Remarks.—As the ubication, so also the momentum produced by accidental action, can be considered both absolutely and respectively. The momentum, considered absolutely, is an act received in a subject—an absolute momentum, an extrinsic term of the virtuality of the motive principle; and, as such a momentum is only one out of the innumerable acts which can proceed from the agent, it has an entity infinitely less than that of the agent. It is evident, in fact, that between a substantial and an accidental act there must be an infinite entitative disproportion, both because no substance can be substantially changed by its accidents, and because the substantial act can never be exhausted, and not even weakened, by the production of accidental acts, as we have established in another place.[158] The momentum is considered respectively when it is compared with another momentum, in which case we can find the relation of the one to the other as to intensity. This intensity is measured by the quantity of the movement to which they give rise when not counteracted.
The unit of intensity is arbitrary in the momentums, as in their principles, for the same reason—that is, because in neither case a natural unit of intensity can be found. The number expressing the relative intensity of a momentum is only virtually discrete, because the momentum is only virtually compounded, since it is not a number of distinct acts, but one act equivalent to many.
Movement and its affections.—The production of a momentum entails movement. The general definition of movement, according to Aristotle and S. Thomas, is Actus existentis in potentia ut in potentia, or, as we would say, an actual passage from one potential state to another. Now, all created being is potential in two manners: first, on account of its passive receptivity; secondly, on account of its affectibility, which is a consequence of its passivity, as we have explained in the “Principles of Real Being.”[159] Hence the momentum of movement, inasmuch as it is received in the patient, actuates its passive potency; and inasmuch as its reception entails a certain mode of being, it affects its resultant potentiality. But besides this double potentiality, which is intrinsic to the subject, there is another potentiality which refers to an extrinsic term, and for this reason movement is considered both as it is a modification of its subject, ratione subjecti, and as it points at an extrinsic term, ratione termini.
With regard to its subject, movement is usually divided into immanent and transient. It is called immanent when it results from immanent acts, as when the soul directs its attention to such or such an object of thought; and it is called transient when it brings about a change in a subject distinct from the agent, as when a man moves a stone, or when the sun moves the earth. But this is inaccurate language; for what is transient in these cases is the action, not the movement.
With regard to its term, movement is divided into two kinds—that is, movement to a place, motus ad ubi, and movement towards a certain degree of perfection or intensity of power, motus virtutis.[160] The first is called local movement, of which we will speak presently. The second is subdivided into intension, remission, and alteration. Intension and remission are the acquisition or loss of some degree of perfection or of intensity with regard to power and qualities; alteration is the passage from one kind of quality or property to another. Thus, in water, heat is subject to intension and remission; but when the cohesive force of the molecules is superseded by the expansive force of vapor, there is alteration.
It is important to notice that there is no motus virtutis in primitive elements of matter. The exertion of their power varies indeed according to the Newtonian law, but the power itself is always exactly the same, as its principle is the substantial act, which cannot be modified by accidental action. It is only in material compounds that the motus virtutis can be admitted, for the reason that the active powers and qualities in them are a result of composition; hence a change in the mode of the composition brings about a change in the resultant. So also in spiritual substances there is no motus virtutis, because their active faculties are always substantially the same. True it is that the intellect has also its passivity with regard to intelligible species, and that it acts by so much the more easily and perfectly in proportion as it is better furnished with intelligible species distinctly expressed and arranged according to their logical and objective connection. But this cannot mean that the active power of the intellect can be increased, but only that it can be placed in more suitable conditions for its operations. And the like is to be said of all acquired habits; for they give a greater facility of acting, not by intensifying the intrinsic power, but by placing the active faculty in such conditions as are more favorable for its operation.
But let us revert to local movement. This movement may be defined as the act of gliding through successive ubications. Such a gliding alters the relations of one body to another, as is evident, but it involves no new intrinsic modification of the subject. As long as the subject continues to move under the same momentum, its intrinsic mode of being remains uniformly the same, while its extrinsic relations to other bodies are in continual change. Hence the local movement of any point of matter merely consists in the act of extending from ubication to ubication, or, as we may say, in the evolution of the intensity of the momentum into continuous extension. The reason of this evolution is that the momentum impressed on a subject has not only a definite intensity, but also a definite direction in space; whence it follows that the subject which receives the momentum receives a determination to describe a line in a definite direction, which it must follow, owing to its inertia, with an impetus equal to the intensity of the momentum itself. And in this manner a material point, by the successive flowing of its ubication, describes a line in space, or evolves the intensity of its momentum into extension.
Hence, of local movement we can predicate both intensity and extension. The intensity is the formal principle, which, by actuating the inertia or mobility of the subject, evolves itself into extension. The extension is the actual evolution of the momentum, and constitutes the essence of local movement, which is always in fieri. And this is what is especially pointed out in Aristotle’s words: Motus est actus existentis in potentia, ut in potentia. The actus refers to the intensity, which is not in fieri, but has a definite actuality; whilst the in potentia ut in potentia clearly refers to the evolution of extension, which is continually in fieri under the influx of said act. Accordingly, local movement is both intensive and extensive. But this last epithet is to be looked upon as equivalent to “extending,” not to “extended”; for it is the line drawn, or the track of the movement already made, that is properly “extended,” whereas the movement itself is the act of extending it.
The formal intensity of local movement is called velocity. We say the formal intensity, because movement has also a material intensity. The formal intensity regards the rate of movement of each element of matter taken by itself, and it is greater or less according as it evolves a greater or a less extension in equal times. The material intensity regards the quantity of matter which is moving with a given velocity, and is measured by the product of the velocity into the mass of the moving body. This product is called the momentum of the body, or its quantity of movement.
Local movement is subject to three affections—viz., intension, remission, and inflexion. In fact, since local movement consists in extending with a certain velocity in a certain direction, it is susceptible of being modified either by a change of velocity, which will intensify or weaken it, or by a change of direction—that is, by inflexion. So long, however, as no agent disturbs the actual movement already imparted to a body, the movement must necessarily continue in the same direction and with the same velocity; for matter, owing to its inertia, cannot modify its own state. This amounts to saying that the tendency uniformly to preserve its rate and its direction is not an accidental affection, but the very nature, of local movement.