LECTURE 2 (201 a 9-b 5)

THE DEFINITION OF MOTION

             283. Having set forth certain things which are necessary for investigating the definition of motion, the Philosopher here defines motion.

             First he defines it generally, and secondly, specifically, where he says, 'What then motion is . . .' (202 b 24).

             Concerning the first part he makes two points. First he shows what motion is. Secondly, where he says, 'The mover, too . . .' (202 a 2), he asks whether motion is the act of the mover or of the mobile object.

             Concerning the first part he makes three points. First he sets forth the definition of motion. Secondly, where he says, 'Examples will elucidate . . .' (201 a 15), he explains the parts of the definition. Thirdly, where he says, 'Further it is evident . . .' (201 b 6), he shows that the definition has been well formulated.

             Concerning the first part he makes two points. First he sets forth the definition of motion. Secondly, where he says, '. . . namely, of what . . .' (201 a 11), he gives examples.

             284. Concerning the first part it must be noted that some would define motion by saying that motion is a passage from potency to act which is not sudden. These thinkers have erred in formulating the definition because they have placed in the definition of motion certain things which are posterior to motion. For a 'passage' is a certain species of motion. Moreover 'sudden' has time in its definition, for that is sudden which happens in an indivisible time. But time is defined by motion.

             285. And so it is altogether impossible to define motion by what is prior and better known other than as the Philosopher here defines it.

             For it has been said that each genus is divided by potency and act. Now since potency and act pertain to the first differences in being, they are naturally prior to motion. And the Philosopher uses these to define motion.

             It must be noted, therefore, that to be only in act is one thing, to be only in potency is another thing, and to be a mean between potency and act is a third thing. That, then, which is in potency only is not yet moved. That which is in perfect act is not moved, but has already been moved. That, therefore, is moved which is a mean between pure potency and act, which is, indeed, partly in potency and partly in act, as is clear in alteration. For when water is hot only in potency, it is not yet moved. On the other hand, when it is already heated, the motion of heating is finished. But when it participates in something of heat although imperfectly, then it is being moved to heat. For that which is becoming hot gradually participates in heat more and more. Therefore, motion is that imperfect act of heat existing in the heatable, not, indeed, insofar as it is only in act, but insofar as already existing in act it has an ordination to further act. For if this ordination to further act were removed, that act [which it already has], however imperfect, would be the end of the motion and not the motion, as happens when something is partially heated. Furthermore, an ordination to further act belongs to that which exists in potency to it.

             Likewise, if the imperfect act were considered only in its ordination to further act, insofar as it has the nature [ratio] of a potency, it would not have the nature [ratio] of motion, but of a principle of motion. For heating can begin from the tepid as well as from the cold.

             Hence, motion is neither the potency of that which exists in potency, nor the act of that which exists in act. Rather motion is the act of that which exists in potency, such that its ordination to its prior potency is designated by what is called 'act', and its ordination to further act is designated by what is called 'existing in potency'.

             Hence, the Philosopher has defined motion most adequately by saying that motion is the entelechy, i.e., the act, of that which exists in potency insofar as it is such.

             286. Next where he says, '. . . namely, of what . . .' (201 a 11), he exemplifies this in all the species of motion. Thus, alteration is the act of the alterable insofar as it is alterable.

             Since motions in quantity and in substance do not have one name, as motion in quality is called alteration, he sets forth two names for motion in quantity. He says that the act of that which can increase and its opposite, i.e., that which can diminish (for which there is no one common name), is increase and decrease. In like manner generation and corruption [is the act] of the generable and the corruptible, and change of place [is the act] of that which can change in place.

             Motion is taken here commonly to mean mutation, and not in the strict sense in which motion is distinguished from generation and corruption, as will be said in Book V.

             287. Next where he says, 'Examples will elucidate . . .' (201 a 15), he explains the individual parts of the definition. He does this first insofar as motion is called an 'act'. Secondly, where he says, 'The same thing . . .' (201 a 19), he explains the meaning of 'that which exists in potency'. Thirdly, where he says, 'What I mean . . .' (201 a 29), he explains the meaning of 'insofar as it is such'.

