But you may also recollect that in the same lecture I pointed out that inevitable beliefs, though we cannot avoid holding them in some shape, are, and have been, held in many shapes; shapes which vary with the changes in our general outlook on men and things. In what shape, then, should our belief in regularity now be held?
The shape in which it is very commonly formulated is something of this kind: “everything is caused; and the same causes are always followed by the same effects.” This is the so-called “law of universal causation.” It has been treated as an assured truth by philosophers of many different schools, though not always for the same reasons; and, so far as the physical universe is concerned, the modern world accepts it without demur. It is, nevertheless, open to criticism from two points of view. It asserts somewhat more about the course of nature than experience suggests, and somewhat less than science requires. Let me take the two points separately.
When I was dealing with ethics I had occasion to point out that if the primitive manifestations of loyalty and love are products of selection, they have developed by a kind of internal momentum, to a point far beyond that to which selection can possibly have carried them. Something of the same kind has happened in the case of the causal postulate. Selection, we must suppose, has produced the capacity for acquiring habitual expectations; and habitual expectation is induction without reasoning. Like induction, it would not only be useless, but harmful, if no regularity existed; if at any moment the future ceased to bear some resemblance to the past. But the regularity asserted by the law of universal causation is far in excess of this requirement. The law applies to regions which never come within the range of finite experience; and, as regards regions which do come within that range, experience hardly confirms it. We may, of course, attribute the apparent irregularities in nature to our ignorance or our errors; and this, in fact, is what we always do. We must (we think) have observed wrongly or insufficiently; or it may be that a clearer insight would show how apparent aberrations really illustrate some larger law, or depend on conditions at present beyond our ken. Such explanations are easy; and, what is more, they are true. There is no complaint to be made of a verdict in favour of absolute uniformity except that it outruns the evidence. None surely, who understand the meaning of the words they use, will dare to assert that nature appears regular. What they may assert is, that the more you examine it, the more regular it appears. The reign of law is always extending. New provinces are always being added to its domains. Anomalies vanish as knowledge grows; and the absolute uniformity which we now only know by faith, we may some day know by sight.
To this “credo” (with reservations) I readily subscribe. But it sounds a little strange in the mouths of some who preach it. Does it not imply that we interpret our experiences in the light of a preconceived scheme of things; that we force our observations into a mould which they do not naturally fit? If, in unravelling a cypher, I come across passages which are unintelligible, I attribute the check to my own ignorance or dullness. Why? Because I know independently that the cypher has a meaning, if only I could find it. But the empirical agnostic professes to know nothing about the world, except what he has observed himself or what other people have observed for him. Why, then, should he suppose perfect regularity to exist when no perfect regularity appears? Why is he not content to accept what he finds, namely, a regularity which is real but incomplete?
It is no reply to say that patient genius is constantly detecting order in apparent chaos. So it is. And when this happens, by all means rearrange your map of the universe accordingly. But do not argue that chaos is therefore non-existent. The belief in universal causation is not based on argument, nor yet on observation. It depends on what I have described as intuitive probability. And if we refuse to regard nature as liable to lapses from perfect uniformity, this is not because such a theory is unthinkable, not because it is contrary to experience, not because it is incompatible with knowledge, not because it is fatal to purposeful action; for it is none of these things. We reject it because it is out of harmony with the ideal we have formed of what the material universe ought to be and is: and so strong is this speculative prepossession that there is no experimental evidence which would convince a man of science that, when physical causes were the same, physical consequences could be different.
III
But this observation brings me to my second commentary on the formula of universal causation. If, as I have contended, it goes beyond what mere experience suggests, it also falls short of what scientific inference requires. The uniformity it postulates lacks a certain kind of “structure” which is absolutely necessary if the past is to be explained and the future foreseen. It is not enough for this purpose that the course of Nature should be determined. It must be determined after a particular pattern; its uniformity must conform to a particular type.
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.
We are so familiar with this “fibrous” structure of the natural world that it seems almost a matter of course. Mill, for example, assumes it, unconsciously no doubt, through all his exposition of inductive methods: and if he had not assumed it, these methods would have come tumbling about his ears in irreparable ruin. But assuredly neither he nor any other logician has a right to make such an assumption in silence. In spite of many speculative difficulties, there is no principle more vital to knowledge, practical and theoretical, than the principle of “negligibility”; the principle which asserts that sequences can be isolated and repeated, and that vast bodies of contemporaneous facts and happenings may be wholly neglected. It is much more important than the principle of causation, if by causation is meant, not a working, though possibly imperfect, regularity, but the speculative completeness implied by the phrase “universal causation” as commonly interpreted.