Some other passages where Aquinas says that generation and corruption are not motions:

SCG II.89.6: on the theory that full human soul is virtually contained in seed, "It would further follow both that the substantial form would be brought from potency to act, not all at once but in successive stages, and that generation is a continuous motion, just as alteration is. Now, all these consequences are impossible in nature." [tr. from dhspriory modified]

[from #64 on the SDP thread} In his commentary on Aristotle's Physics, Aquinas distinguishes between Aristotle's discussions of motion in a strict sense in Book V and later, where Ari limits motion to locomotion, alteration and growth/decay, and Ari's use of a more common sense of "motion" in Book III, where Ari speaks of it as an alternative of "change/mutation," as though it can include generation and corruption (In Phys. III l. 2, C286). In other places in Aquinas' commentary, where Aristotle restricts motion to the three above species, Aquinas notes those three as motion in the strict sense. John Wippel, 
The Metaphysical Thought of Thomas Aquinas (Cath U Press 2000), 445-6 wonders whether Aquinas uses the term "motion" in this broad sense in the First Way, so as to include generation and corruption. Wippel comes down to this: "I am inclined to limit motion as it appears as the starting point of the first way to some form of motion taken strictly, but to suggest that in the course of justifying the principle of motion -- whatever is moved is moved by something else -- Thomas uses motion broadly enough to apply to any reduction from potentiality to actuality." Wippel thinks Aquinas may have allowed a similar, broader understanding of motion in SCG I.13. Then Wippel concludes, "Nonetheless, the starting point for both of these arguments appears to be motion taken strictly and, to be specific, local motion. Perhaps it is because the first way in the Summa theologiae begins with readily observable phenomena--local motion and alteration--that Thomas describes it as the 'more manifest way'."

I believe Wippel's conclusion is correct. 1) In SCG I.13, Aquinas refers expressly and often in those arguments to Books VII and VIII of the Physics. By the time Aristotle wrote the second half of the Physics, he had excluded generation and corruption from motions. It's that second half of the Physics that Aquinas applies in his proofs from motion.
 2) The A-T presentation of motion gives it a telos, an end. If the substance passes out of existence, it is not in entelechy, fulfillment, which is supposed to be the state that a thing attains when it attains an end. 

[from #73 on SDP thread]
 In his proofs from motion, Aquinas employs Aristotle's expositions from this latter part of the Physics, where Aristotle had limited motion to locomotion, alteration, and growth/decay. Check Aquinas' frequent references to the Physics in SCG I.13. There, in arguments from motion, Aquinas does not employ passages from earlier books of the Physics or from other early works of Aristotle in which generation and decay are listed among motions. In his arguments from motion, Aquinas cites from Aristotle's Physics only bks. VI-VIII, where motion (kinesis) is limited to locomotion, alteration, and growth/decay.
 
[from #89]
Cf. lectiones 2-4 of his commentary on Physics Book V. Aquinas goes into a lot of detail explaining how Aristotle restricts kinesis or motus to changes in location, quality or quantity and how Ari denies that changes in substance are species of kinesis. Aquinas explains that Aristotle in bk V makes motus a species of mutatio.

It seems to me that even if that doesn't make it explicit it provides ample reason to consider motion broadly enough as to include it. So I don't see how it leads to the answer as No. And secondly even if we accept from above that act/potency has wider scope than "motion" it hardly follows from it that argument can't be formulated in terms of that.

@ Calhoun and Jeremy, I understand what is being done in proofs that seek to argue from change as actualization of a potency by something else in act. And I understand that someone may seek to reconstruct the First Way so as to include changes in substance, as Feser and others have so reconstructed it.

Thomas' own meaning in the First Way is at the least of importance for the history of philosophy. So the question matters for attempting to understand his thought. The question may also be important for the soundness of the argument if there are problems with explaining generation and corruption as outcomes of hierarchical series ordered per se. I haven't thought that through.

Back to the interpretation of Thomas: not all will be interested in more historical questions, and that's fine. For those who are: the more I poke into this, the more I am convinced that we do not have grounds for supposing that he means "motion" in his First Way to include generation and corruption. 

I've mentioned his commentary on the Physics before and quoted pieces. Some longer quotations are below the dotted line below, for those who have time to read them. As I've said, Aquinas accepts Aristotle's restriction of motion to locomotion, alteration, and growth/decay, and he even offers supporting arguments in C663-664. He cites these latter books of the Physics often in his arguments from motion in SCG and ST.

Consider as well:
1. Aquinas' examples of motion in the First Way and in SCG I.13 do not include generation and corruption of substances. They only include locomotion, alteration, or growth/decay.
2. In SCG I.13.19, Aquinas says that the species and genera of motion are finite in number, and he enumerates the three standard species without mentioning gen/corr: "If, however, the mover is moved by another species of motion, so that (namely) the altering cause is moved according to place, and the cause moving according to place is increased, and so forth, since the genera and species of motion are finite in number, it will follow that we cannot proceed to infinity. There will thus be a first mover, which is not moved by another. Will someone say that there will be a recurrence, so that when all the genera and species of motion have been completed the series will be repeated and return to the first motion? This would involve saying, for example, that a mover according to place would be altered, the altering cause would be increased, and the increasing cause would be moved according to place." It seems strange that, if he wanted 'motion' to include generation/corruption, he should leave them out just here.

So there is considerable reason to suppose Aquinas thinks of "motion" under Aristotle's mature formulation, since he knew that formulation well by the time he wrote the SCG and ST. The texts supply no positive reason to suppose he is including them. 


