[S]uppose we come genuinely to doubt whether, e.g., 7 x 4 = 28 owing to a genuine doubt whether we were right in believing yesterday that 7 x 4 = 28, a doubt which can in fact only arise if we have lost our hold of, i.e. no longer remember, the real nature of our consciousness of yesterday, and so think of it as consisting in believing. Plainly, the only remedy is to do the sum again. Or, to put the matter generally, if we do come to doubt whether it is true that A is B, as we once thought, the remedy lies not in any process of reflexion but in such a reconsideration of the nature of A and B as leads to the knowledge that A is B.
With these considerations in mind, consider the parallel which, as it seems to me, is presented—though with certain differences—by Moral Philosophy. The sense that we ought to do certain things arises in our unreflective consciousness, being an activity of moral thinking occasioned by the various situations in which we find ourselves. At this stage our attitude to these obligations is one of unquestioning confidence. But inevitably the appreciation of the degree to which the execution of these obligations is contrary to our interest raises the doubt whether after all these obligations are really obligatory, i.e., whether our sense that we ought not to do certain things is not illusion. We then want to have it proved to us that we ought to do so, i.e., to be convinced of this by a process which, as an argument, is different in kind from our original and unreflective appreciation of it. This demand is, as I have argued, illegitimate.
Hence in the first place, if, as is almost universally the case, by Moral Philosophy is meant the knowledge which would satisfy this demand, there is no such knowledge, and all attempts to attain it are doomed to failure because they rest on a mistake, the mistake of supposing the possibility of proving what can only be apprehended directly by an act of moral thinking. Nevertheless the demand, though illegitimate, is inevitable until we have carried the process of reflexion far enough to realise the self-evidence of our obligations, i.e., the immediacy of our apprehension of them. This realisation of their self-evidence is positive knowledge, and so far, and so far only, as the term Moral Philosophy is confined to this knowledge and to the knowledge of the parallel immediacy of the apprehension of the goodness of the various virtues and of good dispositions generally, is there such a thing as Moral Philosophy. But since this knowledge may allay doubts which often affect the whole conduct of life, it is, though not extensive, important and even vitally important.
In the second place, suppose we come genuinely to doubt whether we ought, for example, to pay our debts owing to a genuine doubt whether our previous conviction that we ought to do so is true, a doubt which can, in fact, only arise if we fail to remember the real nature of what we now call our past conviction. The only remedy lies in actual [sic] getting into a situation which occasions the obligation, or—if our imagination be strong enough—in imagining ourselves in that situation, and then letting our moral capacities of thinking do their work. Or, to put the matter generally, if we do doubt whether there is really an obligation to originate A in a situation B, the remedy lies not in any process of general thinking, but in getting face to face with a particular instance of the situation B, and then directly appreciating the obligation to originate A in that situation.
(H. A. Prichard, “Does Moral Philosophy Rest on a Mistake?” Mind, n.s., 21 [January 1912]: 21-37, at 35-7 [italics in original])