Philosophical knowledge, if what we have been saying is correct, does not differ essentially from scientific knowledge; there is no special source of wisdom which is open to philosophy but not to science, and the results obtained by philosophy are not radically different from those reached in science. Philosophy is distinguished from science only by being more critical and more general. But when I say that philosophy is critical, I do not mean that it attempts to criticise knowledge from outside, for that would be impossible: I mean only that it examines the various parts of our supposed knowledge to see whether they are mutually consistent and whether the inferences employed are such as seem valid to a careful scrutiny. The criticism aimed at is not that which, without reason, determines to reject, but that which considers each piece of apparent knowledge on its merits and retains whatever still appears to be knowledge when this consideration is completed. That some risk of error remains must be admitted, since human beings are fallible. Philosophy may claim justly that it diminishes the risk of error, and that in some cases it renders the risk so small as to be practically negligible. To do more than this is not possible in a world where mistakes must occur; and more than this no prudent advocate of philosophy would claim to have performed. I want to end with a few words about man’s place in the universe. It has been customary to demand of a philosopher that he should show that the world is good in certain respects. I cannot admit any duty of this sort. One might as well demand of an accountant that he should show a satisfactory balance sheet. It is just as bad to be fraudulently optimistic in philosophy as in money matters. If the world is good, by all means let us know it; but if not, let us know that. In any case, the question of the goodness or badness of the world is one for science rather than for philosophy. We shall call the world good if it has certain characteristics that we desire. In the past philosophy professed to be able to prove that the world had such characteristics, but it is now fairly evident that the proofs were invalid. It does not, of course, follow that the world does not have the characteristics in question; it follows only that philosophy cannot decide the problem. Take for example the problem of personal immortality. You may believe this on the ground of revealed religion, but that is a ground which lies outside philosophy. You may believe it on the ground of the phenomena investigated by psychical research, but that is science, not philosophy. In former days, you could believe it on a philosophical ground, namely, that the soul is a substance and all substances are indestructible. You will find this argument, sometimes more or less disguised, in many philosophers. But the notion of substance, in the sense of a permanent entity with changing states, is no longer applicable to the world.