LECTURE 1 (200 b 12-201 a 8)

NATURAL SCIENCE TREATS MOTION AND THOSE THINGS WHICH ARE CONSEQUENT UPON MOTION. CERTAIN DIVISIONS WHICH ARE NECESSARY FOR THE INVESTIGATION OF THE DEFINITION OF MOTION

             275. Having treated the principles of natural things [Book I] and the principles of this science [Book II], he begins here to pursue his intention by treating the subject of this science, which is mobile being simply.

             This discussion is divided into two parts. First he treats motion in itself [Books III-IV-V-VI]. Secondly, where he says, 'Everything that is in motion . . .' (241 b 24), he treats motion in relation to movers and mobile objects [Books VII].

             The first part is divided into two parts. First he treats motion itself [Books III-IV]. Secondly where he says, 'Everything which changes . . .' (224 a 21), he treats the parts of motion [Book V].

             Concerning the first part he makes two points. First he states his intention. Secondly, where he says, 'We may start . . .' (200 b 26), he develops his position.

             Concerning the first part he makes two points. First he states his main intention. Secondly, where he says, 'When we have determined . . .' (200 b 15), he points out certain related things which he intends to treat as consequences.

             276. Concerning the first part he uses the following argument. Nature is a principle of motion and mutation, as is clear from the definition given in Book II. (How motion and mutation differ will be explained in Book V. Thus it is clear that if one does not know motion, one does not know nature, since motion is placed in the definition of nature. Since, then, we intend to set forth the science of nature, it is necessary to know about motion.

             277. Next where he says, 'When we have determined . . .' (200 b 15), he adds certain things which are concomitant with motion. He uses two arguments, the first of which is as follows.

             Whoever treats a certain thing must treat those things which are consequent upon it. For a subject and its accidents are considered in one science.

             But the infinite follows upon motion intrinsically. This is clear from the following.

             Motion is among the things which are continuous, as will be explained in Book VI. But the infinite falls within the definition of the continuous.

             And he adds 'first', because the infinite which is found in the addition of number is caused by the infinite which is found in the division of the continuous. He shows that the infinite falls in the definition of the continuous. For those who define the continuous often use the infinite, inasmuch as they say that the continuous is that which is infinitely divisible.

             He says 'often' because there is another definition of the continuous which is set forth in the Categories, i.e., a continuum is that whose parts are joined at one common boundary.

             These two definitions differ. For since a continuum is a certain whole, it is defined by its parts. But parts have a twofold relation to the whole, one according to composition, insofar as the whole is composed of parts, and secondly, according to resolution, insofar as the whole is divided into parts.

             Therefore, this definition of the continuous is given by way of resolution. The definition given in the Categories is by way of composition.

             Therefore, it is clear that the infinite follows upon motion intrinsically.

             Certain other things, however, follow upon motion extrinsically as external measures, for example, place and the void and time.

             For time is the measure of motion itself. Place is a measure of the mobile object according to truth. And the void is a measure according to the opinion of some. Thus he adds that there can be no motion apart from place, the void and time.

             Nor does it matter that not all motion is local motion. For nothing is moved unless it exists in place. For every sensible body is in place, and only sensible bodies are moved.

             Local motion is also the first motion, and when it is taken away, so also are the others, as will be made clear below in Book VIII.

             Therefore, it is clear that the four things mentioned above follow upon motion. Hence they pertain to the consideration of the natural philosopher for the reason [ratio] already given.

             278. [These things pertain to the consideration of the natural philosopher] for another reason which he subsequently adds, namely, the above mentioned things are common to all natural things.

             Thus in natural science all natural things must be treated. Now certain of these things must be treated first. For speculation about what is proper is posterior to speculation about what is common, as was said in the beginning. And among these common things motion must be treated first, because the others are consequent upon it, as was said.

             279. Next where he says, 'We may start . . .' (200 b 26), he pursues what he has proposed.

             First he treats motion and the infinite, which follows upon motion intrinsically. Secondly, where he says, 'The physicist must have a knowledge . . .' (208 a 27), [Book IV], he treats the other three things, which follow upon motion extrinsically.

             The first part is divided into two parts. First he treats motion and secondly, the infinite, where he says, 'The science of nature is concerned . . .' (202 b 30).

             Concerning the first part he makes two points. First he sets forth certain things which are necessary to investigate the definition of motion. Secondly, where he says, 'We have now . . .' (201 a 9), he defines motion.

