Now science, as we have just seen, is tolerant of this notion of latency or potentiality, and is ready enough to use it in aid of beliefs in conservation. It was so used in connection with heat when heat was regarded as a material substance. It is still so used in connection with energy, which is sometimes described as an immaterial substance. But (as I have already noted) it has never been so used in connection with matter. The reason, I suppose, is that the conservation of matter is much more a belief of common sense than the conservation of energy. Energy is a conception which has but recently been disengaged from other conceptions, like force and momentum, and has but recently been associated with heat, with chemical reactions, with changes of physical phase, and with electro-magnetic phenomena. It is, therefore, a remote and somewhat abstract product of scientific reflection; and science may do what it will with its own.
The notion of matter, on the other hand, is the common possession of mankind. Whatever difficulties it may present to reflective analysis, it presents none to our work-a-day beliefs. We are quite ready to regard it as indestructible; but we are not ready to combine this conviction with the view that it possesses no single characteristic which may not be temporarily etherealised into a “potentiality.” On such terms the eternal and unchanging identity of this or that parcel of matter would seem a difficult and elusive doctrine, inappropriate to the familiar and substantial world in which we suppose ourselves to live. A belief in the conservation of matter has therefore always, or almost always, carried with it a belief in the unchanging continuity of at least some material qualities; though as to what these qualities are there has been much dispute.
Descartes, though not consistent, found unchanging continuity in the attribute of size; so also did Hobbes. I presume that the older atomists, who explained the appearances of matter by the shape of its constituent atoms, would have regarded both atomic form and atomic magnitude as persistent. But it was the assumption that the same piece of ponderable matter always possessed the same gravitating power, and that the same gravitating power was always associated with the same mass, which, in the hands of Lavoisier, made so great a revolution in eighteenth-century chemistry. Matter might change its size, its shape, its colour, its phase, its power of acting and reacting; but its mass and the quality which caused its weight it could not change; these characteristics were always associated with each other, and were never in abeyance.
To Lavoisier this double principle seemed self-evident. It was not a hypothesis that required testing, but a touchstone by which other hypotheses might safely be tested. If, in the course of some chemical operation, weight increased, then no further proof was required to show that mass had increased also, and that matter had been added. If, on the other hand, weight diminished, then no further proof was required to show that mass had diminished also, and that matter had been subtracted. Whatever other qualities matter might gain or lose, mass and gravity were indestructible and unchanging.
Men of science seemed, on the whole, content silently to assume these principles of conservation without inconveniently raising the question of evidence. Philosophers have not always been so cautious. Kant supposed himself to have demonstrated them a priori. Schopenhauer followed suit. Spencer declared their contraries to be inconceivable. Mill said they were proved by experience. In short, all these eminent thinkers vied with each other in conferring upon this doctrine the highest honours permitted by their respective philosophies. But apparently they were hasty. Recent discoveries have changed our point of view. Mass (it seems) is no longer to be regarded as unchanging. When bodies move at speeds approaching the velocity of light their mass rapidly increases; so that this quality, which is peculiarly characteristic of matter, must be removed from the category of those which persist unchanged, and placed in the category of those which change but can always be restored. Are we so to class gravitation? Would the weight of a body moving nearly at the speed of light increase as, in like circumstances, its inertia increases? If the answer is “no,” then the link is broken which has for long been thought to connect gravity and mass. If the answer is “yes,” then what Kant regarded as certain a priori is false; what Spencer regarded as “inconceivable” is true; another carrier of “persistence” is lost, and some fresh characteristic must be found which will remain unchanged through all time, and under all conditions.
If this characteristic should turn out to be electric charge, what a curious light it will throw upon our tendency to “beliefs of conservation”[15]. After long seeking for some indestructible attribute of matter; after taking up and rejecting size, shape, weight, mass, and (perhaps) impenetrability, we shall at last find the object of our quest in a conception which has (I suppose) been clearly realised only within the last hundred years, about which our senses tell us nothing, and of which the general run of educated mankind are still completely ignorant!
III
It is possible, but not, I hope, probable, that some hasty reader may suppose that in this and the preceding lectures I am recommending a new method or instrument of discovery. “If you want to reach truth, follow your unreasoned inclination,” may be his summary of my doctrine: brief—but also unjust.
Of the manner in which discoveries are going to be made I say nothing, for I know nothing. I am dealing with the past: and in the historic movements of scientific thought I see, or think I see, drifts and currents such as astronomers detect among the stars of heaven. And, as the law of gravitation will hardly (I suppose) explain the last, so observation, experiment, and reasoning will hardly explain the first. They belong to the causal, not to the cognitive, series; and the beliefs in which they issue are effects rather than conclusions.
Those who feel little sympathy for such a view may be inclined to regard the relatively faint inclinations dealt with in this lecture as ordinary scientific hypotheses confirmed by ordinary scientific methods. This view, as I have already observed, is not applicable to the inevitable beliefs dealt with in earlier lectures. Whatever philosophers may say after the event the conviction that we live in an external world of things and persons, where events are more or less regularly repeated, has never been treated as a speculative conjecture about which doubt was a duty till truth was proved. Beliefs like these are not scientific hypotheses, but scientific presuppositions, and all criticism of their validity is a speculative after-thought. The same may be said, though with less emphasis and some qualification, about beliefs fostered by the intellectual tendencies considered in this chapter. These, as we have seen, are many. They are often inconsistent; they are never inevitable; and they perpetually change their form under the pressure of scientific discovery. Atomism in one shape follows atomism in another; doctrines of conservation rise, fall, and rise again; incredulity about “action at a distance” breeds explanations whose failure (in the case of gravity) leaves the hope of final success untouched.