3. The Kinds of Motion
In Book III Aristotle does not attempt a classification of motion by its kinds. This is done at the beginning of Book V.12 As for the special distinction between generation and the movements of alteration and growth, this discussion is remitted to its more proper place, viz., Book I of De Generatione et Corruptione.
When, in Book V, Aristotle does unfold his division of motion into its kinds, he goes into considerable detail. For our purpose, however, it will be enough to chart the essentials.
Aristotle, after some preliminaries, declares that everything which changes must do so in one of four ways, namely:
"Subject," in this instance, as St. Thomas points out, does not mean that which underlies or supports form, but, as Aristotle himself notes, whatever is expressed affirmatively.
The last mentioned possibility, from nonsubject to nonsubject, is dismissed without further ado; it cannot be a species of change, as it lacks an opposition of terms, and every true change falls within contraries or contradictories. The transition from nonsubject to subject is primarily substantial generation; and the change from subject to nonsubject is primarily substantial corruption. Neither of these, as we shall further remark in a moment, is properly speaking motion. This leaves the passage from subject to subject, which is motion proper. But where, in the list of predicaments, is such change found? 'Wherever, says Aristotle, contraries (not to be confused with contradictories) occur: hence in quantity, quality, and place.
Aristotle comes to this conclusion, not by actually proving that motion exists in each of these three categories - this he deems evident - but rather by arguments which eliminate motion from the others. The arguments, in essential, are as follows:
a) First, motion properly speaking is not in the category substance. The reason is that motion involves contrariety, and no mode of being is in strict definition the contrary of substance.13 Besides, motion requires an actual subject that is common to its two terms, and this actual subject is wanting between the terms of a substantial generation or a substantial corruption.
b) Secondly, motion is not in the category relation. A relation is between two terms. A change in one can cause a change to the other, though the other may not have changed in itself. Thus, a thing may be equal in size to another, but if the first becomes smaller or larger, the second is no longer its equal, though there has been no change in the second. A new relation is established without a change in one of the terms. But, and this is the point, such an occurrence is not possible in a predicament of being where motion or movement exists; a change in the same predicament necessarily means a change in the subject. So, for example, a new color cannot supplant an old one without the colored object being altered. Hence, motion or movement is not in the category of relation. Moreover, the absence of motion in relation entails its absence in position [situs] and possession [habitus], both of which imply relation.
c) Thirdly, motion is not in the categories action and passion. Motion in these would be equivalent to motion of motion, the possibility of which Aristotle takes the better part of a chapter to disprove." By the same reasoning, motion is not in the category when [quando]. "When" fixes the time, but time, being the measure of motion, is founded in it, so that motion in time would again be motion of motion. Consequently, the others having been eliminated, only three categories or predicaments remain in which motion, in the proper sense, occurs: quantity, quality, and place. To be borne in mind here is the distinction between motion or movement proper [motus] and change [mutatio], a more comprehensive notion. Thus, a substantial generation or corruption is change, but not, precisely, motion. This, in the light of the foregoing paragraphs, should now be clear. And if motion is only in three categories, there are strictly speaking only three species or distinct kinds of motion, namely: - motion according to quantity, which means motion by increase and decrease. This, in Aristotelian conception, pertains only to living things, specifically to their increase or decrease in size; - motion according to quality, the proper name for which is alteration; and - motion according to place [ubi], which is local motion. These motions, though distinct kinds, interpenetrate. They are an interacting network whose functioning commands the course of all nature. The control center is local motion, the most perfect of all, and the only kind that affects all bodies, not excepting celestial bodies. By inducing the arrangement of bodies within and varying their contacts without, this motion governs the whole complex of all other motions and changes. Placed in contact, bodies alter, which is the motion of alteration; are engendered and destroyed, which is generation and corruption; and, if living bodies, gain or lose their natural quantity, which is motion by increase and decrease. Having declared the kinds of motion, Aristotle continues, in the remainder of Book V and in Book VI, to explore the subject of motion from a great number of other aspects. Treated are, for example, the unity of motion, the contrariety of motion, its continuity, its first moment, its term, its rest. We shall not enter into these matters. Instead, we shall conclude this chapter with an account of the essential ideas in his theory of local motion. This, as we have said, commands the cosmic procession, and a few remarks on it now will stand us in good stead in the chapters to follow.15