Authors/Thomas Aquinas/physics/L3/lect2

Lecture 2 Definition of Motion

Latin	English
	Lecture 2 Definition of Motion
lib. 3 l. 2 n. 1 Postquam philosophus praemisit quaedam, quae sunt necessaria ad inquisitionem definitionis motus, hic definit motum: et primo in generali; secundo in speciali, ibi: quid quidem igitur motus et cetera. Circa primum duo facit: primo ostendit quid sit motus; secundo inquirit cuius actus sit motus, utrum moventis aut mobilis, ibi: movetur autem movens et cetera. Circa primum tria facit: primo ponit definitionem motus; secundo manifestat partes definitionis, ibi: quod autem hoc sit motus etc.; tertio ostendit definitionem esse bene assignatam, ibi: quod quidem igitur hoc sit et cetera. Circa primum duo facit: primo ponit definitionem motus; secundo exemplificat, ibi: ut alterabilis quidem et cetera.	283. After first setting down certain things necessary for investigating the definition of motion, the Philosopher now defines motion: In general; More specifically, at 325. With regard to the first, he does two things: He shows what motion is; He inquires whether motion belongs to the mover or to the mobile thing, at 299. As to the first of these, he does three things: He gives the definition of motion; He explains the parts of the definition, at 287. He shows that it is a good definition, at 291. As to the first, he does two things: He gives the definition of motion; He gives examples, at 286.
lib. 3 l. 2 n. 2 Circa primum sciendum est, quod aliqui definierunt motum dicentes, quod motus est exitus de potentia in actum non subito. Qui in definiendo errasse inveniuntur, eo quod in definitione motus posuerunt quaedam quae sunt posteriora motu: exitus enim est quaedam species motus; subitum etiam in sua definitione recipit tempus: est enim subitum, quod fit in indivisibili temporis; tempus autem definitur per motum.	284. As to the first, one must understand that some have defined motion by saying that motion is “a going-out from potency to act which is not sudden.” But they are found to be in error, because they have placed in the definition certain elements that are posterior to motion: for a “going-out” is a species of motion; “sudden,” likewise, involves time in its definition—the “sudden” is that which occurs in the indivisible of time [i.e., the instant]; time, however, is defined in terms of motion.
lib. 3 l. 2 n. 3 Et ideo omnino impossibile est aliter definire motum per priora et notiora, nisi sicut philosophus hic definit. Dictum est enim quod unumquodque genus dividitur per potentiam et actum. Potentia autem et actus, cum sint de primis differentiis entis, naturaliter priora sunt motu: et his utitur philosophus ad definiendum motum. Considerandum est igitur quod aliquid est in actu tantum, aliquid vero in potentia tantum, aliquid vero medio modo se habens inter potentiam et actum. Quod igitur est in potentia tantum, nondum movetur: quod autem iam est in actu perfecto, non movetur, sed iam motum est: illud igitur movetur, quod medio modo se habet inter puram potentiam et actum, quod quidem partim est in potentia et partim in actu; ut patet in alteratione. Cum enim aqua est solum in potentia calida, nondum movetur: cum vero est iam calefacta, terminatus est motus calefactionis: cum vero iam participat aliquid de calore sed imperfecte, tunc movetur ad calorem; nam quod calefit, paulatim participat calorem magis ac magis. Ipse igitur actus imperfectus caloris in calefactibili existens, est motus: non quidem secundum id quod actu tantum est, sed secundum quod iam in actu existens habet ordinem in ulteriorem actum; quia si tolleretur ordo ad ulteriorem actum, ipse actus quantumcumque imperfectus, esset terminus motus et non motus, sicut accidit cum aliquid semiplene calefit. Ordo autem ad ulteriorem actum competit existenti in potentia ad ipsum. Et similiter, si actus imperfectus consideretur tantum ut in ordine ad ulteriorem actum, secundum quod habet rationem potentiae, non habet rationem motus, sed principii motus: potest enim incipere calefactio sicut a frigido, ita et a tepido. Sic igitur actus imperfectus habet rationem motus, et secundum quod comparatur ad ulteriorem actum ut potentia, et secundum quod comparatur ad aliquid imperfectius ut actus. Unde neque est potentia existentis in potentia, neque est actus existentis in actu, sed est actus existentis in potentia: ut per id quod dicitur actus, designetur ordo eius ad anteriorem potentiam, et per id quod dicitur in potentia existentis, designetur ordo eius ad ulteriorem actum. Unde convenientissime philosophus definit motum, dicens quod motus est entelechia, idest actus existentis in potentia secundum quod huiusmodi.	