LECTURE 4 (202 a 3-21)

MOTION IS THE ACT OF THE MOBILE OBJECT AS THE SUBJECT IN WHICH AND THE ACT OF THE MOVER AS THE CAUSE BY WHICH

             297. After the Philosopher has defined motion, he here explains whose act motion is, i.e., whether it is the act of the mobile object or of the mover.

             It can be said that he sets forth here another definition of motion which is related to the definition already given as the material is related to the formal and as a conclusion is related to a premise.

             The definition is as follows: motion is the act of the mobile object insofar as it is mobile.

             This definition is drawn as a conclusion from the one already given. Since motion is the act of that which exists in potency insofar as it is such, and since that which exists in potency insofar as it is such is the mobile object, and not the mover, since the mover insofar as it is such is a being in act, it follows that motion is the act of the mobile object insofar as it is such.

             298. Concerning this, therefore, he makes three points. First, he shows that motion is the act of the mobile object. Secondly, where he says, '. . . and the actuality of that . . .' (202 a 14), he shows how motion is related to the mover. Thirdly, where he says, 'This view has a dialectical difficulty' (202 a 22), he raises a difficulty.

             Concerning the first part he makes two points. First, he states the definition of motion, namely, motion is the act of the mobile object. Secondly, where he says, 'The solution of the difficulty . . .' (202 a 11), he mentions something which, because of what he has said, could be a difficulty.

             Concerning the first part he makes three points. First he inquires into the definition of motion. Secondly, where he says, 'Hence we can define . . .' (202 a 6), he concludes to the definition. Thirdly, where he says, . . . the cause of the attribute . . .' (202 a 7), he explains the definition.

             Moreover, in order to inquire into the definition of motion, he states that the mover also is moved. And concerning this he makes two points.

             First he proves that every mover is moved. Secondly, where he says, 'For to act . . .' (202 a 4), he shows how the mover is moved.

             299. He shows with two arguments that the mover is moved.

             First, since everything which is first in potency and later in act is moved in some way, and since the mover is found to be at first a mover in potency and afterwards a mover in act, a mover of this sort is moved. He says that every mover, when it is so constituted that at some time it is mobile in potency, i.e., in potency to motion, is moved, as was said. This is apparent from what he has already said. For he said that motion is the act of that which exists in potency, and this occurs in every natural mover. Hence, it was said above that every physical mover is moved.

             300. Secondly, where he says, '. . . and whose immobility . . .' (202 a 3), he shows the same thing in another way.

             Motion is in anything whose immobility is rest. For rest and motion, since they are opposites, come to be with reference to the same thing. But the immobility of a mover, i.e., its cessation from moving, is called rest. For certain things are said to rest when they cease to act. Therefore, every such mover, i.e., one whose immobility is rest, is moved.

             301. Next where he says, 'For to act . . .' (202 a 4), he shows how the mover is moved.

             This does not happen to the mover because of the fact that it moves, but because of the fact that it moves by contact. For to move is to act in such a way that something is moved. That, moreover, which is thus acted upon by a mover is moved. But this sort of action works by contact, for bodies act by touching. Hence it follows that the mover should also be acted upon, for that which touches is acted upon.

             But this must be understood [to be true] when the contact is mutual, i.e., when something touches and is touched, as happens in those things which share in matter, each of which is acted upon by the other while they touch each other. For celestial bodies, which do not share in matter with inferior bodies, act upon inferior bodies in such a way that they are not acted upon by them, and they touch and are not touched, as is said in De Generatione et Corruptione, I.

             302. Next where he says, 'Hence we can define . . .' (202 a 6), he sets forth the definition of motion, concluding from the foregoing that, although the mover is moved, nevertheless motion is not the act of the mover but of the mobile object insofar as it is mobile.

             He subsequently clarifies this by reason of the fact that being moved is accidental to the mover, and does not belong to it per se. Hence, if a thing is moved insofar as its act is motion, it follows that motion is not the act of the mover, but of the mobile object, not, indeed, insofar as it is a mover, but insofar as it is mobile.

             He shows from what was said above that being moved is accidental to the mover. For the act of the mobile object, which is motion, occurs because of the contact of the mover. And from this it follows that the mover is acted upon at the same time that it acts, and thus to be moved belongs to the mover per accidens.

             He shows as follows that being moved does not belong to the mover per se. Some form always seems to be the mover, such as the form which is in the genus of substance in a change which is according to substance, and a form which is in the genus of quality in alteration, and a form which is in the genus of quantity in increase and decrease. Forms of this sort are causes and principles of motions, because every agent moves according to its form. For every agent acts insofar as it is in act, as man in act produces man in act from man in potency. Hence, since each thing is in act through its form, it follows that form is a moving principle. And thus to move belongs to a thing insofar as it has a form by which it is in act. Hence, since motion is the act of that which exists in potency, as was said above, it follows that motion does not pertain to a thing insofar as it is a mover, but insofar as it is mobile. And so it is stated in the definition of motion that motion is the act of the mobile object insofar as it is mobile.

             303. Next where he says, 'The solution of . . .' (202 a 11), he clarifies a certain difficulty which arises from what has been said. For someone might question whether motion is in the mover or in the mobile object.

             This difficulty is answered by what has already been said. For it is obvious that the act of anything is in that whose act it is. And thus it is obvious that the act of motion is in the mobile object because it is the act of the mobile object, caused in it, however, by the mover.

             304. Next where he says, '. . . and the actuality . . .' (202 a 14), he shows how motion is related to the mover.

             First he states his intention, saying that the act of the mover is not other than the act of the mobile object. Hence, although motion is the act of the mobile object, it is also in some way the act of the mover.

             305. Secondly, where he says, '. . . for it must be . . .' (202 a 15), he explains his position.

             Concerning this he makes three points. First, he shows that the mover has an act, as also does the mobile object. For whatever is named according to potency and act has some act belonging to it. Now that which is moved is called mobile by reason of its potency, insofar as it can be moved, and it is called moved by reason of its act, insofar as it is moved in act. In the same way the mover is called a mover by reason of its potency, insofar as it can move, and it is called a motion in its very action, i.e., insofar as it actually acts. Therefore, it is necessary that act pertain to both the mover and the mobile object.

             306. Secondly, where he says, 'But it is on the moveable . . .' (202 a 17), he shows that the act of the mover and of the moved is the same. For a thing is called a mover insofar as it does something, and moved insofar as it is acted upon. But what the mover causes by acting and what the moved receives by being acted upon are the same. He says that the mover acts upon the mobile object, i.e., it causes the act of the mobile object. Hence it is necessary that one act be the act of each, i.e., of the mover and the moved. For what is from the mover as from an agent cause and what is in the moved as in a patient and a recipient are the same.

             307. Thirdly, where he says, '. . . just as one to two . . .' (202 a 19), he clarifies this with an example.

             In respect to things the distance from one to two and from two to one is the same. But these differ according to nature [ratio]. For insofar as we begin the comparison from two and proceed to one, it is called double, whereas if we begin the comparison from the other end, it is called a half. And in like manner the space of rising and of descending is the same, but because of a difference in starting points and finishing points it is called ascent or descent. And the same applies to the mover and the moved. For motion insofar as it proceeds from the mover to the mobile object is the act of the mover, but insofar as it is in the mobile object from the mover, it is the act of the mobile object.