Objection 1. It would seem that the sensitive soul is not transmitted with the semen, but created by God. For every perfect substance, not composed of matter and form, that begins to exist, acquires existence not by generation, but by creation: for nothing is generated save from matter. But the sensitive soul is a perfect substance, otherwise it could not move the body; and since it is the form of a body, it is not composed of matter and form. Therefore it begins to exist not by generation but by creation.
Objection 2. Further, in living things the principle of generation is the generating power; which, since it is one of the powers of the vegetative soul, is of a lower order than the sensitive soul. Now nothing acts beyond its species. Therefore the sensitive soul cannot be caused by the animal's generating power.
Objection 3. Further, the generator begets its like: so that the form of the generator must be actually in the cause of generation. But neither the sensitive soul itself nor any part thereof is actually in the semen, for no part of the sensitive soul is elsewhere than in some part of the body; while in the semen there is not even a particle of the body, because there is not a particle of the body which is not made from the semen and by the power thereof. Therefore the sensitive soul is not produced through the semen.
Objection 4. Further, if there be in the semen any principle productive of the sensitive soul, this principle either remains after the animal is begotten, or it does not remain. Now it cannot remain. For either it would be identified with the sensitive soul of the begotten animal; which is impossible, for thus there would be identity between begetter and begotten, maker and made: or it would be distinct therefrom; and again this is impossible, for it has been proved above (Question 76, Article 4) that in one animal there is but one formal principle, which is the soul. If on the other hand the aforesaid principle does not remain, this again seems to be impossible: for thus an agent would act to its own destruction, which cannot be. Therefore the sensitive soul cannot be generated from the semen.
On the contrary, The power in the semen is to the animal seminally generated, as the power in the elements of the world is to animals produced from these elements--for instance by putrefaction. But in the latter animals the soul is produced by the elemental power, according to Genesis 1:20: "Let the waters bring forth the creeping creatures having life." Therefore also the souls of animals seminally generated are produced by the seminal power.
I answer that, Some have held that the sensitive souls of animals are created by God (65, 4). This opinion would hold if the sensitive soul were subsistent, having being and operation of itself. For thus, as having being and operation of itself, to be made would needs be proper to it. And since a simple and subsistent thing cannot be made except by creation, it would follow that the sensitive soul would arrive at existence by creation.
But this principle is false--namely, that being and operation are proper to the sensitive soul, as has been made clear above (Question 75, Article 3): for it would not cease to exist when the body perishes. Since, therefore, it is not a subsistent form, its relation to existence is that of the corporeal forms, to which existence does not belong as proper to them, but which are said to exist forasmuch as the subsistent composites exist through them.
Wherefore to be made is proper to composites. And since the generator is like the generated, it follows of necessity that both the sensitive soul, and all other like forms are naturally brought into existence by certain corporeal agents that reduce the matter from potentiality to act, through some corporeal power of which they are possessed.
Now the more powerful an agent, the greater scope its action has: for instance, the hotter a body, the greater the distance to which its heat carries. Therefore bodies not endowed with life, which are the lowest in the order of nature, generate their like, not through some medium, but by themselves; thus fire by itself generates fire. But living bodies, as being more powerful, act so as to generate their like, both without and with a medium. Without a medium--in the work of nutrition, in which flesh generates flesh: with a medium--in the act of generation, because the semen of the animal or plant derives a certain active force from the soul of the generator, just as the instrument derives a certain motive power from the principal agent. And as it matters not whether we say that something is moved by the instrument or by the principal agent, so neither does it matter whether we say that the soul of the generated is caused by the soul of the generator, or by some seminal power derived therefrom.
Reply to Objection 1. The sensitive soul is not a perfect self-subsistent substance. We have said enough (25, 3) on this point, nor need we repeat it here.
Reply to Objection 2. The generating power begets not only by its own virtue but by that of the whole soul, of which it is a power. Therefore the generating power of a plant generates a plant, and that of an animal begets an animal. For the more perfect the soul is, to so much a more perfect effect is its generating power ordained.
Reply to Objection 3. This active force which is in the semen, and which is derived from the soul of the generator, is, as it were, a certain movement of this soul itself: nor is it the soul or a part of the soul, save virtually; thus the form of a bed is not in the saw or the axe, but a certain movement towards that form. Consequently there is no need for this active force to have an actual organ; but it is based on the (vital) spirit in the semen which is frothy, as is attested by its whiteness. In which spirit, moreover, there is a certain heat derived from the power of the heavenly bodies, by virtue of which the inferior bodies also act towards the production of the species as stated above (115, 3, ad 2). And since in this (vital) spirit the power of the soul is concurrent with the power of a heavenly body, it has been said that "man and the sun generate man." Moreover, elemental heat is employed instrumentally by the soul's power, as also by the nutritive power, as stated (De Anima ii, 4).
Reply to Objection 4. In perfect animals, generated by coition, the active force is in the semen of the male, as the Philosopher says (De Gener. Animal. ii, 3); but the foetal matter is provided by the female. In this matter, the vegetative soul exists from the very beginning, not as to the second act, but as to the first act, as the sensitive soul is in one who sleeps. But as soon as it begins to attract nourishment, then it already operates in act. This matter therefore is transmuted by the power which is in the semen of the male, until it is actually informed by the sensitive soul; not as though the force itself which was in the semen becomes the sensitive soul; for thus, indeed, the generator and generated would be identical; moreover, this would be more like nourishment and growth than generation, as the Philosopher says. And after the sensitive soul, by the power of the active principle in the semen, has been produced in one of the principal parts of the thing generated, then it is that the sensitive soul of the offspring begins to work towards the perfection of its own body, by nourishment and growth. As to the active power which was in the semen, it ceases to exist, when the semen is dissolved and the (vital) spirit thereof vanishes. Nor is there anything unreasonable in this, because this force is not the principal but the instrumental agent; and the movement of an instrument ceases when once the effect has been produced.
Source: Summa Theologica (New Advent)