285. Consequently it is entirely impossible to define motion in terms of what is prior and better known otherwise than the Philosopher here does. For it has been pointed out already that every genus is divided by potency and act. Now potency and act, since they are among the first differences of being, are naturally prior to motion, and it is these that the Philosopher uses to define motion. Consider, therefore, that something is in act only, something is in potency only, something else is midway between potency and act. What is in potency only is not yet being moved; what is already in perfect act is not being moved but has already been moved. Consequently, that is being moved which is midway between pure potency and act, which is partly in potency and partly in act—as is evident in alteration. [or when water is only potentially hot, it is not being moved; when it has now been heated, the motion of heating is finished; but when it possesses “ some heat, through imperfectly, then it is being moved—for whatever is being heated gradually acquires heat step by step. Therefore this imperfect act of heat existing in a heatable object is motion—not, indeed, by reason of what the heatable object has already become, but inasmuch as, being already in act, it has an order to a further act. For should this order to a further act be taken away, the act already present, however, imperfect, would be the term of motion and not motion itself—as happens when something becomes half-heated. This order to a further act belongs to the thing that is in potency to it. Similarly, if the imperfect act were considered solely as ordered to a further act, under its aspect of potency, it would not have the nature of motion but of a principle of motion—for heating can begin from either a cold or a lukewarm object. The imperfect act, therefore, has the character of motion both insofar as is compared, as potency, to a further act, and insofar as it is compared, as act, to something more imperfect. Hence, motion is neither the potency of a thing existing in potency, nor the act of a thing in act, but it is the act of a thing in potency; where the word “act” designates its relation to a prior potency, and the words “of a thing in potency” designates its relation to a further act. Whence the Philosopher most aptly defines motion as the entelechy, i.e., the act, of a thing existing in potency insofar as it is in potency.
lib. 3 l. 2 n. 4 Deinde cum dicit: ut alterabilis quidem etc., exemplificat in omnibus speciebus motus: sicut alteratio est actus alterabilis inquantum est alterabile. Et quia motus in quantitate et in substantia non habent unum nomen, sicut motus in qualitate dicitur alteratio, quantum ad motum in quantitate ponit duo nomina: et dicit quod actus augmentabilis et oppositi, scilicet diminuibilis, quibus non est unum commune nomen, est augmentum et diminutio. Et similiter generabilis et corruptibilis, generatio et corruptio; et mutabilis secundum locum, loci mutatio. Accipit enim hic motum communiter pro mutatione, non autem stricte secundum quod dividitur contra generationem et corruptionem, ut dicetur in quinto.	286. Then [195 201 a11] he gives examples from all the species of motion—as, for example, alteration is the act of the alterable insofar as it is alterable. And because motion in quantity and in substance does not have a single name in the same way as motion in quality is called “alteration,” he gives two different names for the motions in quantity, and says that the act of the increasable, and of its opposite, i.e., the decreasable, for which two there is no common name, is “increase” and “decrease.” Similarly, the acts of the generable and of the corruptible are “generation” and “corruption”; and the act of what is mutable in regard to place is called “change of place.” In this section the Philosopher uses the word “motion” for any kind of change and avoids the strict usage in which “motion” is distinct from “generation” and “corruption,” as will be said in Book V.
