ch 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 Eph. 2:3: "We . . . were by
nature children of wrath."

Reply Obj. 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 Obj. 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 Obj. 3: Although the guilt is not actually in the semen, yet
human nature is there virtually accompanied by that guilt.

Reply Obj. 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 Obj. 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.
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SECOND ARTICLE [I-II, Q. 81, A