That all material substances possess activity, passivity, and inertia is quite certain on experimental grounds. No conclusion is better established in science than that all the particles of matter act on one another according to a fixed law, and receive from one another their determination to move from place to place, while they are incapable of setting themselves in movement or modifying the movement received from without. Now, it is clear that they cannot act without being active, nor receive the action without being passive, nor be incapable of modifying their own state without being inert.
This shows that activity and inertia do not exclude one another. A particle of matter is said to be active inasmuch as it has the power of causing the movement of any other particle; and is said to be inert inasmuch as it has no power of giving movement to itself. It is plain that these two things are very far from being contradictory. Those philosophers, therefore, who have apprehended an irreconcilable opposition between the two, must have attached to the term “inertia” a meaning quite different from that recognized by physical science.
Balmes remarks, in his Fundamental Philosophy,[131] that there is nothing perfectly still either on earth or in the heavens; and for this reason he expresses the opinion [pg 582] that all bodies have a constant tendency to move. And as he cannot see how such a tendency can be reconciled with the inertia of matter, he comes to the conclusion that bodies are not inert. But it is scarcely necessary to remark that the constant tendency to move which we observe in bodies is the result of universal attraction, and not of a self-acting power inhering in the matter of which the bodies consist; and therefore such a tendency does not in the least interfere with the inertia of matter. A simple reference to the laws of motion suffices to convince the most superficial student that such is the case.
Malebranche goes to the other extreme. He supposes that bodies have no activity of any kind, and that accordingly all the phenomena we witness in the physical world are produced by God alone. This theory, as every one will acknowledge, is supremely extravagant and unphilosophical. It leads to idealism and to pantheism. To idealism, because, if bodies do not act, there is no reason why they should exist; as nothing can be admitted to exist throughout creation which has no aptitude to manifest in its own reality a reflex of the Creator's perfections. And since manifestation is action, no created being can be destitute of active power. This argument drawn from the end of creation may be supplemented by another drawn from the impossibility of our knowing the existence of bodies if they do not act. For, if bodies do not act on our senses, we cannot refer to bodies for the causality of our sensations; and thus the only link by which we have the means of connecting our subjective impressions with exterior objects will be destroyed.
Hence, if bodies are not active, there is no reason why they should be admitted to exist, and we are accordingly condemned to an absurd idealism. Nor can we escape pantheism. For, if the impressions we receive from outside are caused by God alone, we cannot but conclude that whatever we see outside of us has no other objectivity than that of the divine substance itself appearing under different forms. Now, this is a pantheistic doctrine. Therefore the theory which denies the activity of bodies leads to pantheism. We will say nothing more about this preposterous doctrine and its absurd consequences. Plain common sense, without need of further argument, condemns whatever calls in question the reality and objectivity of our knowledge concerning the exterior world.
But, while we admit with all the physicists, and indeed with all mankind, that material substance is competent, through its natural activity, to cause local motion, we must guard against the opinion of the materialists, who pretend that the active power of matter is also competent, under certain conditions and through certain combinations, to produce thought. Nothing, perhaps, can be more inconsistent with reason than this assumption. Were matter not inert, the hypothesis might deserve examination; but an inert thinking substance is such an enormity that it cannot, even hypothetically, be entertained. The thinking faculty evidently implies self-acting power, whereas inertia evidently excludes it; and therefore, so long as we keep in mind that matter is inert, we cannot, without evident inconsistency, extend the range of its activity to immanent operations, but must confine it to the extrinsic [pg 583] production of local motion. Let us here remark that, of all the arguments usually employed in psychology against the materialistic hypothesis, this one drawn from the inertia of matter is the most valuable as it is the most simple and incontrovertible.
The inertia of matter is so universally admitted that it is hardly necessary to say a word about it. No fact, indeed, is more certain in science than that matter, when at rest, cannot but remain so until it receive from without a determination to move; and likewise that, when determined to move with any velocity and in any direction, it cannot but move with that velocity and in that direction until it receive some other determination from without. This incapability of changing its own state constitutes, as we have already stated, the inertia of matter, and is the very foundation of mechanical science.
As to the natural passivity of material substance, we need only say that it consists in its capability of receiving, when it is acted on, any accidental determination to move in any direction and with any velocity. That matter has this passivity is an obvious experimental truth; and that matter has no other passivity except this one we shall prove in another place.
Meanwhile, it is evident from the preceding considerations that all matter is active, passive, and inert. The principle of activity, in every being, is its essential act, and the principle of passivity its essential term, which is a real passive potency;[132] hence the activity and the passivity of matter are a necessary result of the essential constitution of material substance, and are therefore essential properties of the same. The inertia of matter is also a necessary result of the essential constitution of material substance; for the only reason why an element of matter cannot give motion to itself is to be found in the mutual relation of its essential principles, which is of such a nature that the principle of passivity cannot be influenced by any exertion of the active principle, of which it is the intrinsic term. Now, this relation, for which we shall fully account hereafter, belongs to the essence of the substance as truly and as necessarily as the essential principles themselves. Hence the inertia of matter is an essential property of matter no less than its activity and passivity.