lib. 3 l. 2 n. 5 Deinde cum dicit: quod autem hoc sit motus etc., manifestat singulas particulas definitionis: et primo quantum ad hoc quod motus dicitur actus; secundo quantum ad hoc quod dicitur existentis in potentia, ibi: quoniam autem etc.; tertio quantum ad hoc quod additur, inquantum huiusmodi, ibi: dico autem hoc et cetera. Circa primum utitur tali ratione. Id quo aliquid fit actu, prius in potentia existens, est actus; sed aliquid fit actu dum movetur, prius adhuc in potentia existens; ergo motus est actus. Dicit ergo ex hoc manifestum esse quod motus sit hoc, idest actus, quia aedificabile dicit potentiam ad aliquid; cum autem aedificabile secundum hanc potentiam quam importat, reducitur in actum, tunc dicimus quod aedificatur: et iste actus est aedificatio passiva. Et similiter est de omnibus aliis motibus, sicut doctrinatio, medicatio, volutatio, saltatio, adolescentia, idest augmentum, et senectus, idest diminutio. Considerandum est enim quod antequam aliquid moveatur, est in potentia ad duos actus, scilicet ad actum perfectum, qui est terminus motus, et ad actum imperfectum, qui est motus: sicut aqua antequam incipiat calefieri est in potentia ad calefieri et ad calidum esse; cum autem calefit, reducitur in actum imperfectum, qui est motus; nondum autem in actum perfectum, qui est terminus motus, sed adhuc respectu ipsius remanet in potentia.	287. Then [196 201 a15] he explains the several words of the definition: He explains the use of the word “act”; He explains “of a thing existing in potency,” at 288. He explains “insofar as it is such,” at 289. As to the first he uses this reasoning. That by which something previously existing in potency becomes actual is an act. But something becomes actual when it is being moved, although previously it was in potency. Therefore motion is an act. He says therefore that it is plain that motion is an act from the fact that the “buildable” implies a potency to something, but when the “buildable” according to this potency which it implies, is being reduced to act, we then say it is “being built”—and this act is “building” taken passively. And the same thing is true in all other motions such as indoctrination, healing, rolling, jumping, youth (i.e., increase),old age (i.e., decrease). For it must be remembered that before something is being moved it is in potency to two acts: to a perfect act which is the term of the motion, and to an imperfect act which is motion itself. Thus water, before it begins to be heated, is in potency to being heated and to having been heated: when it is being heated it is being reduced to the imperfect act which is motion but not yet to perfect act which is the term of the motion—rather, in respect to this it still remains in potency.
lib. 3 l. 2 n. 6 Deinde cum dicit: quoniam autem quaedam etc., ostendit quod motus sit actus existentis in potentia, tali ratione. Omnis enim actus, eius est proprie actus in quo semper invenitur, sicut lumen nunquam invenitur nisi in diaphano, et propter hoc est actus diaphani. Sed motus semper invenitur in existente in potentia; est igitur motus actus existentis in potentia. Ad manifestationem igitur secundae propositionis, dicit quod quia quaedam eadem sunt et in potentia et in actu, licet non simul aut secundum idem, sicut aliquid est calidum in potentia et frigidum actu; ex hoc sequitur quod multa agunt et patiuntur ad invicem, inquantum scilicet utrumque est in potentia et actu respectu alterius secundum diversa. Et quia omnia corpora naturalia inferiora communicant in materia, ideo in unoquoque est potentia ad id quod est actu in altero: et ideo in omnibus talibus aliquid simul agit et patitur, et movet et movetur. Et ex hac ratione quibusdam visum est quod simpliciter omne movens moveatur: sed de hoc manifestum erit magis in aliis. Ostendetur enim in octavo huius et in XII Metaphys., quod est quoddam movens immobile, quia non est in potentia sed in actu tantum. Sed quando id quod est in potentia, actu quodammodo existens, agit aut ipsum aut aliud inquantum est mobile, idest reducitur in actum motus, sive sit motum a se sive ab alio, tunc est motus actus eius. Et inde est quod illa quae sunt in potentia, sive agant sive patiantur, moventur; quia et agendo patiuntur, et movendo moventur: sicut ignis, cum agit in ligna, patitur inquantum ingrossatur per fumum, quia flamma non est nisi fumus ardens.	288. Then [197 201 a19] he shows that motion is the act “of a thing existing in potency.” For every act is strictly the act of that in which it is always found—as light is never found but in the transparent, for which reason it is the act of the transparent. But motion is found always in a thing existing in potency. Therefore, motion is the act of a thing existing in potency. To explain the second proposition he says that, since certain same things are both in potency and in act, although not at the same time, nor in the same respect—as, for example, something is hot actually and cold potentially it follows that many things mutually act and are acted upon insofar, namely, as both are in potency and in act with respect to the other under different aspects. And because all lower natural bodies share the same matter, there is therefore in each of them a potency to what is actual in another. Hence, in all such bodies something simultaneously acts and is acted upon, both moves and is moved. This fact had led some to say absolutely that every mover is likewise being moved. This point will be cleared up in a later place. For it will be shown in Physics VIII (l.9 ff.) and in Metaphysics XII (l.7) that there exists an immobile mover, since it is not in potency but in act only. But when that which is in potency, yet existing in a certain way in act, either acts itself or is acted upon by another so far as it is movable, i.e., is reduced to the act of motion, whether moved by itself or by another, at such time motion is its act. That is why things in potency, whether they act or are acted upon, are moved, since when acting they are acted upon and when moving they are being moved—just as fire, when it acts on logs, it acted upon, insofar as it becomes more dense through smoke, flame being nothing more than smoke afire.
