Physics Gamma: Motion

Book Alpha (A) had examined the characteristics of physics, the science (or study) of "nature". Book Beta (B) looked at what 'nature' means, and what would be involved in it. There Aristotle noted that all things that exist by nature appear to have in themselves (whatever that means) a principle (source) of motion and of standstill. He then characterized nature as a principle and a cause of being moved, or of standstill (rest) in the things to which it belongs primarily and in virtue of that (i.e. each such) thing, but not accidentally.

The question then arises: what is motion? Physics Gamma looks at this.

It is important to distinguish here between what Aristotle means by the word translated as 'motion' (kinesis) and what he means by the word translated as 'change' (metabole). He does not discuss the difference until Book Epsilon (E), (225a-b, 229b25-230b20), but Greek-speaking people in Aristotle's time would have been aware of the rudiments of the distinction. The basic distinction is this:

- "Change" (metabole) includes "generation" (genesis, coming to be), "destruction" (phthora, perishing, ceasing to be), and "motion" (kinesis). That is, "motion" is a kind of "change".

- "Motion" includes "locomotion" (change of place), "alteration" (change of aspect, e.g. change of color, change of texture, change of mood), "increase" (getting larger), and "decrease" (getting smaller). Only a change from a subject (hupokeimenon, thing laid down) to a subject is a motion. In other words, if it's said that what undergoes a certain change was there before the change, is there during the change, and remains after the change, then that change is a motion. We say that when a piece of wood changes color with age, the wood was there before, during, and after the change. That change would then be called an alteration - a kind of motion. If the wood is left to rot outside and disintegrates into the soil until no wood is left, we would say that the piece of wood no longer exists, that it did not survive the change. That change would be considered a destruction, which would not be a motion. (The chemical components of the wood might still exist, but they would no longer form wood. Thus from the standpoint of the molecules that had formed the wood, there has been a motion - they are no longer bound together. From the standpoint of the wood, there has been a destruction - there is no more wood. Thus the answer to the question of whether a motion, a change of another kind, or no change at all has taken place depends on what you're taking as your hupokeimenon, and in what context you're considering it.)


Book Gamma, Chapters 1 and 2
No motion exists apart from things (200b33). That is, if you say that a motion is going on, you're saying that something is moving. This something does not need to be a material object, for Aristotle; he regards change of mood and the process of learning (becoming conscious of new things) as motions too, for example.


What is motion?
Aristotle characterizes motion as the actuality of the potentially existing qua existing potentially (201a10).

Please do continue to breathe; this is not as unfathomable as it sounds. Let us look at an example of the amplifications he gives: the actuality of the alterable qua alterable is an alteration. If a non-blue thing has the potential to become blue (i.e. to become an existent blue thing), then the actuality of this potential is the going from non-blue to blue. The actuality of the movable with respect to place [qua movable with respect to place] is a locomotion. If something is movable (has the potential to be moved), the actuality of that potential, the actuality of that thing's being movable, is a locomotion from one place to another.

-->If a thing can do something, then motion is the thing's doing what it can.


As suggested by Aristotle's examples, motions will occur according to the "natures" of things (i.e. in virtue of the sources of motion and rest that they may have "in" them). This does not mean that art, luck, and chance play no role; but only that art, luck, and chance involve the "natural" motions of things. For example, if a house is to be "buildable", or if stones are able to be built up into a house, the motion of building the house will on one level of course be a matter of art, but on another level the motion takes place in virtue of the stones' capacity for being moved, which is a matter of "nature". (Also, it could be said that a human has by nature the potential to learn and use arts, so the motion of the building of a house is also the putting-in-action of that capacity, and hence in that sense too a motion.)


-->Rest is the absence of motion in that which can be in motion (Ch. 2). That is, if a thing can do something and is not now doing it, the thing is at rest (with respect to that particular capacity). Numbers, for example, are not at rest when they don't move, because they can't move in the first place. (What will count as absence of motion in a given thing will of course depend on what count as the potentials or capacities the thing is supposed to have. This in turn will depend in part on how the thing is identified or characterized or defined, or on what the thing is said to be. Notice that Aristotle has not laid down any rules for how these identifications and judgments are to be made.)


Chapter 3
Here Aristotle raises a problem concerning cases where one thing is said to act on another. Suppose that thing A is a vacuum cleaner and thing B is dust. When the vacuum cleaner sucks up dust, motion is involved. There are 2 different potentials involved: A can suck things up, and B can get drawn into a vacuum cleaner. When the motion occurs (the vacuum cleaner sucks up the dust), the 2 potentials are getting "actualized", i.e. A and B are doing what they can do. Is the motion then one "actuality", one achieving, or two? We say in one sense that the motion is one event, for "vacuuming" requires a functional vacuum cleaner and something that it is to "vacuum up". But in another sense it is two things, for 2 potentials are getting actualized.

His response to the problem is to say that two things can be involved in one actuality if they are interacting; and that two actualities can belong to one motion. For A's acting on B and B's getting acted upon by A are one motion, but the "formula" (definition, defining characterization, account: logos) of A's acting upon B is different from the "formula" of B's getting acted upon by A - just as the road from Athens to Thebes is the same road as the road from Thebes to Athens, but the direction differs.

What is the point of all of this? First of all, this shows that motions are thought of as occurring within contexts; once things are identified, we don't speak of their motions as occurring without qualification. Also, the stage is set for the question of whether that which moves or acts on something else must also itself move. If we had had to conclude that the "actuality" of A making B move was the same in all respects as the "actuality" of B getting moved by A, we would be forced to infer that all things that cause motion in other must themselves move. That inference may not be correct, or may not be able to be known to be so, as we will see. But we were not forced to make the inference, so we are free to investigate the question of the sources and limits of motion.


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