LECTURE 3 (201 b 6-202 a 2)

THE DEFINITION OF MOTION HAS BEEN WELL FORMULATED

             291. Having set forth the definition of motion, and having explained the individual parts of the definition, he here shows that the definition is well formulated.

             He does this first directly, and secondly, indirectly, where he says, 'The soundness of this . . .' (201 b 16).

             292. With reference to the first part he uses the following argument. Whatever is in potency is at some time in act. But the buildable is in potency. Therefore, there is an act of the buildable insofar as it is buildable. Now this act is either the house or the activity of building. But a house is not the act of the buildable insofar as it is buildable, because the buildable insofar as it is such is reduced to act when it is being built. For when the house already exists, it is not being built. Therefore, it follows that building is the act of the buildable insofar as it is such. Building, however, is a certain motion. Motion, therefore, is the act of that which exists in potency insofar as it is such. And the same argument applies to other motions.

             It is clear, therefore, that motion is an act such as was said, and that a thing is moved when it is in such an act, and neither before nor after. For before, when there is only potency, the motion has not yet begun. And after the motion it has already altogether ceased to be in potency because it is in perfect act.

             293. Next where he says, 'The soundness of this definition . . .' (201 b 16), he shows indirectly that the definition is well formulated. For motion cannot be defined otherwise.

             Concerning this he makes three points. First he states his intention. Secondly, where he says, 'One could not easily . . .' (201 b 19), he sets forth and rejects the definitions of motion given by others. Thirdly, where he says, 'The reason why . . .' (201 b 24), he explains why some have defined motion in this way.

             He says, therefore, first that it is clear that motion has been well defined for two reasons: first, because the definitions by which some have defined motion are absurd; secondly, because of the fact that motion cannot be defined otherwise. The reason [ratio] for this is that motion cannot be placed in any genus other than the genus of the act of that which exists in potency.

             294. Next where he says, 'One could not easily . . .' (201 b 19), he rejects the definitions of motion given by others.

             It must be noted that some have defined motion in three ways.

             For they said that motion is otherness, because of the fact that that which is moved is always different.

             Next they said that motion is inequality, because that which is moved always approaches nearer and nearer to its terminus.

             They also said that motion is that which is not, i.e., non-being. For that which is moved, while it is being moved, does not yet have that to which it is moved. Thus that which is being moved to whiteness is not yet white.

             However the Philosopher rejects these definitions in three ways.

             He does this first in respect to the subject of motion. For if motion were otherness or inequality or non-being, then that in which these things are would necessarily be moved, because that in which motion is is moved. But it is not necessary that things which are other be moved because of the fact that they are other. And the same is true of things which are unequal and of things which are not. Therefore, it follows that otherness and inequality and non-being are not motion.

             Secondly, he shows the same thing in respect to the terminus to which. For motion and mutation are not toward otherness more than toward likeness, nor toward inequality more than toward equality, nor toward non-being more than toward being. For generation is a mutation toward being, and corruption is toward non-being. Therefore, motion is not otherness more than likeness, nor inequality more than equality, nor non-being more than being.

             Thirdly, he shows the same thing in respect to the terminus from which. For as some motions are from otherness and from inequality and from non-being, other motions are from the opposites of these. Therefore, motion should not be placed in these genera rather than in their opposites.

             295. Next where he says, 'The reason why . . .' (201 b 24), he explains why the ancients defined motion in the way mentioned above.

             Concerning this he makes two points. First, he explains what he has said. Secondly, where he says, 'The reason in turn . . .' (201 b 28), he explains a certain point which he had assumed.

             He says, therefore, first that the reason why the ancients held that motion is in the genera mentioned above (i.e., otherness, inequality and non-being) is that motion seems to be something indeterminate, i.e., incomplete and imperfect, not having, as it were, a determinate nature. And because motion is indeterminate it seems that it must be placed in the genus of privatives. For when Pythagoras posited two orders of things, in each of which he placed ten principles, the principles which were in the second order he called indeterminate, because they are privatives. For these things are not determined by a form in the genus of substance, nor by a form of quality, nor by any special form existing in one of these genera, nor even by a form of one of the other predicaments.

             Thus in one order the Pythagoreans placed these ten: the finite, the unequal, the one, the right, the masculine, rest, the straight, light, the good, and the equilateral triangle. In the other order they placed the infinite, the equal, the many, the left, the feminine, motion, the oblique, darkness, evil, and the scalene.

             296. Next where he says, 'The reason in turn . . .' (201 b 28), he explains why motion is placed among the indeterminate things.

             He says that the reason for this is that motion cannot be placed either under potency or under act. For if it were placed under potency, then whatever would be in potency, e.g. to quantity, would be moved according to quantity. And if it were contained under act, then whatever would be a quantity in act would be moved according to quantity. Indeed it is true that motion is an act, but it is an imperfect act, a mean between potency and act. And it is clear that it is an imperfect act because that whose act it is is being in potency, as was said above. Hence it is difficult to grasp what motion is. For it seems at first glance that it is either act simply or potency simply or that it should be contained under privation, as the ancients held that it is contained under non-being or under inequality. But none of these is possible, as was pointed out above. Hence only the above mentioned mode of defining motion remains, namely, motion is an act such as we have said, i.e., the act of that which exists in potency.

             However, it is difficult to understand such an act because of the mixture of act and potency. Nevertheless the existence of such an act is not impossible, but may occur.