His position is as follows: that which in the monad is activity or substantial, is, in sensible matter, motion. That which in the monad is lack of a given activity, that which constitutes its subordinate position in the hierarchy of monads, is, in the sphere of material things, inertia. That which in the spiritual world is the individuality of monads, making each forever ideally distinct from every other, is, in the phenomenal realm, resistance or impenetrability. The perfect continuity of monads in the mundus intelligibilis has also its counterpart in the mundus sensibilis in the diffusion or extension of physical things.
Instead of following out this analogy directly, it will rather be found convenient to take up Leibniz’s thought in its historical connection. We have already alluded to the fact that he began as a Cartesian, and that one of the first ideas which repelled him from that system of thought was the notion that the essence of matter is extension. His earliest philosophical writings, as he was gradually coming to the thoughts which thereafter dominated him, are upon this point. In general, his conclusions are as follows: If matter were extension, it would be incapable of passion or of action. Solidity, too, is a notion entirely opposed to the conception of mere extension. The idea of matter as extension contradicts some of the known laws of motion. It requires that the quantity of motion remain unchanged whenever two bodies come in contact, while as matter of fact it is the quantity of energy, that which the motion is capable of effecting, that remains unchanged; or, as he more often puts the objection, the Cartesian notion of matter requires that matter be wholly indifferent to motion, that there be nothing in it which resists motion when imparted. But, says Leibniz, there is something resisting, that to which Keppler gave the name “inertia.” It is not found to be true if one body impacts upon another that the second moves without diminishing the velocity or changing the direction of the first. On the other hand, just in proportion to the size of the second body, it resists and changes the motion of the first, up to the point of causing the first to rebound if small in comparison. And when it was replied that the retardation was due to the fact that the force moving the first body had now to be divided between two, Leibniz answered that this was simply to give up the contention, and besides the notion of extension to use that of force. If extension were the essence of matter, it should be possible to deduce all the properties of matter, or at least to account for them all, from it. But since, as just seen, this does not enable us to account for any of them, since for any of its concrete qualities we have to fall back on force, it is evident where the true essence of matter is to be found.
Leibniz has another argument of a logical nature, as those already referred to are of a physical: “Those who claim that extension is a substance, reverse the order of words as well as of thoughts. Besides extension there must be a subject which is extended; that is to say, something to which it belongs to be repeated or continued. For extension is nothing but a repetition or continued multiplication of that which is spread out,—it is a plurality, a continuity, a co-existence of parts. Consequently, extension does not suffice to explain the nature of the repeated or manifold substance, of which the notion is anterior to that of its repetition.” Extension, in other words, is nothing substantial, it is not something which can exist by itself; it is only a quality, a property, a mode of being. It is always relative to something which has extension. As Leibniz says elsewhere: “I insist that extension is only an abstraction, and requires something which is extended. It presupposes some quality, some attribute, some nature in a subject which is extended, diffused, or continued. Extension is a diffusion of this quality. For example, in milk there is an extension or diffusion of whiteness; in the diamond an extension or diffusion of hardness; in body in general a diffusion of antitypia or materiality. There is accordingly in body something anterior to extension.”
From the physical side, therefore, we find it impossible to account for the concrete properties of material phenomena from extension; on the logical we find that the idea of extension is always relative to that which is extended. What is that which is to be considered as the bearer of extension and the source of physical qualities? We are led back to the point at which we left the matter in the [last chapter]. It is force, and force both passive and active. Leibniz uses the term “matter” in at least three senses: it is the metaphysical element of passive force in the monad; it is the monad itself considered as, upon the whole, externally conditioned or unconscious; and it is the phenomenon resulting from the aggregation of the monads in the second sense. The first is naked matter, and is a pure abstraction; the second is the monad as material, as opposed to the monad, as soul; the third is clothed, or second matter, or, concretely, body, corpus. The first is unreal by itself; the second is one phase of substance; the third is not substantial, but is a reality, though a phenomenal one. It is from the substantial monad that we are to explain the two things now demanding explanation,—that element in bodies (matter in third sense) which is the source of their physical properties, and that which is the subject, the carrier, so to speak, of extension.
That of which we are in search as the source of the physical qualities of bodies is motion. This is not force, but its “image.” It is force, says Leibniz, that “is the real element in motion; that is to say, it is that element which out of the present state induces a change in the future state.” As force, in other words, is the causal activity which effects the development of one “representation” of a monad out of another, so motion, in the realm of phenomena, is not only change, but change which is continuous and progressive, each new position being dependent upon the foregoing, and following out of it absolutely without break.
Motion, therefore, is the manifestation of the ideal unity of substance,—a unity not of mere static inherence, but of a continuous process of activity. It is from this standpoint that Leibniz accounts for the so-called transference of motion from one body to another upon contact. The ordinary view of this, which looks at it as if one body loses the motion which another body gains, Leibniz ridicules, saying that those who hold this view seem to think that motion is a kind of thing, resembling, perchance, salt dissolved in water. The right view, on the other hand, does away with all appearance of mystery in the carrying over of motion from one body to another, for it recognizes that continuity is the very essence of motion, and that we do not have two things and a third process, but that the two bodies are phases or elements in one and the same system of movement.
