LECTURE 1 (241 b 24-242 a 15)

WHATEVER IS MOVED MUST BE MOVED BY ANOTHER

             884. After the Philosopher in the preceding books has treated motion in itself and its consequences and its parts, he begins here to treat motion in comparison to movers and mobile objects. This discussion is divided into two parts.

             First he proves the existence of a first motion and a first mover. Secondly, in Book VIII where he says, 'It remains to consider . . .' (250 b 11), he investigates the nature of the first motion and the first mover.

             The first part is divided into two parts. First he proves the existence of a first motion and a first mover. And secondly, since things which are of one order are related to each other, he treats the comparison of motions to each other, where he says, 'A difficulty may be raised . . .' (248 a 10).

             Concerning the first part he makes three points. First he sets forth what is needed to prove his position. Secondly, where he says, 'Since everything . . .' (242 a 16), he proves his position. Thirdly, where he says, 'That which is the first movement . . .' (243 a 4), he explains a certain thing which he had assumed.

             885. He states, therefore, first that whatever is moved must be moved by another. In some things this is obvious. For there are certain things which do not have within themselves the principle of their own motion. Rather the principle of their motion is extrinsic, as in things which are moved through force. If, therefore, there is something which does not have within itself the principle of its own motion, but the principle of its motion is extrinsic, it is clear that it is moved by another. But if there is some mobile object which would have within itself the principle of its own motion, about this there can be some question as to whether it is moved by another. And therefore concerning this he tries to show that it is moved by another. Let A B be a mobile object which is not moved by another. A B is moved in itself and primarily and not because some part of it is moved. For otherwise it would not be moved in itself but in respect to a part. Furthermore, if something moves itself and is not moved by another, it must be moved primarily and per se. For example, if something is not hot from another, it must be hot primarily and per se.

             Granting this, he proceeds to prove his position in two ways. First he eliminates that from which it especially seems that a thing is not moved by another. Secondly, where he says, 'In the second place . . .' (241 b 35), he shows directly that nothing can be moved by itself.

             A thing especially seems not to be moved by another when it is not moved by something exterior but by an interior principle.

             He says, therefore, first that A B is thought to be moved by itself because the whole is moved, and it is not moved by something exterior. It is as if one were to say that a mobile thing, of which one part is moved and another part moves, would move itself because one cannot distinguish which part may be moving and which part may be moved. For example, let J K L be such a mobile object. The part J K moves the part K L, but it is not known which of these parts moves and which is moved. He wishes, moreover, that the first mobile object, A B, the whole of which is moved by an interior principle, be understood as a living body the whole of which is moved by a soul. However, he wishes that the mobile object J K L be understood as some body which is not wholly moved. Rather one bodily part is moving, and the others are moved. In this mobile object it is clear that that which is moved is moved by another. And from this he wishes to show the same thing of a living body which seems to move itself. For this is proper to it insofar as one part moves another, that is, the soul moves the body, as will be explained more fully in Book VIII.

             886. Next where he says, 'In the second place . . .' (241 b 35), he shows directly with the following argument that everything which is moved is moved by another.

             Whatever is moved by itself does not rest from its own motion because of the rest of any other mobile object. He accepts this as though known per se. From this he concludes further that if a mobile object comes to rest because of the rest of another object, then it is moved by that other object. Furthermore, granting this, he concludes that whatever is moved must be moved by another. That this follows from these premises he proves as follows.

             The mobile object which we have supposed to be moved by itself, namely A B, must be divisible. For whatever is moved is divisible, as was proven above. Therefore, since it is divisible, no difficulty arises if it should be divided. Let it be divided, therefore, at the point C so that one part of it is B C and the other part A C. If, therefore, B C is part of A B, then when the part B C is at rest, the whole of A B must be at rest. If, therefore, the whole is not at rest, but a part is at rest, then the whole is moved and one part is at rest. But since one part is given as being at rest, then it cannot be granted that the whole is moved, except by reason of the other part. Thus when B C, which is one part, is at rest, then A C, which is the other part, is being moved. But no whole of which only one part is moved is moved primarily and per se. Therefore, A B is not moved primarily and per se, which was supposed. Hence, when B C is at rest, the whole of A B must be at rest. And so that which is being moved will stop, that is, will cease to be moved, because of the rest of another. But it has been stated above that if a thing comes to rest and ceases to be moved because of the rest of another, then it is moved by that other. Therefore A B is moved by another.

             And the same reasoning applies to any other mobile object, because everything which is moved is divisible. And for the same reason, when a part is at rest, the whole must be at rest. It is clear, therefore, that whatever is moved is moved by another.

             887. There has been much objection to this proof of Aristotle.

             Galenus objects to Aristotle's statement that if only one part of a mobile object is moved and the others are at rest, then the whole is not moved per se. He says that this is false because things which are moved with respect to a part are moved per se.