lib. 3 l. 2 n. 7 Deinde cum dicit: dico autem hoc inquantum etc., manifestat hanc particulam, inquantum huiusmodi: et primo per exemplum; secundo per rationem, ibi: manifestum autem et in contrariis et cetera. Dicit ergo primo quod necessarium fuit addi inquantum huiusmodi, quia id quod est in potentia, est etiam aliquid actu. Et licet idem sit subiectum existens in potentia et in actu, non tamen est idem secundum rationem esse in potentia et esse in actu, sicut aes est in potentia ad statuam et est actu aes, non tamen est eadem ratio aeris inquantum est aes et inquantum est potentia ad statuam. Motus autem non est actus aeris inquantum est aes, sed inquantum est in potentia ad statuam: alias oporteret quod quandiu aes esset, tandiu aes moveretur, quod patet esse falsum. Unde patet convenienter additum esse inquantum huiusmodi.	289. Then [198 201 a29] he explains this part of the definition, “insofar as it is such”: By an example; By giving a reason, at 290. He says therefore first that the phrase, “insofar as it is such,” had to be added, because what is in potency is at the same time something in act. And although the subject which is both in potency and in act may be the same, nevertheless to be in potency and to be in act is not contained under the same notion. Thus, although brass is a statue in potency but is brass actually, nevertheless the notion of the brass as brass is not the same as the notion of the brass as it is in potency to a statue. Now motion is not an act of the brass insofar as it is brass but insofar as it is in potency to a statue; otherwise. during the whole time that it was brass it would be undergoing motion, which is clearly false. That is why it is necessary to add “insofar as it is such.
lib. 3 l. 2 n. 8 Deinde cum dicit: manifestum autem et in contrariis etc., ostendit idem per rationem sumptam a contrariis. Manifestum est enim quod aliquod idem subiectum est in potentia ad contraria, sicut humor aut sanguis est idem subiectum se habens in potentia ad sanitatem et aegritudinem. Manifestum est autem quod esse in potentia ad sanitatem, et esse in potentia ad aegritudinem, est alterum et alterum (et hoc dico secundum ordinem ad obiecta): alioquin si idem esset posse laborare et posse sanari, sequeretur quod laborare et sanari essent idem. Differunt ergo posse laborare et posse sanari secundum rationem, sed subiectum est unum et idem. Patet ergo quod non est eadem ratio subiecti inquantum est quoddam ens, et inquantum est potentia ad aliud: alioquin potentia ad contraria esset una secundum rationem. Et sic etiam non est idem secundum rationem color et visibile. Et ideo necessarium fuit dicere quod motus est actus possibilis inquantum est possibile: ne intelligeretur esse actus eius quod est in potentia, inquantum est quoddam subiectum.	290. Then [199 201 a34] he explains the same thing by using an argument based on the nature of contraries. For it is clear that a given subject is in potency to contraries—as a humor or the blood is in potency to health and to sickness. But to be in potency to health is one thing and to be in potency to sickness is another, if one considers their objects. Otherwise, if to be able to be sick and to be able to be well were the same thing, it then would follow that being sick and being well would be the same. Hence to be able to be sick and to be able to be healthy are different notions, although their actual subject is one and the same thing. It is plain, therefore, that there is not one and the same notion of the subject as it is a certain being, and as it is in potency to something else. Otherwise, potency to contrary things would fall under one and the same notion. In like manner, the notion of that which is “color” and that which is “visible” are not one and the same. Thus it was necessary to say that motion is the act of the possible “insofar as it is possible”—to prevent supposing that it is the act of what is in potency insofar as it is merely some subject.