SECOND ARTICLE [I, Q. 118, Art. 2]
Whether the Intellectual Soul Is Produced from the Semen?
Objection 1: It would seem that the intellectual soul is produced from the semen. For it is written (Gen. 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.
Obj. 2: Further, as shown above (Q. 76, A. 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.
Obj. 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.
Obj. 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.
Obj. 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."