Commentary Aristotle Physics
first, he sets out certain things necessary for his proposition;
second, he proves his proposition, at “the same matter also serves” (217a26; [554]).
553. Now (217a21), he lays down four preliminary statements that he takes from the subjects, that is, the presuppositions of natural science that were already explained in book 1.
The first of these is that the matter of contraries is one (for example, of the hot and the cold, or of any other natural contrariety), for contraries are apt to affect the same thing.
The second is that whatever is in act has to have come into being from what was in potency.
The third is that matter is not separable from contraries such as to exist without them, but yet, according to motion, the matter is distinct from the contraries.
The fourth is that matter is not, by virtue of being, now under one contrary and then under another, different, but numerically one.
554. Then, at “the same matter also serves” (217a26; [554]), from these preliminaries, he proves his point in this way. The matter of contraries is one in number. But the large and the small are contraries in respect of quantity. Therefore, the matter of the large and the small is numerically the same.
And this is clear in substantial change. For when air is generated from water, the same matter that previously was under the water came to be under air, not receiving anything that it previously lacked, but rather that which was previously in potency in the matter was reduced to act. And the same is true in reverse when water is generated from air.
But there is this difference: when air is generated from water, there is a change from small to large, because the quantity of air generated is larger than the quantity of water from which it was generated.
But, when water is made from air, there is produced, contrariwise, a change from largeness to smallness. Therefore, when a large amount of air is reduced to a smaller amount by condensation, or from a small amount to a larger amount by rarefaction, it is the same matter that becomes both in act (namely, large and small) while being previously in potency to them.
Therefore, condensation does not take place by certain parts moving into others, or rarefaction by inhering parts being extracted, as those thought who posited a void within bodies. Rather, it is because the matter of the same parts now has greater quantity and then lesser. Hence, to become rare is nothing other than for matter to receive greater dimensions by being reduced from potency to act, and the opposite is becoming dense. For just as matter is in potency to definite forms, so it is in potency to definite quantity. Hence rarefaction and condensation do not proceed to infinity in natural things.
555. Then, at “for as the same matter” (217a33), he makes the same thing clear from examples. And, because rarefaction and condensation pertain to the motion of alteration, he gives an example of other alterations. And he says that, just as the same matter is changed from cold to hot and from hot to cold, because both were in the matter potentially, so also something passes from hot to hotter not because some part of the matter that was previously not hot becomes hot, but because the entire matter is reduced into the act of being more or less hot.
He gives another example of a quality in the matter of quantity. And he says that, if the circumference and convexity of a larger circle are brought into that of a smaller circle, they become more curved. This happens not because an ambit, or curvature, begins to exist in some part that previously was not curved but straight, but because the same that was previously less curved becomes more curved.
For in alterations of this sort, things do not become more and less by difference (that is, by subtraction) or addition, but by the changing of one and the same thing from perfect to imperfect, or from imperfect to perfect. This is evident in the case of what is absolutely and uniformly such and such: it is impossible to find in it any part lacking that quality, just as it is impossible in a flame to find any part lacking heat and whiteness, that is, clarity. So also, prior heat comes to a later heat not because a part previously not hot became hot, but because what was less hot became hotter.
So too, the largeness and smallness of a sensible body is not extended or increased in rarefaction and condensation by the matter receiving some addition, but by the matter that was previously in potency to large and small being changed from one to the other. Therefore, the rare and the dense are not produced by the addition of penetrating parts or by their removal, but by there being one matter of the rare and of the dense.
556. Then, at “the dense is heavy” (217b11), he manifests his proposition by the effects of the rare and of the dense. For from a difference in rarity and density, there follows a difference in other qualities, such as in heaviness and lightness or hardness and softness. Consequently, rarity and density diversify qualities, not quantities.
He says therefore, that lightness follows rarity, and heaviness density. And with good reason, for rarity arises from matter receiving greater dimensions, density from matter receiving lesser dimensions. Consequently, if you take diverse bodies of equal quantity, one being rare and the other dense, then the dense has more matter. Now, it was said above in the treatise on place that the contained body is related to the container, as is matter to form; consequently, a heavy body that tends toward the middle, as contained, is with good reason more dense, because it has more matter. Just as, therefore, the circumference of a larger circle, when it is restricted to a smaller circle, does not receive concavity in a part not previously concave, but rather a part previously concave was reduced to a greater concavity, and just as any part of fire that anyone may take will be hot, so also it is the whole body that becomes rare or dense by the conduction, that is, contraction or expansion of one and the same matter, accordingly as it is moved to greater or smaller dimensions.
This is clear from what follows from rarity and density, which are qualities. For the heavy and the hard follow from density. Why heaviness follows density has already been explained. Why hardness follows is easy to explain: that is hard that is better able to resist both pressure and cleavage, but what has more matter is less divisible, because it is less obedient to something acting upon it, on account of its being more remote from act. Contrariwise, lightness and softness follow upon rarity. But the heavy and the hard fail to coincide in some things, for lead is heavier, but iron is harder. The reason for this is that lead has more of the element of earth in it, but what there is of water in it is less perfectly congealed and distributed.
557. Then, at “from what has been said” (217b20), he concludes his chief proposition. And he says it is clear from the foregoing that there is no separate empty space. It is not anything existing absolutely outside a body, or in a rarefied thing after the manner of empty holes, or in potency in a rarefied body, as say those who did not posit a void that exists in bodies as something separated from the fullness of the body. Consequently, in no way is there a void, unless someone simply wants to call matter the void, since it is somehow the cause of heaviness and lightness, and consequently the cause of motion in respect of place. For density and rarity are causes of motion according to the contrariety of heavy and light, but in regard to the contrariety of hard and soft, passible and non-passible are the causes, for the soft is that which easily suffers division and the hard contrariwise, as was said. However, this does not pertain to local motion, but rather to alteration.
And so he concludes that it has been determined in what way the void exists and in what way it does not.