             But Galenus was deceived by the equivocation of 'per se'. For 'per se' sometimes is used insofar as it is opposed only to that which is per accidens. And thus that which is moved with respect to a part is moved per se, as Galenus thought. Sometimes, however, it is used insofar as it is opposed both to that which is per accidens and to that which exists in respect to a part. And this is called not only 'per se' but also 'primarily'. And it is thus that Aristotle understands per se here. This is clear because when he concluded that A B is not moved per se, he added that it was agreed that it is moved by itself primarily.

             888. But of greater urgency is the objection of Avicenna. He says that this argument proceeds from an impossible supposition, from which an impossible conclusion follows, and not from that which was given; namely, that a thing is moved by itself. For if we grant that a mobile object is moved primarily and per se, it is natural for it to be moved with respect to both the whole and the parts. If, therefore, it is granted that some part of it is at rest, there will be an impossible position. And from this position there follows the impossibility which Aristotle brings out; namely, that the whole would not be moved primarily and per se, as was given.

             One might answer this objection, however, by saying that although it is impossible for a part to be at rest with respect to a determinate nature, insofar as it is a body of a certain species, for example, the heavens or fire, nevertheless, this is not impossible if the common nature [ratio] of body is considered. For a body, insofar as it is a body, is not prevented from being at rest or in motion.

             But Avicenna rejects this answer in two ways. First because with equal reason it can be said that a whole body is not prevented from being at rest because it is a body, just as was said of a part. And so for the proof of the proposition it was superfluous to assume the division of the mobile object and the rest of the part. The second reason is that a proposition is simply reduced to impossibility if the predicate is repugnant to the subject by reason of the specific difference, even though it is not repugnant to it by reason of its genus. For it is impossible for a man to be irrational even though he is not prevented from being irrational because he is an animal. So, therefore, it is simply impossible for a part of a moving body to be itself at rest. For this is contrary to the nature [ratio] of such a body, although this is not contrary to the common nature [ratio] of body.

             889. Therefore, rejecting this answer, Averroes answers otherwise. He says that a condition can be true when the antecedent is impossible and the consequent is impossible. For example, if man is an ass, he is an irrational animal. It must be granted, therefore, that if a mobile object moves itself, it is impossible for either the whole or a part to be at rest, just as it is impossible that fire is not hot because it is the cause of its own heat. Hence this condition is true: if a part of a mobile object which moves itself is at rest, then the whole is at rest. Moreover, if Aristotle's words are weighed carefully, he never uses the rest of a part except in an expression having the force of a conditional proposition. For he does not say, 'B C is at rest.' Rather he says, 'If B C is at rest, then A B must be at rest'; and again, 'When a part is at rest, the whole is at rest.' And from this true condition Aristotle demonstrates the proposition.

             But Averroes says that this demonstration is not a simple demonstration. Rather it is a demonstration signi or demonstration quia in which such conditions are used.

             However, this answer is reliable insofar as Averroes speaks about the truth of the condition. But it seems that it must be said that it is not a demonstration quia, but propter quid. For it contains the reason why it is impossible for a mobile object to move itself.

             To see this it must be understood that a thing's moving of itself is nothing other than its being the cause of its own motion. That which is itself the cause of something must primarily agree with it. For that which is first in any genus is the cause of the things which come afterward. Thus fire, which is the cause of heat for itself and for others, is the first hot thing. However, Aristotle has shown in Book VI that there is no first in motion or in time or in magnitude or in the mobile object because of their divisibility. Therefore, there cannot be discovered a first whose motion does not depend on something prior. For the motion of a whole depends on the motion of its parts and is divided into them, as was proven in Book VI. Therefore, Aristotle thus shows the reason why no mobile object moves itself. For there cannot be a first mobile object whose motion does not depend on its parts; just as if I were to show that a divisible thing cannot be the first being because the being of whatever is divisible depends on its parts. And thus this condition is true: 'if a part is not moved, the whole is not moved', just as this condition is true: 'if a part is not, the whole is not'.

             890. Hence even the Platonists, who held that things move themselves, said that nothing corporeal or divisible moves itself. Rather, to move itself is limited to spiritual substance which knows itself and loves itself. Without exception all operations are called motions. Even Aristotle in De Anima, III, says that sensation and understanding are motions insofar as motion is the act of the perfect. But here he is speaking of motion as the act of the imperfect, that is, the act of that which exists in potency. In respect to this motion the indivisible is not moved, as was proven in Book VI and as is here assumed. And thus it is clear that Aristotle, who holds that whatever is moved is moved by another, does not differ in meaning but only in words from Plato, who holds that some things move themselves.