20 century. But this common picture of Prichard underestimates his place in the history of ethics, which I believe is central. This is not because he defended completely distinctive ideas; his most important views were shared by other philosophers of his period, from Henry Sidgwick to A.C. Ewing. But it is often Prichard who stated those views most forcefully and defended them best. These views can be summarized in a slogan Prichard himself did not use: “duty is underivative.” But the idea that duty is underivative can apply at three different levels. The first concerns the normative realm as a whole; here it expresses the non-naturalist view that normative truths are sui generis, neither reducible to nor derivable from non-normative truths such as those of science. Duty is underivative in the sense that truths about how we ought to act, in the broadest sense of “ought,” are self-standing. The second level focuses more narrowly on moral judgements, as one kind of normative judgement. Now the claim is that truths about how we ought morally to act are also underivative, not only from non-normative truths but also from any other normative truths; there are no non-moral “oughts” or values from which moral “oughts” derive. The final level is that of specific deontological duties such as to keep promises, not harm 1 others, and so on. For Prichard these duties do not derive from a more general consequentialist duty to promote good consequences. The main reason we ought to keep our promises or not harm others is just that we ought to; those duties, like the normative realm as a whole and moral duty in general, are self-standing. Prichard accepted all three of these claims, though he did not distinguish the first two from each other as many present-day philosophers do. My main interest is in his defense of the second claim, about moral duty in general; this appears in his famous argument that it is a mistake to ask “Why ought I to do what I morally ought to do?” because the only possible answer is “Because you morally ought to.” But I will begin by examining his defense of the third claim, about deontological duties, because it sheds light on his methodology in discussing the other two. This is the subject of Section 1; later sections will address claims one and two...