Objection 1. It would seem that the intellectual soul is produced from the semen. For it is written (Genesis 46:26): "All the souls that came out of [Jacob's] thigh, sixty-six." But nothing is produced from the thigh of a man, except from the semen. Therefore the intellectual soul is produced from the semen.
Objection 2. Further, as shown above (Question 76, Article 3), the intellectual, sensitive, and nutritive souls are, in substance, one soul in man. But the sensitive soul in man is generated from the semen, as in other animals; wherefore the Philosopher says (De Gener. Animal. ii, 3) that the animal and the man are not made at the same time, but first of all the animal is made having a sensitive soul. Therefore also the intellectual soul is produced from the semen.
Objection 3. Further, it is one and the same agent whose action is directed to the matter and to the form: else from the matter and the form there would not result something simply one. But the intellectual soul is the form of the human body, which is produced by the power of the semen. Therefore the intellectual soul also is produced by the power of the semen.
Objection 4. Further, man begets his like in species. But the human species is constituted by the rational soul. Therefore the rational soul is from the begetter.
Objection 5. Further, it cannot be said that God concurs in sin. But if the rational soul be created by God, sometimes God concurs in the sin of adultery, since sometimes offspring is begotten of illicit intercourse. Therefore the rational soul is not created by God.
On the contrary, It is written in De Eccl. Dogmat. xiv that "the rational soul is not engendered by coition."
I answer that, It is impossible for an active power existing in matter to extend its action to the production of an immaterial effect. Now it is manifest that the intellectual principle in man transcends matter; for it has an operation in which the body takes no part whatever. It is therefore impossible for the seminal power to produce the intellectual principle.
Again, the seminal power acts by virtue of the soul of the begetter according as the soul of the begetter is the act of the body, making use of the body in its operation. Now the body has nothing whatever to do in the operation of the intellect. Therefore the power of the intellectual principle, as intellectual, cannot reach the semen. Hence the Philosopher says (De Gener. Animal. ii, 3): "It follows that the intellect alone comes from without."
Again, since the intellectual soul has an operation independent of the body, it is subsistent, as proved above (Question 75, Article 2): therefore to be and to be made are proper to it. Moreover, since it is an immaterial substance it cannot be caused through generation, but only through creation by God. Therefore to hold that the intellectual soul is caused by the begetter, is nothing else than to hold the soul to be non-subsistent and consequently to perish with the body. It is therefore heretical to say that the intellectual soul is transmitted with the semen.
Reply to Objection 1. In the passage quoted, the part is put instead of the whole, the soul for the whole man, by the figure of synecdoche.
Reply to Objection 2. Some say that the vital functions observed in the embryo are not from its soul, but from the soul of the mother; or from the formative power of the semen. Both of these explanations are false; for vital functions such as feeling, nourishment, and growth cannot be from an extrinsic principle. Consequently it must be said that the soul is in the embryo; the nutritive soul from the beginning, then the sensitive, lastly the intellectual soul.
Therefore some say that in addition to the vegetative soul which existed first, another, namely the sensitive, soul supervenes; and in addition to this, again another, namely the intellectual soul. Thus there would be in man three souls of which one would be in potentiality to another. This has been disproved above (Question 76, Article 3).
Therefore others say that the same soul which was at first merely vegetative, afterwards through the action of the seminal power, becomes a sensitive soul; and finally this same soul becomes intellectual, not indeed through the active seminal power, but by the power of a higher agent, namely God enlightening (the soul) from without. For this reason the Philosopher says that the intellect comes from without. But this will not hold.
First, because no substantial form is susceptible of more or less; but addition of greater perfection constitutes another species, just as the addition of unity constitutes another species of number. Now it is not possible for the same identical form to belong to different species.
Secondly, because it would follow that the generation of an animal would be a continuous movement, proceeding gradually from the imperfect to the perfect, as happens in alteration.
Thirdly, because it would follow that the generation of a man or an animal is not generation simply, because the subject thereof would be a being in act. For if the vegetative soul is from the beginning in the matter of offspring, and is subsequently gradually brought to perfection; this will imply addition of further perfection without corruption of the preceding perfection. And this is contrary to the nature of generation properly so called.
Fourthly, because either that which is caused by the action of God is something subsistent: and thus it must needs be essentially distinct from the pre-existing form, which was non-subsistent; and we shall then come back to the opinion of those who held the existence of several souls in the body--or else it is not subsistent, but a perfection of the pre-existing soul: and from this it follows of necessity that the intellectual soul perishes with the body, which cannot be admitted.
There is again another explanation, according to those who held that all men have but one intellect in common: but this has been disproved above (Question 76, Article 2).
We must therefore say that since the generation of one thing is the corruption of another, it follows of necessity that both in men and in other animals, when a more perfect form supervenes the previous form is corrupted: yet so that the supervening form contains the perfection of the previous form, and something in addition. It is in this way that through many generations and corruptions we arrive at the ultimate substantial form, both in man and other animals. This indeed is apparent to the senses in animals generated from putrefaction. We conclude therefore that the intellectual soul is created by God at the end of human generation, and this soul is at the same time sensitive and nutritive, the pre-existing forms being corrupted.
Reply to Objection 3. This argument holds in the case of diverse agents not ordered to one another. But where there are many agents ordered to one another, nothing hinders the power of the higher agent from reaching to the ultimate form; while the powers of the inferior agents extend only to some disposition of matter: thus in the generation of an animal, the seminal power disposes the matter, but the power of the soul gives the form. Now it is manifest from what has been said above (105, 5; 110, 1) that the whole of corporeal nature acts as the instrument of a spiritual power, especially of God. Therefore nothing hinders the formation of the body from being due to a corporeal power, while the intellectual soul is from God alone.
Reply to Objection 4. Man begets his like, forasmuch as by his seminal power the matter is disposed for the reception of a certain species of form.
Reply to Objection 5. In the action of the adulterer, what is of nature is good; in this God concurs. But what there is of inordinate lust is evil; in this God does not concur.
Source: Summa Theologica (New Advent)