I shall be told, perhaps, that on this view of the nature of Causality we ought to speak of night as the cause of day. So perhaps we should, if the result to which we are led by a more limited experience were not corrected by the results of a larger experience. To say nothing of the valuable correction afforded by the polar winter and the polar summer, we have learned by a more comprehensive experience to replace the law that day follows night by the wider generalisation that the visibility of objects is invariably coincident upon the presence of some luminous body and not upon a previous state of darkness. But between cases of what we call mere succession and what is commonly called causal sequence the difference lies merely in the observed fact that in some cases the sequence varies, while in others no exception has ever been discovered. No matter how frequently we observe that a sensation of red follows the impact upon the aural nerve of a shock derived from a wave of ether of such and such a length, we see no reason why it should do so. We may, no doubt, make a still wider generalization, and say that every event in Nature is invariably preceded by some definite complex of conditions, {34} and so arrive at a general law of the Uniformity of Nature. And such a law is undoubtedly the express or implied basis of all inference in the Physical Sciences. When we have once accepted that law (as the whole mass of our experience in the purely physical region inclines us to do), then a single instance of A B C being followed by D (when we are quite sure that we have included all the antecedents which we do not know from other experience to be irrelevant) will warrant our concluding that we have discovered a law of nature. On the next occasion of A B C's occurrence we confidently predict that D will follow. But, however often we have observed such a sequence, and however many similar sequences we may have observed, we are no nearer to knowing _why_ D should follow ABC: we can only know that it always does: and on the strength of that knowledge we infer, with a probability which we do no doubt for practical purposes treat as a certainty, that it always will. But on reflection we can see no reason why a wave of ether of a certain length should produce red rather than blue, a colour rather than a sound. There, as always, we discover nothing but succession, not necessary connexion.