At first sight this statement may seem rather obscure. What (you will ask) is this “structure” or pattern whose absence would be so disastrous to knowledge? It is a structure (I reply) which makes it possible to break up the flow of events into intelligible repetitions. It is not enough that the condition of the world at any moment should be strictly determined by its condition at the preceding moment. Such a world would, I suppose, completely conform to the doctrine of uniformity, and obey both in spirit and in letter the law of universal causation. Yet, unless it also conformed to the additional canon I have just laid down, it would provide no basis either for scientific knowledge or for practical decision. The same consequent would always succeed the same antecedent, if and when it recurred. But, unless we accept the cyclic theories of the Stoics, it never would recur. The completest knowledge of the past would tell us nothing about the future; not because the succession of events was arbitrary or (as the word is commonly misused) miraculous; but because each cross-section of the stream of Time (that is to say, the sum of all contemporaneous facts and events) had to be considered as a single cause, completely determining the whole cross-sections immediately in front of it; and, as a single effect, completely determined by the whole cross-section immediately behind it. Such a world might have a history, but it could never have a science. The reason is plain. Science requires uniformities even more than uniformity; and a universe such as I have just described has uniformity but no uniformities. The very phrase “laws of nature” shows that it is these subordinate uniformities for which we look. The whole efforts of the skilled investigator are directed towards so isolating the sequences he is examining that his experiments shall become (as the phrase goes) _crucial_. If no such isolation could be effected, it would never be possible to point to some “phenomenon” and say of it “Here is a cause,” and to some other “phenomenon” and say of it “Here is its effect.” The world, in short, must have a structure which connects its successive phases in such a way that definite parts of all that exists or happens are knit with peculiar closeness to definite parts of what existed or happened before. It is on these connecting strands that we mainly fix our gaze; they are often difficult to trace, they are sometimes hopelessly entangled; but when we can bring them into clear vision, then, and not till then, we triumphantly say that we have discovered a law of nature.