Objection 1. It would seem that the first sin of our first parent is not contracted by others, by way of origin. For it is written (Ezekiel 18:20): "The son shall not bear the iniquity of the father." But he would bear the iniquity if he contracted it from him. Therefore no one contracts any sin from one of his parents by way of origin.
Objection 2. Further, an accident is not transmitted by way of origin, unless its subject be also transmitted, since accidents do not pass from one subject to another. Now the rational soul which is the subject of sin, is not transmitted by way of origin, as was shown in the I, 118, 2. Therefore neither can any sin be transmitted by way of origin.
Objection 3. Further, whatever is transmitted by way of human origin, is caused by the semen. But the semen cannot cause sin, because it lacks the rational part of the soul, which alone can be a cause of sin. Therefore no sin can be contracted by way of origin.
Objection 4. Further, that which is more perfect in nature, is more powerful in action. Now perfect flesh cannot infect the soul united to it, else the soul could not be cleansed of original sin, so long as it is united to the body. Much less, therefore, can the semen infect the soul.
Objection 5. Further, the Philosopher says (Ethic. iii, 5): "No one finds fault with those who are ugly by nature, but only those who are so through want of exercise and through carelessness." Now those are said to be "naturally ugly," who are so from their origin. Therefore nothing which comes by way of origin is blameworthy or sinful.
On the contrary, The Apostle says (Romans 5:12): "By one man sin entered into this world, and by sin death." Nor can this be understood as denoting imitation or suggestion, since it is written (Wisdom 2:24): "By the envy of the devil, death came into this world." It follows therefore that through origin from the first man sin entered into the world.
I answer that, According to the Catholic Faith we are bound to hold that the first sin of the first man is transmitted to his descendants, by way of origin. For this reason children are taken to be baptized soon after their birth, to show that they have to be washed from some uncleanness. The contrary is part of the Pelagian heresy, as is clear from Augustine in many of his books [For instance, Retract. i, 9; De Pecc. Merit. et Remiss. ix; Contra Julian. iii, 1; De Dono Persev. xi, xii.]
In endeavoring to explain how the sin of our first parent could be transmitted by way of origin to his descendants, various writers have gone about it in various ways. For some, considering that the subject of sin is the rational soul, maintained that the rational soul is transmitted with the semen, so that thus an infected soul would seem to produce other infected souls. Others, rejecting this as erroneous, endeavored to show how the guilt of the parent's soul can be transmitted to the children, even though the soul be not transmitted, from the fact that defects of the body are transmitted from parent to child--thus a leper may beget a leper, or a gouty man may be the father of a gouty son, on account of some seminal corruption, although this corruption is not leprosy or gout. Now since the body is proportionate to the soul, and since the soul's defects redound into the body, and vice versa, in like manner, say they, a culpable defect of the soul is passed on to the child, through the transmission of the semen, albeit the semen itself is not the subject of the guilt.
But all these explanations are insufficient. Because, granted that some bodily defects are transmitted by way of origin from parent to child, and granted that even some defects of the soul are transmitted in consequence, on account of a defect in the bodily habit, as in the case of idiots begetting idiots; nevertheless the fact of having a defect by the way of origin seems to exclude the notion of guilt, which is essentially something voluntary. Wherefore granted that the rational soul were transmitted, from the very fact that the stain on the child's soul is not in its will, it would cease to be a guilty stain binding its subject to punishment; for, as the Philosopher says (Ethic. iii, 5), "no one reproaches a man born blind; one rather takes pity on him."
Therefore we must explain the matter otherwise by saying that all men born of Adam may be considered as one man, inasmuch as they have one common nature, which they receive from their first parents; even as in civil matters, all who are members of one community are reputed as one body, and the whole community as one man. Indeed Porphyry says (Praedic., De Specie) that "by sharing the same species, many men are one man." Accordingly the multitude of men born of Adam, are as so many members of one body. Now the action of one member of the body, of the hand for instance, is voluntary not by the will of that hand, but by the will of the soul, the first mover of the members. Wherefore a murder which the hand commits would not be imputed as a sin to the hand, considered by itself as apart from the body, but is imputed to it as something belonging to man and moved by man's first moving principle. In this way, then, the disorder which is in this man born of Adam, is voluntary, not by his will, but by the will of his first parent, who, by the movement of generation, moves all who originate from him, even as the soul's will moves all the members to their actions. Hence the sin which is thus transmitted by the first parent to his descendants is called "original," just as the sin which flows from the soul into the bodily members is called "actual." And just as the actual sin that is committed by a member of the body, is not the sin of that member, except inasmuch as that member is a part of the man, for which reason it is called a "human sin"; so original sin is not the sin of this person, except inasmuch as this person receives his nature from his first parent, for which reason it is called the "sin of nature," according to Ephesians 2:3: "We... were by nature children of wrath."
Reply to Objection 1. The son is said not to bear the iniquity of his father, because he is not punished for his father's sin, unless he share in his guilt. It is thus in the case before us: because guilt is transmitted by the way of origin from father to son, even as actual sin is transmitted through being imitated.
Reply to Objection 2. Although the soul is not transmitted, because the power in the semen is not able to cause the rational soul, nevertheless the motion of the semen is a disposition to the transmission of the rational soul: so that the semen by its own power transmits the human nature from parent to child, and with that nature, the stain which infects it: for he that is born is associated with his first parent in his guilt, through the fact that he inherits his nature from him by a kind of movement which is that of generation.
Reply to Objection 3. Although the guilt is not actually in the semen, yet human nature is there virtually accompanied by that guilt.
Reply to Objection 4. The semen is the principle of generation, which is an act proper to nature, by helping it to propagate itself. Hence the soul is more infected by the semen, than by the flesh which is already perfect, and already affixed to a certain person.
Reply to Objection 5. A man is not blamed for that which he has from his origin, if we consider the man born, in himself. But it we consider him as referred to a principle, then he may be reproached for it: thus a man may from his birth be under a family disgrace, on account of a crime committed by one of his forbears.
Source: Summa Theologica (New Advent)