In the following essay I briefly explicate the definition of motion as given by Aristotle. This is an excerpt from a larger work that has been modified to stand on its own. This will be the first in a series of articles that is concerned with motion. In particular we will discuss inertia and its validity as a principle, especially in light of the definition of motion and the principle, "omnia quod movetur ab alio movetur"

Defining motion poses a daunting task for any thinker. This is for two reasons: First, motion is not very much of a being. As will be shown below, it has only a kind of potential, or imperfect being and this is the the reason that Aristotle notes that “it is difficult to grasp what motion is.”i . Second, it is very much a principle of our coming to understand nature, and as such, it is one of those things that is more known by nature, and less known to us. On account of these difficulties, definitions tend to fail in one of two ways. They end up being circular, such as Descartes' and Newton ii after him, or they end up neglecting the continuity of motion as notably done by Bertrand Russell.iii Aristotle's definition is the only definition that obviate these difficulties and comes to the truth. As St. Thomas says, “It is altogether impossible to define motion through what is prior and more known accept as the philosopher defines it here.”*
It is nonetheless true that Aristotle's definition of motion, “the actuality of what exists in potency, as such,”iv provides its own challenges. Prima facie, this definition seems incomprehensible. Indeed, Descartes remarked famously about this passage, “Who understands theses words?”v In addition to the difficulties that we are confronted with above, Aristotle's definition is hard because he is defining motion by what is prior to motion. While it seems reasonable enough that we must define motion in terms of something prior to motion, it challenges us, because there does not seem to be motion in the definition. This difficulty can be surmounted, only if we are able to understand how act and potency are prior to motion.
One final note about Aristotle's definition of motion before we proceed: he is not only defining locomotion, that is, motion from place to place. He certainly sees this as the paradigm of motion and will even say that it is at the base of all other kinds of motion, but he will consider growth and diminution, alteration, and locomotion as kinds of motion. Therefore he will speak of motion in very general terms, which is at once an excellence of his account, and a challenge to we who would understand him.
When beginning his inquiry into the definition of motion Aristotle lays out those things that “are concomitant to motion.”vi This should make us realize that a complete understanding of what motion is, will be impossible until we have considered the continuous, the infinite, place, void, and time.vii Having given us this context Aristotle makes a division between potency and act. “There is, then, something which is only in actuality and something in potency and actuality.”viii This is the first of three divisions that he makes in his search for the definition of motion. As Glen Coughlin points out in his appendix on the definition of motion, it is important to note what is meant by potential.ix He is dividing act from potency and for this division to be helpful, then there must be things that stand in opposition to one another. For instance, it does not aid our inquiry to say that I have the potency to stand because I am standing.x Therefore, we must understand potency together with privation.
Aristotle proceeds to explain the basis for relations as excess and defect, action and passion, the mover and the mobile.xi He leaves this aside at this time, but he will return to this consideration when discussing the relation between the mover and the moved. We shall also take this up below.
The second division is made when Aristotle states, “motion is not beyond things.”xii That is to say, that it is not beyond any of the categories. The final distinction points out that in every genus there is a being and privation of that being.xiii
We see from the first division that motion must belong to what can be in potency and act. This is clear if we consider that motion is a kind of change, which is obvious from experience. What does not have potency (and this is potency in the sense of privation, as was said) is unable to change, for there is, as it were, no where for it to go. Therefore, motion must pertain to what can be in potency and act.
The second distinction, that motion belongs to the categories must be understood in two ways. First, that motion cannot be some super genus since “one can grasp nothing common.”xiv Second, we must understand that motion is not some eleventh category since there is “nothing beyond the things named.”xv We may therefore conclude from this that motion is going to be understood in the context of a genus.
The third distinction concerning the being and privation in each genus, indicates where to look for motion in a genus. That is, it is going to be somewhere between the being and privation, the white and the black or the perfect and imperfect, etc. It must be clear that when Aristotle says that “the species of motion and change are as many as are those of being,” it does not mean that there is motion in every genus, as he will later deny, but he is claiming that as many categories as admit of motion, so many will the species of motion be. There will not be one kind of motion for multiple categories.
From the above we may conclude that motion belongs to those things that are in act and potency in some genus, according to the mode of that genus. We must take this together with our common experience of motion. We know motion as that which is between its two ends, “that from which” and “that towards which.” For we see the apple moving from green to red, or the child growing to his full height when a man, or the cup being here, and then being there. Such is out experience of motion.
It is worthwhile to point out that to a large extent our current question focuses on this claim precisely, that is, must motion have an end. For now, it is important to see what we experience simply of motion, such as in the examples given. Moreover, even inertial motion must be defined in terms of non-motion. Finally, inertial motion is certainly less known than the above examples and is consequently not profitable to examine at this point. We must proceed from what is more known to what is less known.xvi Therefore we will begin here and see if we need to clarify our definition later.
What we know about motion at this point is its ends. For I know that the mobile is here or there, but when it is here or there I do not say that it is in motion. Therefore, since we know the ends of motion and we must always proceed from what is more known to the less known, then we must define motion in terms of its ends. For the sake of generality we will names these ends, “that from which” and “that towards which”
The end “from which” is what the mobile posses now, before it has gone somewhere. It is the end that is held in act. The end towards which is what is held in potency, the mobile is not there. We also see that there are any number of points in between these two at which the mobile could be said to be if its motion did not continue on. The mobile has no intrinsic order away or towards either end if it comes to rest at one of these points. If I get up and walk to the doorway and stop, I do not belong to either room more than the other. It is only in motion that I have an order towards or away from some point.
From these considerations we see that motion is a kind of potency, since it does not posses its end. We see also that there is a kind of actuality to motion. The points at which the motion could stop, are closer to the end, so long as we take the order provided by the motion, than the point from which the motion began. Therefore, as long as it is moving it is becoming the end towards which.
Now, the end that towards which is what the mobile holds in potency, before and during the motion. Once the mobile holds the end towards which in act the motion is completed. However, we may still define this in terms of the end from which as the end from which. In order to do this in the language of act and potency, we say that mobile at the end “towards which” is the actuality of the potency. Or in a parallel formulation to Aristotle's definition, it is the actuality of what existed in potency.
So far, we are still speaking of the ends and not of what lies between them. We therefore want to understand the end “towards which” as it exists in potency, not as it existed . Therefore we must understand that potency insofar as it is still in potency. This is our definition: the actuality of what exists in potency, as it is in potency. As Aristotle phrases it, “the actuality of what exists in potency, as such, is motion.”xvii
We have therefore defined motion. The definition is good insofar as it is in terms of what is prior and what is more known. Indeed St. Thomas claims that it is wholly impossible to define motion in another way, by what is prior and more known, than as the philosopher defines it here.xvii This does not mean that all of our difficulties are resolved. However, we have perhaps narrowed down what the difficulties are. Aristotle's approach seems reasonable enough and he is certainly proceeding from common experience. Our question then must turn towards inertial motion. We must see how inertial motion influences our definition. The definition can change by expanding, and going beyond what have said, certainly. However, any sort of claim made by inertial motion which contradicts what has been said, should straight away be seen as an error, for this would be to contradict our experience. The other logical possibility is that inertial motion is not motion, but this would be strange. We speak about these things now, in a vague and indistinct way. For now, let what has been said suffice for our present consideration, and we shall take up these further difficulties later.


xviii omnino impossibile est aliter definire motum per priora et notiora, nisi sicut philosophus hic definit. (In Physicam, Lib. III, lectio 2)