A difficulty in distinguishing this task, which Ethics may perhaps undertake with some hope of success, from the hopeless task of finding duties, arises from an ambiguity in the use of the term possible. An action may, in one perfectly legitimate sense, be said to be impossible solely because the idea of doing it does not occur to us. In this sense, then, the alternatives which do actually occur to a man would be the only possible alternatives; and the best of these would be the best possible action under the circumstances, and hence would conform to our definition of duty. But when we talk of the best possible action as our duty, we mean by the term any action which no other known circumstance would prevent, provided the idea of it occurred to us. And this use of the term is in accordance with popular usage. For we admit that a man may fail to do his duty, through neglecting to think of what he might have done. Since, therefore, we say that he might have done, what nevertheless did not occur to him, it is plain that we do not limit his possible actions to those of which he thinks. It might be urged, with more plausibility, that we mean by a man's duty only the best of those actions of which he might have thought. And it is true that we do not blame any man very severely for omitting an action which, as we say, he could not be expected to think. But even here it is plain that we recognise a distinction between what he might have done and what he might have thought of doing: we regard it as a pity that he did not do otherwise. And duty is certainly used in such a sense, that it would be a contradiction in terms to say that it was a pity that a man did his duty. (§ 92 ¶ 1)

We must, therefore, distinguish a possible action from an action of which it is possible to think. By the former we mean an action which no known cause would prevent, provided the idea of it occurred to us: and that one among such actions, which will produce the greatest total good, is what we mean by duty. Ethics certainly cannot hope to discover what kind of action is always our duty in this sense. It may, however, hope to decide which among one or two such possible actions is the best: and those which it has chosen to consider are, as a matter of fact, the most important of those with regard to which men deliberate whether they shall or shall not do them. A decision with regard to these may therefore be easily confounded with a decision with regard to which is the best possible action. But it is to be noted that even though we limit ourselves to considering which is the better among alternatives likely to be thought of, the fact that these alternatives might be thought of is not included in what what we mean by calling them possible alternatives. Even if in any particular case it was impossible that the idea of them should have occurred to a man, the question we are concerned with is, which, if it had occurred, would have been the best alternative? If we say that murder is always a worse alternative, we mean to assert that it is so, even where it was impossible for the murderer to think of doing anything else. (§ 92 ¶ 2)

The utmost, then, that Practical Ethics can hope to discover is which, among a few alternatives possible under certain circumstances, will, on the whole, produce the best result. It may tell us which is the best, in this sense, of certain alternatives about which we are likely to deliberate; and since we may also know that, even if we choose none of these, what we shall, in that case, do is unlikely to be as good as one of them, it may thus tell us which of the alternatives, among which we can choose, it is best to choose. If it could do this it would be sufficient for practical guidance. (§ 92 ¶ 3)