These seem to me to be the most obvious objections to the assertion that all our true ideas are useful. It is clear, we saw to begin with, that true ideas, which are sometimes useful, _would_ not be useful at all times. And it seemed almost equally clear that they do sometimes occur at times when they are not useful. Our true ideas, therefore are not useful at every time when they actually occur. But in just this sense in which it is so clear that true ideas which are sometimes useful, nevertheless sometimes occur at times when they are not, it seems pretty plain that true ideas, which occur but once, are, some of them, not useful. If an idea, which is sometimes useful, does sometimes occur to a man at a time when it is irrelevant and in the way, why should not an idea, which occurs but once, occur at a time when it is irrelevant and in the way? It seems hardly possible to doubt that this does sometimes happen. But, if this be so, then it is not true that all our true ideas are useful, even so much as on one occasion. It is not true that none of our ideas are true, except those which are useful. But now, what are we to say of the converse proposition--the proposition that all those among our ideas, which are useful, are true? That we never have a useful idea, which is not true? I confess the matter seems to me equally clear here. The assertion should mean that every idea, which is at any time useful, is true; that no idea, which is not true, is ever useful. And it seems hardly possible to doubt that this assertion is false. It Is, in the first place, commonly held that it is sometimes right positively to deceive another person. In war, for instance it is held that one army is justified in trying to give the enemy a false idea as to where it will be at a given time. Such a false idea is sometimes given, and it seems to me quite clear that it is sometimes useful. In such a case, no doubt, it may be said that the false idea is useful to the party who have given it, but not useful to those who actually believe in it. And the question whether it is useful on the whole will depend upon the question which side it is desirable should win. But it seems to me unquestionable that the false idea is sometimes useful on the whole. Take, for instance, the case of a party of savages, who wish to make a night attack and massacre a party of Europeans but are deceived as to the position in which the Europeans are encamped. It is surely plain that such a false idea is sometimes useful on the whole. But quite apart from the question whether deception is ever justifiable, it is not very difficult to think of cases where a false idea, not produced by deception, is plainly useful--and useful, not merely on the whole, but to the person who has it as well. A man often thinks that his watch is right, when, in fact, it is slow, and his false idea may cause him to miss his train. And in such cases, no doubt, his false idea is _generally_ disadvantageous.