Objection 1. It would seem that the Blessed Virgin cooperated actively in the conception of Christ's body. For Damascene says (De Fide Orth. iii) that "the Holy Ghost came upon the Virgin, purifying her, and bestowing on her the power to receive and to bring forth the Word of God." But she had from nature the passive power of generation, like any other woman. Therefore He bestowed on her an active power of generation. And thus she cooperated actively in Christ's conception.
Objection 2. Further, all the powers of the vegetative soul are active, as the Commentator says (De Anima ii). But the generative power, in both man and woman, belongs to the vegetative soul. Therefore, both in man and woman, it cooperates actively in the conception of the child.
Objection 3. Further, in the conception of a child the woman supplies the matter from which the child's body is naturally formed. But nature is an intrinsic principle of movement. Therefore it seems that in the very matter supplied by the Blessed Virgin there was an active principle.
On the contrary, The active principle in generation is called the "seminal virtue." But, as Augustine says (Gen. ad lit. x), Christ's body "was taken from the Virgin, only as to corporeal matter, by the Divine power of conception and formation, but not by any human seminal virtue." Therefore the Blessed Virgin did not cooperate actively in, the conception of Christ's body.
I answer that, Some say that the Blessed Virgin cooperated actively in Christ's conception, both by natural and by a supernatural power. By natural power, because they hold that in all natural matter there is an active principle. otherwise they believe that there would be no such thing as natural transformation. But in this they are deceived. Because a transformation is said to be natural by reason not only of an active but also of a passive intrinsic principle: for the Philosopher says expressly (Phys. viii) that in heavy and light things there is a passive, and not an active, principle of natural movement. Nor is it possible for matter to be active in its own formation, since it is not in act. Nor, again, is it possible for anything to put itself in motion except it be divided into two parts, one being the mover, the other being moved: which happens in animate things only, as is proved Phys. viii.
By a supernatural power, because they say that the mother requires not only to supply the matter, 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 to Objection 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 to Objection 2. The generative power of the female is imperfect compared to that of the male. And, therefore, just as in the arts the inferior art gives a disposition to the matter to which the higher art gives the form, as is stated Phys. ii, so also the generative power of the female prepares the matter, which is then fashioned by the active power of the male.
Reply to Objection 3. In order for a transformation to be natural, there is no need for an active principle in matter, but only for a passive principle, as stated above.
Source: Summa Theologica (New Advent)