Starting from this idea of motion, then, Leibniz is to account for the actual qualities of matter as found in experience. These are the form, magnitude, cohesion, resistance, and the purely sensible qualities of objects. “First” matter, that is, abstract matter, may be conceived, according to Leibniz, as perfectly homogeneous, a “subtle fluid,” in his words, without any distinction of parts or of solidity. But this is an abstract notion. It is what matter would be without motion. Motion necessarily differentiates this plenum of homogeneity, and thus causes distinctions of figure (that is, boundaries of parts) and varieties of cohesion, or the varying solidity and fluidity of bodies. The latter difference is indeed the ultimate one. The principle of continuity or gradation, as applied to motion, makes it necessary that motions should not be in any two places of exactly the same energy. The result is that the original fluid matter is everywhere differently divided. Motion, entering into the uniform plenum, introduces distinction; it causes so much of the matter as is affected by a given movement to collect together and form in appearance a coherent body, as opposed to surrounding bodies which are affected by different degrees of energy. But even this is only approximate; the same principle of continuity must be applied within any apparently coherent body; its parts, while, in relation to other bodies, they have the same amount of motion, are in relation to one another differently affected. There are no two having exactly the same motion; if they had, there would be no distinction between them; and thus, according to the principle of Leibniz, they would be the same.
It follows at once from this that there is in the universe no body of absolute hardness or solidity, nor of entire softness or fluidity. A perfectly solid body would be one whose system of motions could not be affected by any other system,—a body which by motion had separated itself from motion, or become absolute. This is evidently an idea which contradicts itself, for the very essence of motion is continuity or relation. A body perfectly fluid, on the other hand, would be one in which there was no resistance offered to other motions,—a body, in other words, in which there are no movements that, entering into connection with one another, form a relative opposition to other movements. It would be a body isolated or out of relation with the general system of motions, and hence an impossibility. There is no last term either of solidity or of fluidity.
It equally follows as matter of course that there is no indivisible particle of matter,—no atom. The infinity of degrees of motion implies a corresponding division of matter. As already said, it is only in contrast with other relatively constant systems of motion that any body is of uniform motion; in reality there is everywhere throughout it variety of movement, and hence complete divisibility, or rather, complete division. If Leibniz were to employ the term “atom” at all, it could be only in the sense of the modern dynamical theory (of which, indeed, he is one of the originators), according to which the atom is not defined by its spatial position and outlines, but, by the range of its effects, as the centre of energies of infinite circumference. Correlative to the non-existence of the atom is the non-existence of the vacuum. The two imply each other. The hard, limited, isolated body, having no intrinsic relations with other bodies, must have room to come into external relations with them. This empty space, which is the theatre of such accidental contacts as may happen, is the vacuum. But if bodies are originally in connection with one another, if they are in reality but differentiations of varying degrees of motion within one system of motion, then there is no necessity for the vacuum,—nay, there is no place for it. The vacuum in this case could mean only a break, a chasm, in the order of nature. According to the theory of Leibniz, “bodies” are but the dynamic divisions of the one energy that fills the universe; their separateness is not an independent possession of any one of them or of all together, but is the result of relations to the entire system. Their apparent isolation is only by reason of their actual connections. To admit a vacuum anywhere, would thus be to deny the relatedness of the parts separated by it. The theory of the atom and the vacuum are the two phases of the metaphysical assumption of an indefinite plurality of independent separate realities. The theory of Leibniz, resting as it does on the idea of a perfect unity of interrelated members, must deny both of these aspects. Were we making an extended analysis of the opposed view, it would be necessary to point out that it denies itself. For it is only through the vacuum that the atoms are isolated or independent, and the sole function of the vacuum is to serve as the background of the atoms. The atoms are separated only in virtue of their connection, and the vacuum is what it is—pure emptiness—only on account of that which is in it. In short, the theory is only an abstract and incomplete way of grasping the thought of relation or mediated unity.
We have thus discovered that all motions conspire together, or form a system. But in their unity they do not cease to be motions, or variously differentiated members. Through this differentiation, or mutual reaction of motions, there comes about the appearance of boundaries, of separation. From these boundaries or terminations arise the form and size of bodies. From motion also proceeds the cohesion of bodies, in the sense that each relative system resists dissolution, or hangs together. Says Leibniz, “The motions, since they are conspiring, would be troubled by separation; and accordingly this can be accomplished only by violence and with resistance.” Not only form, size, and stability depend upon motion, but also the sensible, the “secondary” qualities. “It must not be supposed that color, pain, sound, etc., are arbitrary and without relation to their causes. It is not God’s way to act with so little reason and order. There is a kind of resemblance, not entire, but of relation, of order. We say, for example, ‘Light is in the fire,’ since there are motions in the fire which are imperceptible in their separation, but which are sensible in their conjunction or confusion; and this is what is made known in the idea of light.” In other words, color, sound, etc., even pain, are still the perception of motion, but in a confused way. We thus see how thoroughly Leibniz carries back all the properties of bodies to motion. To sum up, motion is the origin of the relative solidity, the divisibleness, the form, the size, the cohesion, or active resistance of bodies, and of their properties as made known to us in immediate sensation.