Cursus Philosophicus Thomisticus, Secunda Pars Philosophiae Naturalis

After treating mobile being in general in the eight books of the Physics, and incorruptible mobile being—that is, heaven—which was treated in the second part of Philosophy,¹ it follows that in this third part he should treat of mobile being that is corruptible and generable, according to all those motions that pertain to generation or are ordered to it. For insofar as generable things can be ordered through local motion to acquiring their natural place and position in the universe, it pertains to the books On Heaven and the World to treat of them. Although the discussion there is principally about heaven, it nevertheless treats adequately of the whole universe, which is also composed of the elements, insofar as they are arranged in it through the position of bodies.

Aristotle comprises everything pertaining to generable and corruptible being in two books. In the first he explains all the changes that pertain to generation and corruption. In the second he proceeds to explain the bodies to which these changes belong—namely, the elements and mixed bodies—insofar as they are the principles of active and passive qualities, according to which these changes, and generation and corruption themselves, take place.

The changes that principally follow upon generation itself or can be ordered to it are seven. The first and principal one is substantial generation itself and its opposite, corruption. The second, which is more closely ordered to generation, is alteration: one generically, but divided into three species—simple alteration, intensification, and remission. The third is nutrition or, speaking more broadly, adgeneration (generation by addition); the fourth, increase; the fifth, reaction and reciprocal passion; the sixth, condensation and rarefaction; the seventh, mixture. The Philosopher treats all these in the first book.

With regard to generable bodies, he considers three things chiefly in the second book: first, the elements—what qualities they possess and how they are contrary to one another; second, mixed bodies—how they are tempered; and third, all bodies that undergo corruption—whether they can be reproduced again simply because they have once ceased to be. The same question can also be asked about anything else that is corrupted.

On the matter and subject of these books

It is clear from what we said in q. 1 of the Physics,² that mobile being is the adequate subject of Philosophy. Although the formal object is one and indivisible, precisely because the science is one according to an indivisible species, its subject or matter is nevertheless multiple: one and the same science treats of different things, although under one formal aspect.

And so we inquire here which part of mobile being pertains to the proper matter and subject of this treatise On Generation, which is a part of the one natural science. Nearly everyone agrees that the being treated in these books is mobile being that is generable and corruptible, since it is treated nowhere else in Philosophy. For the books On the Soul treat of animate mobile being; although it is corruptible, it is not treated there insofar as it is corruptible, but according to its character as living or animate, and according to vital motions. In the books On Heaven, as we said, the discussion concerns only incorruptible motion or the whole universe insofar as it is composed of simple bodies.

There can be a difficulty only as to whether the subject of these books is mobile being toward substantial form, so that substantial generation alone is considered per se, while alteration and the other changes ordered to generation are treated only as a secondary subject and by reduction; or whether the adequate subject is generable being, considered without distinguishing between substantial and accidental generation, although the principal subject is substantially generable being, or substantial generation itself.

In the judgment of St. Thomas, the more correct view appears to be that the subject of these books is generable being in the second sense just mentioned—that is, generable being considered without distinguishing between substantial and accidental generation—although the chief subject is what is generable according to substance, or substantial generation itself. For in the Proemium to the books On Generation,³ he says that these books treat of motions “which are not common to all bodies, but are found only in lower bodies.” These are all the motions except place, which is also found in the higher bodies. “Among these, generation and corruption hold the principal place; for alteration is ordered to generation as to its end, and increase likewise follows upon generation.” St. Thomas’s causal phrase “for alteration,” etc., does not explain why these books treat of alteration, but why alteration is a less principal motion than generation. Thus alteration is treated here not merely by reduction, but as a less principal subject, just as Metaphysics treats of accident; the principal subject, however, is substantial generation, just as in Metaphysics it is substance or God, and in Logic it is demonstration. This is what St. Thomas means when he says that generation holds the principal place among these motions.

This is also clear from reason: alteration, increase, and the other motions are truly and per se species of motion. Therefore Philosophy treats of them as things that pertain to it per se, not by reduction—just as it also treats of local motion in the books On Heaven, and of increase here. Therefore some part of Philosophy must treat of them per se, just as it treats of local motion. But there is no other part in which alteration and increase are treated except this one.

Hence it follows that the subject of these books is mobile being toward an intrinsic form, according to which generation or corruption takes place, or which is ordered to it. This does not mean that they treat of a conflation of substance and accident, so that their subject is some analogical entity. First, their subject is not accident and substance, but mobile being toward substantial or accidental form—which is itself a substance as subject, although it can be terminated in something accidental. Second, accident and substance can be treated in some science as a univocal object in the order of what is knowable, even though they are analogues in the order of being. This is clear in Metaphysics, which is one science and treats of accident and substance, since things that agree analogically can be made manifest by the same light and from the same principles, and thus can agree univocally in the order of what is manifestable and knowable.

You will object: If it pertains to these books to treat per se of increase and nutrition, then it will not pertain per se to the books On the Soul to treat of them—the opposite of what we said at the beginning of the Physics when we distributed the parts of Philosophy according to the species of motion, assigning increase, because it is vital, to the books On the Soul. Alternatively, the same science will treat of increase twice. Second, it follows that this treatise On Generation precedes the treatise On Heaven, since the latter treats of local motion, which is less perfect than the motion of generation: it terminates at place, which is less perfect than quantity, quality, and substance, the terms at which these other motions terminate.

To the first, we reply that these books treat of increase and nutrition not as vital motions proceeding from a living soul, but as certain changes that are partly corruptive or generative. It is no difficulty for the same thing to be treated twice in the same science under different considerations. Thus the books On Heaven treat of the elements insofar as, in this universe, they require a determinate position in a place; these books treat of them insofar as they are generable and corruptible. At the beginning of the Physics we said that increase is treated in the books On the Soul insofar as it is a vital motion proceeding from the soul, not under the aspect of a corruptive motion.

To the second, we reply that at most it proves that the books On Generation are prior in dignity to the books On Heaven, but not in the order of teaching, since one proceeds rather from what is less perfect. I add that although local motion, considered precisely in relation to its term, place, is less perfect than alteration and generation, the local motion that belongs to heaven must nevertheless be treated in Philosophy before the other motions: both because the subject of such motion is something more excellent and higher, namely heaven itself, and because its motion has causality over the other, lower things. Thus, if one must begin with what is more excellent and perfect, one must also begin with this local motion belonging to heaven, because insofar as it causes in this way it is more excellent. Finally, local motion is more universal and belongs to more things than alteration and increase, for it is found in every body; but one must begin from what is more common.

Order of the questions

We shall complete the whole treatise in twelve questions: nine concerning the first book and three concerning the second. The first will concern substantial generation, the second its opposite, corruption, the third alteration in general, the fourth alteration in particular, such as intensification and remission, etc., the fifth reaction and reciprocal passion, the sixth mixture, the seventh condensation and rarefaction, the eighth increase and nutrition, and the ninth the subject of accidents and the principle of individuation.

With regard to the second book, the tenth question will concern the elements, the eleventh mixed bodies, and the twelfth the reproduction of a thing that has passed away.