2. Motion, Mover, Movable
Aristotle's definition of motion is general; it makes no mention of a particular kind, or of the conditions attending every motion. But experience shows that the change from potency to act which characterizes motion cannot be made without the activity of an agent or mover exerting itself on a movable, on something that is formally distinct from the mover. This brings up the relation of motion to mover and movable. Respectively linked to mover and movable are, moreover, action and passion, two predicaments of being which also, it is thought, express the fact of change, including the change that is motion. Hence, there is the further question whether these predicaments are distinct from motion.
Our answers, developed in the order given, will be:
a) Motion is the act of the movable. - We assume, what would seem an obvious fact, that motion involves a receiving subject, a "movable," and an application to the subject from an outside agent, from a "mover." What may not be self-evident is whether motion, which is joined both to mover and movable, is the act of the mover or the movable.
According to Aristotle, when there is motion, what is moved is the passive or receiving subject, and not, properly speaking, the agent. This is also the impression we get from experience, and it is confirmed by the nature of motion itself. Motion, after all, is the act of what is in potency. But which of the two is in potency, the subject or the agent? Obviously the subject. It cannot be the agent, because in the exercise of its agency an agent is in act. If in its activity the agent, too, happens to be modified or moved, this is only by reaction of the receiving subject, whose reaction is accidental to the motion imparted by the agent.
Motion, consequently, is in the movable, that is, in the moved. Yet, as mentioned a moment ago, it is also connected with the agent, but as proceeding from it, as ab hoc, and not as founded in it or in hoc. Motion, in short, is the act of the movable. Or, in Scholastic idiom: motus est actus mobilis.
b) Mover and moved have one and the same act. - If motion is the act of the movable, what becomes of the agent or mover? Is there not act in the mover as well? These questions, it should be apparent, bear on the unity of motion. If the act of the movable and of the mover are two different acts, then every motion is in fact two motions. But this is inadmissible, since the course of motion has a manifest unity. What the agent produces when it moves the movable, and what the movable receives when moved, these are one and the same thing; hence there is only one motion, which, however, is the simultaneous act of mover and movable alike. Motion, accordingly, is the act of the mover so far as it proceeds from the mover into the movable; but it is the act of the movable so far as it is in the movable from the mover.8 Aristotle finds an illustration of this in teaching and learning. The teaching which the teacher imparts, and the pupil receives, are one and the same teaching.9
c) Motion: action or passion, or both? - To say that motion has unity safeguards the true meaning of motion; but, as already intimated, it also raises a serious difficulty in connection with the predicaments action and passion. According to predicamental teaching, the act of the agent is action, and the act of the patient passion. If we say that action, the act of the agent, and passion, the act of the patient, constitute two distinct motions, we counter our previous assertion of the unity of motion. On the other hand, if action and passion are the same motion, can we still maintain that they are two distinct predicaments of being?
In reply, action and passion are indeed the same motion, but from different points of view. Action is motion as proceeding from the agent, whereas passion is motion as resident in the passive or receiving subject. On which St. Thomas has this, as usual perceptive, gloss. "It is clear," he writes, after some careful deciphering, "that the motion of mover and moved is the same motion because motion as motion abstracts from both formalities [i.e., the formalities ab hoc and in hoc]; 10 but action and passion nevertheless differ because in their respective signification they include these opposite formalities." 11
The term "motion," therefore, as St. Thomas indicates, is more abstract and universal than the term "action" or "passion." Motion, taken absolutely, is not in a particular predicament; reductively, or in the last analysis, it is placed in whatever predicamental genus terminates it, either in quantity, quality, or place. But though motion may be considered in the abstract, it is not an abstraction but a concrete reality, and one of the conditions of its production is the causal activity of an agent. From the perspective of this activity motion presents itself with agent and patient, and we may, in consequence, refer it to the separate predicaments of action and passion.