But original sin can nowise be in the flesh as its subject, but only in the soul. Now in this transmission it is to be observed, that whatever accrues from the motion of the will consenting to sin, to any part of man that can in any way share in that guilt, either as its subject or as its instrument, has the character of sin. Thus from the will consenting to gluttony, concupiscence of food accrues to the concupiscible faculty, and partaking of food accrues to the hand and the mouth, which, in so far as they are moved by the will to sin, are the instruments of sin. But that further action is evoked in the nutritive power and the internal members, which have no natural aptitude for being moved by the will, does not bear the character of guilt. Consequently it follows that the flesh is the subject, not of guilt, but of punishment.
Original sin is caused by the semen as instrumental cause.
The corruption of original sin is nowise caused by God, but by the sin alone of our first parent through carnal generation. And so, since creation implies a relation in the soul to God alone, it cannot be said that the soul is tainted through being created. On the other hand, infusion implies relation both to God infusing and to the flesh into which the soul is infused. The common good takes precedence of private good. For the soul is naturally apt to be the subject of sin, in respect of those parts which can be moved by the will.