(2) In order to shew that any action is a duty, it is necessary to know both what are the other conditions, which will, conjointly with it, determine its effects; to know exactly what will be the effects of these conditions; and to know all the events which will be in any way affected by our action throughout an infinite future. We must have all this causal knowledge, and further we must know accurately the degree of value both of the action itself and of all these effects; and must be able to determine how, in conjunction with the other things in the Universe, they will affect its value as an organic whole. And not only this: we must also possess all this knowledge with regard to the effects of every possible alternative; and must then be able to see by comparison that the total value due to the existence of the action in question will be greater. But it is obvious that our causal knowledge alone is far too incomplete for us ever to assure ourselves of this result, that an action is our duty: we can never be sure that any action will produce the greatest value possible. (§ 91 ¶ 1)

Ethics, therefore, is quite unable to give us a list of duties: but there still remains a humbler task which may be possible for Practical Ethics. Although we cannot hope to discover which, in a given situation, is the best of all possible alternative actions, there may be some possibility of shewing which among the alternatives, likely to occur to any one, will produce the greatest sum of good. This second task is certainly all that Ethics can ever have accomplished: and it is certainly all that it has ever collected materials for proving; since no one has ever attempted to exhaust the possible alternative actions in any particular case. Ethical philosophers have in fact confined their attention to a very limited class of actions, which have been selected because they are those which most commonly occur to mankind as possible alternatives. With regard to these they may possibly have shewn that one alternative is better, i.e. produces a greater total of value, than others. But it seems desirable to insist, that though they have represented this result as a determination of duties, it can never really have been so. For the term duty is certainly so used that, if we are subsequently persuaded that any possible action would have produced more good than the one we adopted, we admit that we failed to do our duty. It will, however, be a useful task if Ethics can determine which among alternatives likely to occur will produce the greatest total value. For, though this alternative cannot be proved to be the best possible, yet it may be better than any course of action which we should otherwise adopt. (§ 91 ¶ 2)