Jacques Maritain Center : A First Glance at St. Thomas Aquinas / by Ralph McInerny


Since nature is a principle of change or motion, one cannot know what nature is without knowing what motion is. To give an account of motion without introducing motion into the account is difficult. You may think this account has already been given, but this is not so. True, the analysis of the result of becoming and the Parmenidean problem give us the elements we need for defining motion. But that task lies ahead. Let us take it up.

The definition St. Thomas accepts from Aristotle presupposes recognition that being is divided into the potential and the actual as well as into substance and the feature or accidents of substance. The category of relation is of peculiar importance since the moved is related to the mover.

The Definition

A thing can be completely actual, a thing can be only potentially, and a thing can be in between these two extremes. Something in potency alone is not moving and what is completely actual has already moved. The batter standing at home plate could be at first, he is potentially there. Safely on first, he has fully realized that potentiality. On the way to first, the batter is between the state of pure potentiality and that of pure actuality. Motion is the middle state, an imperfect actualization of a potency.

Looking toward home, motion is an incomplete or imperfect realization of the potency. But that alone could cover the runner if he trips and falls on the way to first. So too, if we stress the potentiality still unrealized, looking toward first, this too alone cover the arrested, fallen player. How then put it? Motion is the act of being in potency insofar as it is in potency.

Motion is an act. An act or actuality is correlative to a potency. The full actualization of the potency is the term of the change. Motion is an imperfect act of the potency.

Hence it is not the potency of something existing in potency nor is it the act of something existing in potency, such that by 'act' the order to the previous potency is designated and by 'existing in potency' the order to further act is designated. (Physics III.2.284)

The definition thus avoids invoking motion or an equivalent in defining motion, but employs the contrast between being able to be something and actually being something, a contrast everybody understands and can give examples of without hesitation. When we first read the definition, it looks like something very abstruse and difficult to understand. But if we say that motion is the act of a player who can be on first insofar as he can be on first, we can see, as Thomas says, that 'act' picks out the fact that the ability he had at home plate is no longer just pure potentiality, and 'insofar as he can be' indicates that the one whose act motion is has motion insofar as he still is in potency to be on first.

You may still object that you know perfectly well what motion is but this account of it seems very obscure. It is true that anyone and everyone recognizes that change occurs, knows that motion is, but this is not to be clear as to what it is. It is because we can easily get clear as to what potential and actual are that we can use them to express what motion is.

It is because motion and change so obviously are, because we are unlikely to doubt that they occur, that the difficulty of the account or definition of motion surprises us. The real difficulty lies in the fact that motion is such a fleeting reality, it is imperfect being. The mind more easily seizes on what is actual. No wonder the attempt to capture motion seems somewhat tortured.

But note this well. The definition commends itself in terms of what anybody already knows. That is how to appraise it. No appeal is being made to some special "philosophical" experience. 'Actual' and 'potential' should not be regarded as technical Aristotelian terms. They are no more technical than our ability to see the difference between 'could be' and 'is.'

Motion is found in a number of different categories of being: in quality, when a thing alters in quality; in quantity, when a thing changes in size or weight; in place, when a thing changes location. The change whereby a substance as such come to be or passes away is not a motion. But more on that later.

Thomas formulates this argument on behalf of the definition . "An act is properly the act of that in which it is always found; motion is always found in something existing in potency; so motion is the act of that which exists in potency." It is the second premise he thinks needs comment.

What is actually hot is potentially cold and vice versa. Subjects like this act on one another with changes going in opposite directions. While what is hot and becoming cold is in potency to being cold, what is cold and becoming hot is in potency to being hot. This mutual interaction can be made as general as the cosmos if we say that there is one common or prime matter in every physical thing. With respect to the ultimate subject, any substance which is actually what it is, is potentially any of the other perfections or acts of matter.

Change thus looks reciprocal. What changes is changed by what it changes. The warming agent is cooled and cooling agent is warmed. Does this mean that every mover is moved? That this cannot be universally true, that there must be some mover that is not itself moved, is one of the crucial claims of Aristotle's Physics. This is his proof of the Prime Mover.

Whose Motion?

The recognition of reciprocal changing indicates the connection between this discussion and that on causes. Motion is the act of the moved, of that which is in potency as such, but we must relate to the mover as well.

Whatever at one time is only potentially and later is actual has changed. But a thing is sometimes a mover only potentially and later actually moves. Such a mover, then , is the subject of change or motion.

Whatever can be said to be at rest can be in motion, because motion and rest are opposites. But when something stops moving something, it is said to be at rest. So the mover must be subject to motion. But motion or change does not belong to the mover as mover, but rather because it moves by contact with the moved. The mover is not moved insofar as it is a mover, but insofar as it is also movable. It just happens that the mover is also moved. The mover is not moved insofar as it is a mover, but the moved is the subject of motion as such, per se.

Motion is from the mover and in the moved, speaking per se.

These clarifications turn out to be of crucial importance for the philosophical outlook that Thomas Aquinas adopts from Aristotle. If motion is the act of the moved, it requires a mover. Whatever is moved is moved by another. Is every mover in turn moved? If there is required an unmoved mover, and if moved movers are such because of sharing a matter with their effects, the unmoved mover will be free of matter.

That further importance of the definition of motion justifies our devoting time to it here.


The Definition of Motion

[3] It has been pointed out that every genus is divided by act and potency. Since they are among the primary differences of being, act and potency are naturally prior to motion and the Philosopher uses them in order to define motion.

It should be considered that something is in act alone or something is in potency alone or it is midway between potency and act. What is in potency alone does not yet move; what is in complete act is not moved by has been moved. Therefore that which moves is midway between pure potency and act, and is indeed partly potential and partly actual, as is clear from alteration. For when water is only potentially hot, it is not yet moved; when it has been heated, its being heated is done, but when it has something of heat, though only imperfectly, than it is moved toward heat; for what is being heated little by little has more and more heat. The imperfect act of heat existing in the heatable is motion, not indeed insofar as it is act alone, but insofar as, according to the act it already has it is ordered to further act. If this order to further act were taken away, however imperfect its actuality; it would be the term of motion, and not motion, as happens when something is lukewarm. The order to further act belongs to that which is in potency to it.

Similarly, if the imperfect act were considered only as ordered to further act, and it compares to that further act as potency, it would not have the mark of motion but of the beginning of motion. Heating can begin from the lukewarm as well as from the cold.

Thus it is that imperfect act has the mark of motion, both insofar as it compares to further act as potency and insofar as it compares to something less perfect as act.

Therefore it is neither a potency of something existing in potency nor the act of something existing in act, but the act of something existing in potency. By 'act' the order to the previous potency is designated, and by 'of something existing in potency' the order to further act.

The Philosopher thus defines motion most properly when he says that motion is the entelechy, that is the act of something existing in potency insofar as it exists in potency.

-- Commentary on 'Physics' Book Three, lesson 2

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