(3) Again, it is even more evident in the case of energy than in the case of mass, that we are dealing with one aspect singled out by abstraction from a whole possessed of other aspects not regarded in a purely mechanical construction. For (a) the capacity for work of an actual body does not always exist in the “kinetic” form of actual motion. There are various forms of non-kinetic energy, such as, e.g., the energy of “position” of a resting body, the heat of a body of higher temperature than its surroundings, which Mechanics treats as equivalent to “kinetic” energy, because they are theoretically capable of being converted into it. And these forms of non-kinetic energy are qualitatively different both from energy of actual motion and from each other. It is by a mere abstraction that we treat them as identical because they are, for certain special purposes, equivalent. The qualitative differences may make no difference with respect to a particular purpose of our own, but they are none the less really there.
Again, the mechanical scheme itself is quite insufficient to explain why or when these different forms of energy are replaced by one another. As has been well said by Professor Ward, the doctrine of the Conservation of Energy asserts no more than that a certain quantitative identity is maintained in all exchanges of energy. But when or in what direction these exchanges shall take place, the principle itself does not enable us to say. Thus, to take a simple example: if I know the mass of a stone lodged on a roof, the height of the roof from the ground, and the acceleration produced by gravity at the spot in question, I can determine the “potential energy” of the stone. But my data tell me nothing as to whether this potential energy will remain for ever in its potential form, or whether the stone will yet be dislodged and its energy converted into kinetic shape, and if so, when. The principles of the mechanical interpretation of nature are thus inadequate to describe the concrete course of events in so simple a case as that of the fall of a stone. If the stone falls, then by the aid of the mechanical postulate I can describe one aspect of the process, namely, the amount of kinetic energy which will be evolved; and again, if certain previous conditions are fulfilled, e.g., if the support gives way, and if the descent of the stone is not previously arrested, the mechanical postulate enables me to infer that the stone will fall and will reach the ground with just this kinetic energy. But I can never escape, so long as I keep within the mechanical scheme, from this necessity of hypothetically assuming as given data which the mechanical scheme itself cannot fully determine.
All these considerations show how the very nature of the mechanical scheme itself justifies our previous conclusion, that it is in all its details simply the expression of a postulate created by our practical need that the course of nature shall admit of calculation with sufficient exactitude for the devising of successful rules for intervention in it, but logically incapable of being without contradiction regarded as the real truth about any concrete natural process. The internal evidence, derived from examination of the fundamental concepts of scientific Mechanism, thus confirms the view we have already adopted on different grounds, that the whole physical order is merely the appearance of a more ultimate reality of a kind akin to our own sentient and purposive life. At the same time, our examination of mechanism may serve to throw some useful light on the often misconceived antithesis between Reality and Appearance. We call the physical order, as conceived by mechanical science, “appearance,” not because we regard it as illusory or deceptive in itself, or because it is not the manifestation of a true reality, but because it takes account only of those particular aspects of Reality which are important and significant for certain very special purposes. What appears to us as the physical order is, indeed, true Reality, and is, in fact, an integral part of the only Reality there is, but it appears to us in this special form and under these special restrictions because we have arbitrarily excluded every other aspect of the concrete facts from our purview by the choice of our initial postulates of descriptive science. By the nature of the special questions we put to our world, in our physical science, we determine in advance for ourselves the general character of the answer we are to receive.
Rigidly scientific investigation, for instance, finds mechanical determination everywhere in the world, and purposive spontaneity nowhere, just because it has previously resolved that it will accept “mechanical explanation” and nothing else as the answer to its questions. So far as we bear in mind the presence of these self-imposed logical limitations throughout our mechanical science, their existence need lead to no illusion or deception. The success of our mechanical postulates shows that, within the sphere of their logical applicability, the course of the world does really conform to them, and thus the results won by their application are genuine truth, so far as they go. It is only when we forget the limits set to the logical applicability of the mechanical postulates, by the special nature of the interests they subserve, and proceed to treat them as logically indispensable conditions of all existence and all knowledge, that the truths of mechanical science are perverted into the illusions and falsehoods of a mechanical philosophy.
Consult further:—F. H. Bradley, Appearance and Reality, chaps. 11 (Phenomenalism), 22 (Nature); H. Lotze, Metaphysic, bk. ii. chaps. 7, 8; E. Mach, Science of Mechanics, chap. 2, § 5, p. 216 ff.; K. Pearson, Grammar of Science, chaps. 7, 8; H. Poincaré, La Science et L’Hypothêse, parts 3 and 4, chaps. 6-10; J. B. Stalle, Concepts and Theories of Modern Physics, chaps. 2-6, 10-12; J. Ward, Naturalism and Agnosticism, vol. i. lects. 2-6.
[161]. And, again, the intermediate links themselves, however imperceptible, have always to be thought of as exhibiting properties identical in kind with those of objects given in direct presentation. As Mill said, a hypothesis which assumes at once an entirely unfamiliar agent and an equally unfamiliar mode or law of operation, would be useless. Thus the imperceptibles of scientific hypothesis belong essentially to the physical order.
[162]. This is the characteristic selected by Prof. Münsterberg as the basis of his own distinction between “physical” or “superindividual” and “psychical” or “individual” objects. See Grundzüge der Psychologie, i. 15-77.
[163]. Cf. Mach, Science of Mechanics, p. 4. Mach, however, erroneously as I think, makes the intercommunication a secondary consequence of the rise of specialised industrial classes.
[164]. I.e., the mechanical view of Nature, to be thoroughly self-consistent, must be purely mechanistic.