which is the menstrual blood, but also
the semen, which, being mingled with that of the male, has an active
power in generation. And since in the Blessed Virgin there was no
resolution of semen, by reason of her inviolate virginity, they say
that the Holy Ghost supernaturally bestowed on her an active power in
the conception of Christ's body, which power other mothers have by
reason of the semen resolved. But this cannot stand, because, since
"each thing is on account of its operation" (De Coel. ii), nature
would not, for the purpose of the act of generation, distinguish the
male and female sexes, unless the action of the male were distinct
from that of the female. Now, in generation there are two distinct
operations--that of the agent and that of the patient. Wherefore it
follows that the entire active operation is on the part of the male,
and the passive on the part of the female. For this reason in plants,
where both forces are mingled, there is no distinction of male and
female.

Since, therefore, the Blessed Virgin was not Christ's Father, but His
Mother, it follows that it was not given to her to exercise an active
power in His conception: whether to cooperate actively so as to be
His Father, or not to cooperate at all, as some say. Whence it would
follow that this active power was bestowed on her to no purpose. We
must therefore say that in Christ's conception itself she did not
cooperate actively, but merely supplied the matter thereof.
Nevertheless, before the conception she cooperated actively in the
preparation of the matter so that it should be apt for the conception.

Reply Obj. 1: This conception had three privileges--namely, that it
was without original sin; that it was not that of a man only, but of
God and man; and that it was a virginal conception. And all three
were effected by the Holy Ghost. Therefore Damascene says, as to the
first, that the Holy Ghost "came upon the Virgin, purifying
her"--that is, preserving her from conceiving with original sin. As
to the second, he says: "And bestowing on her the power to receive,"
i.e. to conceive, "the Word of God." As to the third, he says: "And
to give birth" to Him, i.e. that she might, while remaining a virgin,
bring Him forth, not actively, but passively, just as other mothers
achieve this through the action of the male seed.

Reply Obj. 2: The generative power of the female is imperfect
compared to that of the male. And, therefore, just as in the art