kant’s moral thinking that was significantly influenced by Hume and his fellow sentimentalists.