Can we ever know that something—some state of affairs, or some action taken or contemplated—is evil or wrong, or is it always at best a matter of opinion? It is curious that analytic philosophers, despite their preoccupation with the issue of scepticism and their many discussions of sceptical doubts, have given so little attention to this question. If we look, for example, at Professor Ayer's recent volume The Problem of Knowledge, which consists in effect of a prolonged consideration of various sceptical arguments, we find no reference to the problem of scepticism in moral matters; and this is the more surprising when we reflect that an explicit comparison between this sort of scepticism and scepticism about knowledge generally had been made in a celebrated essay by an earlier philosopher which is now widely thought to have broken fresh ground both in ethics and in theory of knowledge. I refer, of course, to H. A. Prichard's “Does Moral Philosophy rest on a Mistake?” contributed to Mind in 1912. There Prichard claimed to know that something was his duty, in a particular instance, as surely as he knew that seven times four are twenty-eight; in each case, he said, the knowledge is immediate and underivative. To think the contrary was, he argued, to embark on a hopeless quest; for if you cannot claim to know something until you produce criteria for knowing, you can be asked in turn what makes you think the criteria valid, and this will generate a vicious infinite regress. This is not in principle unlike the argument put forward by linguistic philosophers that, unless we allow that the word “know” is properly used in certain paradigm cases, which we take as in order without further questioning, it will lose all application. The interesting thing about Prichard is that he thought the argument applied to morals as readily as to the topics covered by traditional theory of knowledge. But here, as I say, he has not been explicitly followed by his successors.