             With reference to the first part he uses the following argument. That by which something which previously existed in potency comes to be in act is act. But a thing comes to be in act while it is being moved, whereas prior to this, it was existing in potency. Therefore, motion is an act.

             Hence he says that from this it is obvious that motion is an act. For he says that the 'buildable' is a potency for something. However, when the buildable with respect to this potency which it implies is being reduced to act, then we say it is being built. And this act is passive building.

             And it is the same with all other motions, such as learning, healing, rolling, dancing, maturing, i.e., growing, and ageing, i.e., decreasing.

             For it must be noted that before a thing is moved, it is in potency to two acts, namely, a perfect act which is the end of the motion, and an imperfect act which is motion. Thus, before water begins to be heated, it is in potency to being heated and to being hot. When it is being heated, it is reduced to the imperfect act, which is motion. However it is not yet in perfect act, which is the end of motion. Rather with respect to this, it remains in potency.

             288. Next where he says, 'The same thing . . .' (201 a 19), he shows with the following argument that motion is the act of that which exists in potency.

             Every act is the proper act of that in which it is always found. Thus, light is found only in the diaphanous, and because of this it is the act of the diaphanous. But motion is always found in that which exists in potency. Therefore, motion is the act of that which exists in potency.

             In order to clarify the second proposition, he says that the same thing is in potency and in act, although not at the same time, or in the same respect, as when a thing is hot in potency and cold in act. Hence, it follows from this that many things act upon and are acted upon by each other insofar as each is in potency and in act with reference to the other in different respects. And because every inferior natural body shares in matter, then in each there is a potency for that which is in act in another. And so in all such cases a thing both acts and is acted upon, moves and is moved.

             And for this reason it seemed to some that absolutely every mover is moved. But this point will be examined more fully in other places. For it will be shown in Book VIII of this work and in Metaphysics, XII, that there is a certain mover which is immobile, because it is not in potency but is only in act.

             But when that which is in potency, while existing in act in some way, acts either upon itself or upon something else insofar as it is mobile (i.e., when the moved is reduced to act, whether it be moved by itself or by another), then the motion is its act. Hence things which are in potency are moved, whether they act or are acted upon. For by acting they are acted upon, and by moving they are moved. Thus, when fire acts upon wood, it is acted upon insofar as it is encompassed by smoke, for flames are nothing but burning smoke.

             289. Next where he says, 'What I mean . . .' (201 a 29), he explains the meaning of 'insofar as it is such'. He does this first with an example, and secondly, with an argument, where he says, 'This is obvious in contraries . . .' (201 a 34).

             He says, therefore, first that it was necessary to add 'insofar as it is such' because that which is in potency is also something in act. And although the same thing exists both in potency and in act, to be in potency and to be in act are not the same according to nature [ratio]. Thus, bronze is in potency to statue and is bronze in act, yet the nature [ratio] of bronze insofar as it is bronze and insofar as it is a potency for a statue is not the same. Hence motion is not the act of bronze insofar as it is bronze, but insofar as it is in potency to statue. Otherwise, as long as the bronze would exist, it would be moved. And this is clearly false. Hence it is clear that 'insofar as it is such' is aptly added.

             290. Next where he says, 'This is obvious in contraries . . .' (201 a 34), he explains the same thing with an argument taken from the contraries.

             It is clear that the same subject is in potency to contraries, as humour or blood is the same subject which is potentially related to health and sickness. And it is also clear that to be in potency to health and to be in potency to sickness are two different things (I say this according to their ordination to their objects). Otherwise, if to be able to be ill and to be able to be well were the same, it would follow that to be ill and to be healthy would be the same. Therefore, to be able to be ill and to be able to be healthy differ in nature [ratio], but their subject is one and the same.

             It is clear, therefore, that the nature [ratio] of the subject insofar as it is a certain being and insofar as it is a potency to another is not the same. Otherwise potency to contraries would be one according to nature [ratio]. So also colour and the visible are not the same according to nature [ratio].

             Therefore it was necessary to say that motion is the act of the possible insofar as it is possible, lest it be understood to be the act of that which is in potency insofar as it is a certain subject.