This paper examines H.A. Prichard's defense of the view that moral duty is underivative, as reflected in his argument that it is a mistake to ask “Why ought I to do what I morally ought?”, because the only possible answer is “Because you morally ought to.” This view was shared by other philosophers of Prichard's period, from Henry Sidgwick through A.C. Ewing, but Prichard stated it most forcefully and defended it best. The paper distinguishes three stages in Prichard's argument: one (...) appealing to his conceptual minimalism, one an epistemological argument that parallels Moore's response to skepticism about the external world, and one arguing that attempts to justify moral duties on non-moral grounds distort the phenomena by giving those duties the wrong explanation or ground. The paper concludes by considering Prichard's critique of ancient ethics and in particular the ethics of Aristotle. The paper is broadly sympathetic to Prichard's position and arguments; its aim is partly to make a case for him as a central figure in the history of ethics. (shrink)