Hume’s answer is ingenious. The principle of causation cannot be rationally justified, and the necessary connection we predicate of changes in the outside world is not in the things; it is only a feeling in ourselves, and is the result of custom. After seeing the same succession several times, we come somehow to feel differently about it, and that feeling of difference we express by saying that we have before us an instance not of simple succession, but of cause and effect. This is not the place to discuss the difficulties of Hume’s position; it is enough to notice how entirely passive it makes the mind, and how alien such an explanation is from the spirit of inquiry and discovery. If cause is simply the effect of custom on the mind, then the facts either produce that effect or they do not. In neither case is there anything to find out. But the scientist, in investigating causes, however strongly he may hold that he has to observe the facts, knows also that he has a problem to solve, that he has to discover the right way to go about it, must adopt some principle in dealing with the facts. Pure passivity will help him little. Hume’s account of causation, then, is really a denial of even empirical science, and yet it helped to make clear an important truth; for, although we do not get the _principle_ of causation from experience, we have to go to experience to discover causal laws. We do not discover causation by analysing a cause and seeing that it is such that, from its nature, it must produce a certain effect. All knowledge of causation goes back to observed succession, though all cases of observed and even repeated succession are not cases of causation. Hume, therefore, was right in saying that where there could be no observed succession there could be no knowledge of causation. Both the rationalistic and the empirical explanations of science had failed, the one because it could find no room for observation of facts, the other because it could find no room for principles governing that observation; and we shall see that Kant started with a consciousness of this double failure. He saw that Hume’s criticism of causation raised problems for which the rationalist had no answer, and yet that the position reached by Hume was incompatible with the existence of science. The same failure of both rationalism and empiricism had become evident in another sphere--that of morals and religion. The relation of philosophy to science is always twofold. Philosophy is partly concerned with analysing and reflecting on the methods of the different sciences, partly with seeking to adjust the rival and conflicting claims of the two great departments of man’s life--science and religion.