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In his Commentary on Aristotle’s Physics V.2, Aq makes clear that Ari is now using “motion” with a scope restricted to locomotion, alteration and growth/decay, excluding generation and corruption. Those are mutationes but not motus.
  C649 “… when Aristotle defined motion above in Book III, he took the word ‘motion’ as common to all the species of mutation. Here he attributes this meaning to the word ‘mutation.’ And he uses ‘motion’ in a more limited sense as a certain species of mutation.”… C652 there cannot be a species of mutation from non-subject to non-subject. Every mutation is between opposites, but two negations are not opposites; they can be simultaneously true of one and the same thing, as a stone is not sick and not healthy. Mutation per se is only between contraries and contradictories; mutation from negation to negation is only per accidens. C656 “… he shows that neither of the foregoing [sc. generation and corruption, discussed in C654-655] is motion.
   First he shows that generation is not motion, and secondly that corruption is not motion, where he says, ‘So, too, perishing…’
   He proves the first point with two arguments. The first is as follows. That which is simply not a ‘this’ cannot be moved, For that which is not is not moved. But that which is generated simply is not a ‘this’; for it is simply non-being. Therefore, that which is generated simply is not moved. Therefore simple generation is not motion…”
  C657 “He gives the second argument where he says, ‘… and it may be further…’ The argument is as follows.
     Whatever is moved is in place. But that which is not is not in place, because it can be said to be anywhere. Therefore that which is not is not moved. This is the same conclusion as the above. The truth of the first proposition arises from the fact that, since local motion is the first motion, it is necessary that whatever is moved be moved with respect to place, and thus is in place. And when that which is prior is removed, things which are consequent are also removed.
   C658 Next where he says, ‘So, too, perishing …’, he proves that corruption is not motion. Nothing is contrary to motion except motion or rest. But generation, which is neither motion nor rest, as was shown, is contrary to corruption. Therefore corruption is not motion.
  C659 Next where he says, ‘Since, then, every motion …’ he concludes from the above that motion is the remaining part of the division given above.
  Motion is a certain species of mutation because is motion there is something after another, which he said above pertains to the nature [ratio] of mutation. But motion is neither generation nor corruption, which are mutations in respect to contradictories. And since there are only three species of mutation, it follows of necessity that motion is mutation from subject to subject.”
 
In Arist. Phys. V.3 continues the above. C662 “He proves as follows that there is no motion in the genus of substance. Every motion is between contraries, as was said above. But there is no contrary of substance. Therefore there is no motion in respect to substance.” C663 Aq discusses places in Ari (GC II.3, 331a2-6; Caelo I.3, 270a13-23) that create difficulty: where a substance is said to be the contrary of another substance (GC), sc. fire to water; where heavens are not generable or corruptible because they have no contrary (Caelo). C664 The answer is that forms of diverse species can differ from each other w/ respect to excess and defect but not w/ respect to their own proper natures [ratio]. So there need not be a passage in order through intermediaries between one substantial form and another. “Hence Aristotle’s proof that there is no motion in substance because there is no contrariety in substance is demonstrative, and not just a probable argument, as the Commentator seems to indicate. Moreover, that there is no motion in substance can also be proved by means of another argument which he gave above, namely, the subject of substantial form is being in potency only.” [from the Blackwell, Spath, Thirlkel translation]
 

@ficino,

From Aquinas's 

COMMENTARY ON
ARISTOTLE'S
GENERATION AND CORRUPTION

1. As the Philosopher says in On the Soul III, the sciences are divided off in the same manner as things are — for all habits are distinguished by their objects, from which they are specified. Now the things considered by Natural Science are motion and mobile being. Thus the Philosopher says in Physics II that whatever things move, they themselves being moved, these belong to physical speculation. Consequently, it is according to the differences between motions and mobiles that the parts of natural science must be distinguished and ordered.

Now the first motion is local motion, which is more perfect than the other kinds, and common to all natural bodies, as is proved in Physics VII. Therefore, after the study of motions and mobiles in common in the book of the Physics, it was first necessary to treat of bodies as they are moved with local motion. This was in the book On the Heavens, which is the second book of natural science. What remains, therefore, is to consider the other subsequent motions which are not common to all bodies but are found only in lower bodies.

Among these motions, generation and corruption obtain the primacy. For alteration is directed to generation as to its end, and the end is by nature more perfect than what leads to it. Growth, likewise, is subsequent to generation, for growth does not take place without a certain particular generation, namely, that by which food is converted into the thing fed. Thus the Philosopher says in On the Soul II that food nourishes in so far as it is potentially flesh, but it produces increase inasmuch as potentially it is quantified flesh. Therefore, since these motions are in a certain way consequent upon generation, they must be studied along with generation and corruption.

2. Now it should be noted that whatever is found in a number of things should first be considered in common before coming to the specific cases. Otherwise the same thing will be frequently repeated, in that what is common will be repeated in each individual case, as the Philosopher proves in On the Parts of Animals I. Consequently, generation and corruption should be considered in common before coming to the parts [i.e., species] thereof. Likewise, it should be noted that if in any genus there be found some first thing which is the cause of the other things in that genus, the study of the common genus and of that which is first in that genus will belong to the same study. For that first thing is the cause of the entire genus, and anyone who studies some genus must consider the causes of the entire genus. That is why the Philosopher in the Metaphysics at once studies being in general and first being, which is separated from matter. Now in the genus of generable and corruptible things there are found certain first principles, namely, the elements, which are the cause of generation and corruption and alteration in all other bodies. Hence Aristotle in this book, which is the third part of natural science, discusses not only generation and corruption in general and other consequent motions, but also generation and corruption of the elements.

With these prefatory remarks to show Aristotle's intention in this book, we now arrive at its exposition.