             Concerning the first part he makes two points. First he sets forth certain divisions. For the most convenient way of discovering a definition is by division, as is clear from what the Philosopher says in Posterior Analytics, II, and in Metaphysics, VII. Secondly, where he says, 'Again there is no such thing . . .' (200 b 32), he shows that motion falls within the above mentioned divisions.

             280. Concerning the first part he sets forth three divisions. The first is that being is divided by potency and act. This division does not distinguish the genera of beings, for potency and act are found in every genus.

             The second division is that being is divided into the ten genera, of which one is 'a this', i.e., substance, another 'quantity', or 'quality', or one of the other predicaments.

             The third division pertains to one of the genera of beings, namely, relation. For motion seems in some way to belong to this genus, insofar as the mover is referred to the mobile object.

             In order to understand this third division, it must be noted that, since relation has the weakest existence because it consists only in being related to another, it is necessary for a relation to be grounded upon some other accident. For the more perfect accidents are closer to substance, and through their mediation the other accidents are in substance.

             Now relation is primarily founded upon two things which have an ordination to another, namely, quantity and action. For quantity can be a measure of something external, and an agent pours out its action upon another.

             Some relations, therefore, are grounded upon quantity, and especially upon number, to which belongs the first nature [ratio] of measure, as is clear in the double and the half, the multiplied and the divided, and other such things. Moreover the identical and the like and the equal are founded upon unity, which is the principle of number.

             Other relations, however, are founded upon action and passion, either according to the act itself, as heating is referred to the heated, or according to that which has acted, as a father is referred to a son because he has begotten him, or according to the power of the agent, as a master is referred to a servant because he can compel him.

             The Philosopher explains this division clearly in Metaphysics, V. Here he touches upon it briefly, saying that one type of relation is according to excess and defect, and this is founded upon quantity, such as the double and the half. The other type is according to the active and the passive, and the mover and the mobile object, which are referred to each other, as is clear in itself.

             281. Next where he says, 'Again, there is no such thing . . .' (200 b 32), he shows how motion is reduced to the above mentioned divisions.

             Concerning this he makes two points. First he shows that there is no motion outside of the genera of things in which motion occurs. Secondly, where he says, 'Now each of these . . .' (201 a 4), he shows that motion is divided as the genera of things are divided.

             Concerning the first part it must be noted that since motion, as he will explain below, is an imperfect act, and since whatever is imperfect falls under the same genus as the perfect (not, indeed, according to species, but by reduction, as primary matter is in the genus of substance), there cannot be any motion outside of the genera of things in which motion occurs. This is what he means when he says that motion is not 'over and above the things', that is over and above the genera of things in which there is motion, as if it were something extraneous or something common to these genera.

             He makes this clear by the fact that everything that is changed is changed according to substance, or according to quantity, or according to quality, or according to place, as will be shown in Book V.

             Moreover, in these genera there is no common univocal thing which would be their genus and which would not be contained under some predicament. Rather being is common to them by analogy, as will be shown in Metaphysics, IV. Hence it is clear that there is no motion or mutation outside the above mentioned genera. For there is nothing beyond these genera, since they divide being sufficiently well. He will show below how motion is related to the predicament of action or passion.

             282. Next where he says, 'Now each of these . . .' (201 a 4), he shows that motion is divided as the genera of things are divided.

             It is clear that in each genus a thing can be in two ways, either as perfect or as imperfect. The reason [ratio] for this is that privation and possession are the first contraries, as is said in Metaphysics, X. Therefore, since all the genera are divided by differentiating contraries, there must be in every genus a perfect and an imperfect. Thus in substance, something is as the form, and something as the privation; in quality, something is as the white which is perfect, another is as the black which is, as it were, imperfect; in quantity, something is a perfect quantity, another imperfect; and in place, something is above which is, as it were, perfect, another below which is, as it were, imperfect, or else something is light and heavy, which are placed in the category 'where' by reason [ratio] of their inclination. Hence it is clear that motion is divided in as many ways as being is divided.

             For the species of motion differ according to the different genera of beings. Thus growth, which is motion in quantity, differs from generation, which is motion in substance.

             And the species of motion differ within the same genus according to the perfect and the imperfect. For generation is motion in substance toward form, whereas corruption is motion toward privation. And in quantity, growth is toward perfect quantity, and decrease is toward imperfect quantity. Why he does not designate the two species in quality and in 'where' will be explained in Book V.