as it is generally admitted that it is absolutely impossible for us to become directly aware of any one else's consciousness. We cannot have any immediate knowledge of the mental states of others. There is no way of becoming immediately acquainted with any soul except our own, and so the problem of the existence of mental life in our fellow beings or indeed anywhere else seems to be a typical case of those questions the answers of which we shall never know.
Let us conclude this little survey and see what we can learn from it. There are certain questions among our examples of which we could say immediately that we are able to solve them; there are some others where this seems doubtful, and still others where it seemend quite necessary to admit that we shall never know the answer.
But in this latter case we must make a very important distinction. If we ask, for example, what Napoleon did at a particular minute of his life, it is very likely that nobody will ever know the answer. And yet such a question, as I have already pointed out, is not unanswerable in some very essential sense of the word. It just happens to be unanswerable for us because we do not possess any historical evidence concerning the facts; there is no present experience from which we could infer exactly what Napoleon did at that moment, but it is not unknowable in principle. It is just unknowable through accidental circumstances, not because of the intrinsic nature of things.
There is an enormous difference between these two kinds of impossibility. We must distinguish between those problems which are insoluble in principle, and those which man cannot solve because he does not happen to have the technical means necessary for the solution. Whenever he comes across an unanswerable question, the philosopher must ask himself to which of these two classes it belongs. It is likely that no human being will ever know what the back of the moon looks like, because the moon always turns only one and the same side towards the earth. But the impossibility of knowing the far side of the moon is only a practical or technical one; it is due to the fact that man has not yet invented a ship that will take him around the moon, but such an enterprise would by no means be contrary to the laws of nature, and even if it were against the laws of nature we might imagine the laws of the universe to be different in such a way that it would
