The problem of how, if at all, we could set about justifying assertions about what we ought to do in various practical situations is one that has been the major concern of moral philosophers. Such basic questions are indeed endemic in most branches of philosophy. We ask not only if we can ever know what we ought to do but whether we can justify our claims to knowledge of an external world, how we can know the truth of statements about the past, or whether we can ever be sure of the existence of minds other than our own. But in ethics the problem seems more recalcitrant and, indeed, to many nonphilosophers at least, more real. For while skepticism about the existence of an external world or of other minds may seem difficult to refute, to most it is impossible to embrace...