The first question we may ask concerning the views which are thus so clearly and forcibly expressed is this: Does Hume disclose anything beyond observable or frequently observed succession? Obviously not. Let us take a matter of common experience. The flash and the report of a distant cannon are so habitually connected in experience that the occurrence of the one suggests the other through association. Here we are simply describing certain facts of experience in terms of antecedence and sequence. Of any "power" or "strongest necessity" Hume should be, and I take it actually was, the last to see in mere custom the smallest indication. To modify the words of Hobbes without altering his meaning we may say, "What we call custom is nothing else but remembering what antecedents have been followed by what consequents"; and we may add in the phrase of Glanvill, "For the causality itself is insensible".
Hume's primary contention may be thus summarised: All that is disclosed to experience may be expressed in terms of actually observed or observable antecedence and sequence. "The scenes of the universe are continually shifting, and one object follows another in uninterrupted succession; but the power or force which actuates the whole machine is entirely concealed from us, and never discovers itself in any of the sensible qualities of body. When Herbert Spencer said that "by the Persistence of Force we really mean the persistence of some Cause which transcends our knowledge and conception" he was using the terms "force" and "cause" for the "power which actuates the whole machine" was using them, therefore, in a non-scientific or metaphysical sense.
For science occupies Hume's position with some amendment and extension. He dealt largely with the naive expectations of daily life. And his doctrine of the effects of custom and habit led him, no doubt, as Reid pointed out, to exaggerate the importance of the repetition of experience. When the conception of uniformity, as part of the ideal construction of science, has been reached, a single accurate and precise determination of the essential antecedent conditions of any event is sufficient. Hobbes, in a passage which is quoted by Jevons, had already brought out another important feature when he said, "A cause is the sum or aggregate of all such accidents, both in the agents and the patients, as concur in the producing of the effect propounded; all which existing together, it cannot be understood but that the effect existed with them; or that it can possibly exist if any one of them be absent." Mill accepted and endorsed this view. "The real cause," he said, "is the whole of the antecedents; and we have, philosophically speaking, no right to give the name of cause to any one of them exclusively of the others." True and important, "philosophically speaking," as is this identification of the cause with the totality of the antecedent conditions, it is none the less true that "scientifically speaking" it is the aim of physics to isolate the factors of phenomena and to disentangle the threads which are woven into the totality of antecedent conditions. It is this disentanglement which serves, in part at least, to distinguish the ideal scheme of physics from the complex web of natural phenomena, which with ever-increasing success it enables us to interpret. At the same time it should be noted that this method of scientific procedure does not at all invalidate Hobbes's contention. For though physics adopts the method of analysis with a view to isolating the factors of causation, it still remains true that, when its results are applied to a complex phenomenon such as Hobbes had in view, no interpretation can be satisfactory unless all the cooperating antecedents are represented synthetically in due quantitative proportion. Accepting, therefore, the validity of Hobbes's contention that the cause is the totality of the conditions, we may add, as a rider, that science analyses this complexion into its factors and utilises the results of its analysis in synthetic interpretation.
Such, I take it, is the conception of physical causation we reach when we reduce the notion of sequence to its ideal limits. It is the doctrine of Hume translated from the region of practical observation into the region of conceptual thought founded thereon. And in this sense we may say that modern science accepts the doctrine in its essential features. Why the sequence is of that nature which we find it to be in the data of sensory experience, physical science as such, does not, I conceive, attempt to explain. Here are the facts as practically given; that is an end of the matter so far as physical science is concerned.
It may, however, be said that I am attempting to impose on physical science restrictions which the physicist himself will not be ready to accept. Permit me therefore to quote a paragraph from Professor Lamb's Presidential Address before Section A at the Cambridge (1904) meeting of the British Association. It will serve to lead up to the next step in my argument.
We have, most of us [he says] frankly adopted the empirical attitude in physical science; it has justified itself abundantly in the past, and has more and more forced itself upon us. We have given up the notion of causation, except as a convenient phrase; what were once called laws of nature are now simply rules by which we can tell more or less accurately what will be the consequences of a given state of things. We cannot help asking, however: How is it that such rules are possible? A rule is invented in the first instance to sum up in a compact form a number of past experiences; but we apply it with little hesitation, and generally with success, to the prediction of new and sometimes strange ones. Thus the law of gravitation indicates the existence of Neptune; and Fresnel's wave surface gives us the quite unsuspected phenomena of double refraction. Why does nature make a point of honouring our cheques in this manner, or, to put the matter in a more dignified form, how comes it that, in the words of Schiller;
" Mit dem Genius stent die Natur im ewigen Bunde Was der eine verspricht leistet die andre gewiss".
The question is as old as science, and modern tendencies have only added point to it. It is plain that physical science has no answer; its policy, indeed, has been to retreat from a territory which it could not securely occupy. We are told in some quarters that it is vain to look for an answer anywhere. But the mind of man is not wholly given over to physical science and will not be content for ever to leave the question alone. It will persist in its obstinate questionings, and however hopeless the attempt to unravel the mystery may be deemed, physical science, powerless to assist, has no right to condemn it.
Let us now see how we stand. Naturalism, interpreting the material universe in terms of mechanism, formulates an ideal construction in terms of causal antecedence and sequence; in this it believes with a faith which is worthy of our admiration, since it is founded on certain selected aspects of experience. When it is modest, which I fear is not always the case, it confesses that its ideal construction cannot as yet always be applied with confidence to the observed facts, but it claims that wherever and whenever, in the existing state of assured knowledge, it can be so applied it fits the actual facts (new facts as well as old) with much accuracy. Let us accept this position and see what follows. The ideal construction of naturalism is admittedly rational and connected. But when this scheme (which is the product of our rational thought) is applied to the data of sensory experience (which are independent of our rational thought and over which our reason has no control) it is found to fit the given changes of configuration. Hence, just in so far as the connections of the ideal scheme coincide with the sequences of sensory experience, may we assume that these sequences have some underlying connection something which makes them of such a kind that they can be rationally treated. Science, however, ignores, though it should not deny, the existence of a "power or force which actuates the whole machine" ; it does not attempt to discuss the question why the antecedences and sequences which it studies are of such a kind as we find them to be. Some of us, nevertheless, as Professor Lamb indicates, are impelled by the very nature of our rational thought to seek an answer to this question. We, too, have our ideal constructions. We, too, have our beliefs which perchance include more than we can definitely prove. I for one believe that the connected and rational character of our ideal schemes of naturalism have their source and origin in the rational and connected character of the reality which underlies objective existence. But the reality beneath our systems of thought is the unity of human purpose which gives to every item significance within the connected scheme. Again, therefore, I am led to ask: May not the reality which is manifested in objective existence that nature which we strive to interpret be the unity of purpose which underlies it too, and gives to the world of phenomena a significance which would otherwise be wholly wanting?
It is commonly urged that in the phenomena observed in living creatures there is a special manifestation of purpose differing from that of which physical science yields far less convincing evidence. To a consideration of some of these phenomena we may now turn.