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?