Commentary Aristotle Physics
Nevertheless, in all divisibles and indivisibles, what Aristotle says here is true, for even changes that are called indivisible are in a sense divisible, not by reason of their extremities, but by reason of something connected to them. And this is what Averroes wanted to say when he said that it is per accidens that some changes occur in non-time.
801. However, there is here another difficulty. For when it comes to alteration, it does not seem to be true that what is being altered is partly in one terminus and partly in the other during the alteration. For the motion of alteration does not take place in such a way that first one part and then another is altered; rather, the entire thing that was less hot becomes hotter.
For which reason, Aristotle even says in De sensu et sensato that alterations are not like local motions. For in the latter, the subject reaches the intermediate before the goal, but such is not the case with things that are altered; for some things are altered all at once and not part by part, for it is the entire water that all at once freezes.
802. But to this it must be replied that, in this book, Aristotle is treating of motion as continuous. And continuity is primarily and per se and strictly found only in local motion, which alone can be continuous and regular, as will be shown in book 8. Therefore, the demonstrations given in book 6 pertain completely to local motion but incompletely to other motions, that is, only to the extent that they are continuous and regular.
Consequently, it must be said that what is mobile in respect of place always enters a new place part by part before it is there in its entirety; but, in alteration, that is only partially true. For it is clear that every alteration depends on the power of the agent that causes the alteration: as its power is stronger, it is able to alter a greater body.
Therefore, since the cause of the alteration has finite power, a body capable of being altered is subject to its power up to a certain limit of quantity, which receives the impression of the agent all at once; hence, the whole is altered all at once, and not part after part. Yet that which is altered can in turn alter something else conjoined to it, although its power in acting will be less forceful, and so on, until the power involved in the series of alterations is depleted. An example of this is fire that all at once heats one section of air that in turn heats another, and thus part after part is altered.
Hence, in De sensu et sensato, after the passage referenced above, Aristotle goes on to say: yet, if the object heated or frozen is large, part after part will be affected. But the first part had to be altered all at once and suddenly by the agent.
Yet, even in things that are altered all at once, it is possible to discover some kind of succession, because, since alteration depends on contact with the cause that alters, the parts closer to the body that causes the alteration will more completely receive at the very beginning an impression from the agent; thus, the state of perfect alteration is reached successively according to an order of parts. This is especially true when the body to be altered has something that resists the power of the altering cause.
Consequently, the conclusion—namely, that what is being changed is, while it is being changed, partly in the terminus from which and partly in the terminus to which (in the sense that one part reaches the terminus to which before another does)—is unqualifiedly and absolutely true in local motion. But, in alteration, it is qualifiedly true, as we have said.
803. Some, on the other hand, have held that the present doctrine is truer when applied to alteration than when applied to local motion, for they hold that the statement “what is being changed is partly in the terminus from which and partly in the terminus to which” is not to be interpreted as meaning that one part of the thing in motion is in one terminus and another in the other, but that reference is being made to the parts of the termini; that is, that what is being moved has part of the terminus from which and part of the terminus to which, as something in motion from white to black is at the very beginning neither perfectly white nor perfectly black, but imperfectly partakes of both. But, in local motion, this does not seem to be true except in the sense that the thing in motion, while it is between the two extremities, somehow partakes of both extremities. For example, if earth were to be moved to the place normal to fire, then, while it was in the region proper to air, it would have a part of each extremity (that is, earth and fire), in the sense that the place of air is above that of earth and below that of fire.
804. But this is a forced explanation and against Aristotle’s opinion. For in the first place, we need only look at the very words of Aristotle. For he says as a conclusion that it follows, therefore, that part of that which is being changed must be at the starting point, and part at the goal. He is speaking therefore about the parts of the mobile, and not about the parts of the termini.
In the second place, it is against Aristotle’s intention. For Aristotle brings to light facts that will prove that what is being changed is divisible—a statement that could not be proved if you held to the interpretation given.
Hence, Avempace said that Aristotle does not intend here to prove that a mobile can be divided into quantitative parts, but according to forms, in the sense that what is being moved from contrary to contrary has, while it is being moved, something from each contrary. But the intention of Aristotle is expressly to show that a mobile can be divided into its quantitative parts, just as any continuum, for he makes use of that fact in the demonstrations that will follow.
Nor can we heed the opinion that such an interpretation will help to prove that a mobile can be divided on the basis of continuity. For the very fact that a mobile, while it is being moved, partakes of each terminus, not perfectly possessing the terminus to which all at once, reveals that change is divisible on the basis of continuity. And thus, since a divisible cannot exist in an indivisible, it follows that the mobile also can be divided as a continuum. For in the matters to follow, Aristotle will clearly prove that motion is divisible, because the mobile is divisible. Hence, if he intended to conclude that a mobile is divisible because motion is divisible, he would be arguing in a circle.
Third, such an interpretation appears to conflict with Aristotle’s own interpretation at here, by “goal of change,” I mean that into which it is first changed during the process of change (234b17).
This shows that he does not intend to say that it is partly in the terminus from which and partly in the terminus to which just because it is midway and, as it were, sharing in both extremities, but because, in regard to one part of itself, it is in one extreme, and according to another part, in what is midway.
805. But, with respect to this explanation of Aristotle, one might wonder why he says that into which it is first changed, for it seems impossible to discover that into which it is first changed, since a magnitude can be divided to infinity.
Therefore, it must be said that that into which it is first changed in local motion is the place next to but not part of the place from which the local motion starts. For if we took it to mean a place that included part of the original place, we would not be assigning the first place into which it is being moved.
The following example will illustrate this. Let AB be the place from which a mobile is being moved, and let BC be the adjacent place equal to AB. Now, since AB can be divided, let it be divided at D, and take a point G near C such that the place GC is equal to BD. It is clear that the mobile will arrive at DG before it reaches BC. Moreover, since AD can be divided, a place prior to DG can be taken, and so on to infinity.
Similarly, in regard to alteration, the first into which something is changed must be considered to be an intermediate; for example, when something is changed from white to black, the first into which the subject is changed is into grey, not into less white.