a. First Objection

239. But against this:

First, because a quality that has power for every degree of alteration preparatory to generation has power for a degree incompossible with the substantial form of the thing to be corrupted - and so it has power for corruption. The proof of the first part [“power for a degree incompossible.”] is that, if the quality does not have power to induce some degree incompossible with the substantial form, then the substantial form of the thing to be corrupted can stand with any degree inducible by the quality, and so the thing to be corrupted can, by no alteration, be corrupted.

240. I reply: the altering thing can never induce a degree incompossible with the alterable thing. The proof comes from the ideas of the terms, because a mover cannot induce a degree incompossible with its movable object (the proof of which is that the movable thing remains throughout the whole motion;     therefore , it remains under every degree induced by the motion); but nothing remains under anything incompossible with it; therefore etc     .

241. But when you infer [n.239] that ‘then the altering thing will, by alteration, never corrupt the substance of the altered thing’, I concede it. And I prove this by another reason a priori, because a naturally prior is not corrupted by something naturally posterior; but whatever is induced by an altering thing, as altering thing, is naturally posterior to the substantial form of the alterable thing; therefore by nothing such that is induced by the altering thing is the substance of the alterable thing corrupted.

242. I reply, therefore, that no substance can be corrupted save by a substance, just as neither can the opposite of a substance to be corrupted be induced save by a substance, as was proved above [nn.202, 206, 212ff.]; but a substance is not corrupted by a contrary quality save dispositively, because generation follows on the dispositive alteration unless something prevents it.

243. But on the contrary:

It would then follow that if the accidents of fire were the greatest (namely as great in quantity as the sphere of fire), and yet they were separated from substance, they could not corrupt a drop of water existing there, which seems impossible.

244. Something else follows, because if the accidents that do the altering have power for any degree whatever of alteration the way they had before, then they have power for the ultimate degree - and consequently, since the ultimate degree, when corrupted, disposes immediately for the corruption of substance, or its corruption is immediately accompanied by the corruption of substance, the result is that the substance will be corrupted;     therefore etc     .

245. To the first [n.243] it can be said: I concede that if the whole substance of fire were destroyed and all the accidents remained there, a drop of water put there would never be destroyed. And the reasons set down before [nn.242, 202, 206] have this as conclusion, because accidents cannot generate any substance, and the corruption of substance can only be into substance; because the corruption is not into nothing, nor into matter, nor into accidents, and especially by virtue of a natural agent, because nature cannot annihilate anything, nor resolve anything into prime matter, nor make accidents without a subject.

246. Lest, however, this be said to be too absurd and against sense, I say that, if any accidents were without a subject, a substance of any quantity and quality next to them would be corrupted, but not by them, rather by the created universal agent, namely the heaven, to which natural philosophers take flight - especially since the form to be here induced is imperfect and is totally in the active power of the heaven. And consequently, with no particular agent impeding but rather disposing, this form can be induced by the heaven, just as the heaven induces certain imperfect forms in the matter of corruptible and generable things. And if there were some form for inducing which the heaven did not suffice, one must flee to the simply first Cause, which would here induce the form for which the natural agent disposes it. Nor is this a fleeing to a miracle, because God as a matter of rule supplies every impotency of the whole creation, as is plain in the animation of an organic body by the intellective soul, where no miracle is posited. So here the whole of created nature would not suffice for the generation or corruption in question; and therefore, when nature makes disposition for it with ultimate disposition, God would as a matter of rule supply the impotence of nature.

247. To the second [n.244] I say that there is no degree of alteration on which would necessarily follow the corruption of the substantial form in the alterable thing; but the degree that is next in one alterable thing can fail to be next in another alterable thing, speaking with respect to the corrupter or generator intrinsic to the thing that does the altering. For let it be that some degree, which is as it were next to the degree that is incompossible with the form of water, would be induced in the water by some agent -corruption of the water would immediately follow if there were from the agent a form by which it could corrupt the water. But if there not be in the agent a form corruptive of the water, then the corruption of the water does not follow from that agent, but perhaps from some universal agent inducing the form for which the matter has been disposed. So here, if the water were altered by separated accidents to the degree that would be proximate to the corruption of water, in the way it is possible for a proximate degree to be there (and I speak thus because, in the case of indivisibles, nothing divisible is proximate to something indivisible), then indeed the corruption of the water would immediately follow from the altering agent, though in this case only extrinsically.