A continuation of the proof that the intellective soul is not physical and thus must come from God. We had a hiatus, which is a good thing, since the number of comments on the last post were so many. Angels are on their way!
1 In order to facilitate the solution of these arguments, certain things must be premised in explanation of the order and process of the generation of man and of animals in general.
2 To be taken into account first of all is the falsity of the opinion of those who say that the vital operations appearing in the embryo before its complete development do not proceed from a soul, or from a soul’s power existing in the embryo, but from the soul of the mother.
If this were true, the embryo would not even be an animal, since every animal consists of soul and body.
Vital operations, moreover, do not issue from an extrinsic active principle, but from an internal power; and in this respect particularly are living things, to which self-movement properly belongs, seen to differ from the non-living. For the thing that is nourished assimilates the nourishment and thus must possess an active power of nutrition; what the agent effects is like to itself. And this fact is much more manifest in the operation of the senses; it is through a power existing in this person, and not in another, that he is enabled to see and to hear. Hence, nourishment and even sensation on the part of the embryo prior to its complete development cannot be attributed to the soul of the mother.
Notes And thus it is immoral to kill the embryo.
3 Nevertheless, it cannot be said that the soul in its complete essence is present in the semen from the very beginning, though its operations are not manifested because of the lack of organs. This is impossible in view of the fact that since the soul is united to the body as its form, it is united only to a body of which it is properly the act. Now, a soul is the “act of an organic body.”
Prior to the organization of the body, therefore, the soul is not in the semen actually, but only potentially or virtually. Thus, Aristotle says in De anima II  that “seeds and fruits are endowed with life potentially so far as they are rid of,” that is, lack, “a soul; whereas the thing of which the soul is the act has indeed the power of life, but is not without a soul.”…
5 But this theory would be even more ridiculous if applied to the rational soul. For, first, the soul cannot possibly be divided as the body is, so as to be present in the separated semen; and second, it would follow that in all extra-copulative emissions of semen, without conception taking place, rational souls would nevertheless be multiplied.
Notes And thus are obviated all those bad jokes about masturbation and killing. Same thing with scratching-your-nose jokes about loosing skin cells.
6 Another theory, likewise inadmissible, is stated as follows. From the moment of severance the soul is not present in the semen actually but virtually, because of the lack of organs and yet this very power of the semen—itself a body potentially endowed with organs though actually without them—is, proportionately to the semen, a potential but not an actual soul.
Moreover, since plant life requires fewer organs than animal life, from the moment that the organic development of the semen suffices for plant life, the aforesaid seminal power becomes a vegetative soul; and later, the organs having been perfected and multiplied still more, the same power is raised to the level of a sensitive soul; and finally, with the perfecting of the organs form, the same soul becomes rational, not indeed, by the action of that seminal power, but through the influx of an external agent.
And for this reason the proponents of the theory suppose Aristotle to have said in the De generatione animalium that “the intellect is from without””[II, 3]. Now, this theory would involve the consequence that numerically one and the same power is at one time a purely vegetative soul, and afterwards a sensitive soul, the substantial form itself thus being perfected successively more and more. It would further follow both that the substantial form would be brought from potentiality to act, not all at once but in successive stages, and that generation is a continuous movement, just as alteration is. Now, all these consequences are impossible in nature.
7 But that theory would entail a consequence still more incongruous, namely, the mortality of the rational soul. For nothing formal in character that accrues to a corruptible thing makes it incorruptible by nature; in that case, the corruptible would be changed into the incorruptible, which is impossible, since they differ in genus, as Aristotle says in Metaphysics X .
In the process described above, however, the substance of the sensitive soul is held to be generated accidentally by the generated body, and hence that substance must necessarily be corruptible with the corruption of the body. Therefore, if the same soul becomes rational through the infusion into it of a kind of light, having the role of a form in its regard, for the sensitive is potentially intellective, then necessarily the rational soul perishes along with the body. But this is impossible, as we proved above, and as the Catholic faith teaches…
12 With these considerations in mind, it is easy to answer the objections.
To the first objection, that the sensitive soul must originate in the same way in man and in irrational animals because animal is predicated of them both univocally, we reply that this is not necessary. For, although the sensitive souls in man and brute are generically alike, they differ specifically, as do the things whose forms they are; since, just as the human animal differs specifically from the other animals by the fact that it is rational, so the sensitive soul of man differs specifically from the sensitive soul of the brute by the fact that it is also intellective. Therefore, in the soul of the brute there is nothing supra-sensitive, and, consequently, it transcends the body neither in being nor in operation; and that is why the brute soul must be generated together with the body and perish with the body.
But in man the sensitive soul is possessed of intellective power over and above the sensitive nature and is therefore raised above the body both in being and in operation; it is neither generated through the generation of the body, nor corrupted through the body’s corruption. Thus, the diversity in mode of origin of the human and of the brute soul is not on the part of the sensitive faculty, from which the generic nature is derived, but on the part of the intellective faculty, whence the specific difference stems. Hence, it cannot be inferred that they are diverse generically, but only specifically.
13 As to the second objection, to say that the thing conceived is an animal before a man does not prove that the rational soul is produced together with the semen. For the sensitive soul, by which it was an animal, does not remain, but is succeeded by a soul both sensitive and intellective in character, by which it is at once animal and man, as we have already made clear…
Notes The refutations of the other objections follow along similar lines and do not add much new. So we will skip toward the end, since by this point all should be convinced the human soul is not transmitted in the semen; but see the original in case you still have doubts.
21 The tenth objection, that the body is conformed to the soul and that, therefore, the soul forms a body like to itself, is partly true and partly false. This statement is true if referred to the soul of the begetter, but false if referred to the soul of the begotten; for, as regards its primary and principal parts, the body is not formed by the power of the latter’s soul, but by that of the former, as we have just shown. So, too, is every matter configured to its form: a configuration which, however, is not brought about by the action of the thing generated, but by the action of the generating form.
22 As to the eleventh objection, it is quite clear, from what has been said, that at the beginning of its separation the semen is only potentially animate; hence, it does not at that time have a soul actually, but virtually. In the process of generation the semen is, by its own power, endowed with a vegetative and a sensitive soul, which do not remain but pass away, being succeeded by a rational soul.
23 Nor, again, is the reasoning in the twelfth objection conclusive. For, if the formation of the body precedes the human soul, it does not follow that the soul is for the sake of the body.
Indeed, a thing is for the sake of another, in two ways. In one way, for the sake of the latter’s operation, or preservation, or anything of the sort which follows upon being; and such things are posterior to that on whose account they are; the clothes are for the man, and tools for the worker. In another way, for the sake of its being; and thus, a thing which is for the sake of another is prior to the latter in time, but posterior in nature. It is in this sense that the body is for the sake of the soul, just as in every case matter is for the sake of the form. But this would not be true if the joining of soul and body did not constitute a thing one in being, as those say who deny that the soul is the form of the body.