Objection 1: It would seem that the woman should not have been formed from the rib of man. For the rib was much smaller than the woman's body. Now from a smaller thing a larger thing can be made only---either by addition (and then the woman ought to have been described as made out of that which was added, rather than out of the rib itself)---or by rarefaction, because, as Augustine says (Gen. ad lit. x): "A body cannot increase in bulk except by rarefaction." But the woman's body is not more rarefied than man's---at least, not in the proportion of a rib to Eve's body. Therefore Eve was not formed from a rib of Adam.
Objection 2: Further, in those things which were first created there was nothing superfluous. Therefore a rib of Adam belonged to the integrity of his body. So, if a rib was removed, his body remained imperfect; which is unreasonable to suppose.
Objection 3: Further, a rib cannot be removed from man without pain. But there was no pain before sin. Therefore it was not right for a rib to be taken from the man, that Eve might be made from it.
On the contrary, It is written ( Gn. 2:22 ): "God built the rib, which He took from Adam, into a woman."
I answer that, It was right for the woman to be made from a rib of man. First, to signify the social union of man and woman, for the woman should neither "use authority over man," and so she was not made from his head; nor was it right for her to be subject to man's contempt as his slave, and so she was not made from his feet. Secondly, for the sacramental signification; for from the side of Christ sleeping on the Cross the Sacraments flowed---namely, blood and water---on which the Church was established.
Reply to Objection 1: Some say that the woman's body was formed by a material increase, without anything being added; in the same way as our Lord multiplied the five loaves. But this is quite impossible. For such an increase of matter would either be by a change of the very substance of the matter itself, or by a change of its dimensions. Not by change of the substance of the matter, both because matter, considered in itself, is quite unchangeable, since it has a potential existence, and has nothing but the nature of a subject, and because quantity and size are extraneous to the essence of matter itself. Wherefore multiplication of matter is quite unintelligible, as long as the matter itself remains the same without anything added to it; unless it receives greater dimensions. This implies rarefaction, which is for the same matter to receive greater dimensions, as the Philosopher says (Phys. iv). To say, therefore, that the same matter is enlarged, without being rarefied, is to combine contradictories ---viz. the definition with the absence of the thing defined.
Wherefore, as no rarefaction is apparent in such multiplication of matter, we must admit an addition of matter: either by creation, or which is more probable, by conversion. Hence Augustine says (Tract. xxiv in Joan.) that "Christ filled five thousand men with five loaves, in the same way as from a few seeds He produces the harvest of corn"---that is, by transformation of the nourishment. Nevertheless, we say that the crowds were fed with five loaves, or that woman was made from the rib, because an addition was made to the already existing matter of the loaves and of the rib.
Reply to Objection 2: The rib belonged to the integral perfection of Adam, not as an individual, but as the principle of the human race; just as the semen belongs to the perfection of the begetter, and is released by a natural and pleasurable operation. Much more, therefore, was it possible that by the Divine power the body of the woman should be produced from the man's rib.
From this it is clear how to answer the third objection.
Whether the woman was formed immediately by God?
Objection 1: It would seem that the woman was not formed immediately by God. For no individual is produced immediately by God from another individual alike in species. But the woman was made from a man who is of the same species. Therefore she was not made immediately by God.
Objection 2: Further, Augustine (De Trin. iii, 4) says that corporeal things are governed by God through the angels. But the woman's body was formed from corporeal matter. Therefore it was made through the ministry of the angels, and not immediately by God.
Objection 3: Further, those things which pre-exist in creatures as to their causal virtues are produced by the power of some creature, and not immediately by God. But the woman's body was produced in its causal virtues among the first created works, as Augustine says (Gen. ad lit. ix, 15). Therefore it was not produced immediately by God.
On the contrary, Augustine says, in the same work: "God alone, to Whom all nature owes its existence, could form or build up the woman from the man's rib."
I answer that, As was said above (A[2], ad 2), the natural generation of every species is from some determinate matter. Now the matter whence man is naturally begotten is the human semen of man or woman. Wherefore from any other matter an individual of the human species cannot naturally be generated. Now God alone, the Author of nature, can produce an effect into existence outside the ordinary course of nature. Therefore God alone could produce either a man from the slime of the earth, or a woman from the rib of man.
Reply to Objection 1: This argument is verified when an individual is begotten, by natural generation, from that which is like it in the same species.
Reply to Objection 2: As Augustine says (Gen. ad lit. ix, 15), we do not know whether the angels were employed by God in the formation of the woman; but it is certain that, as the body of man was not formed by the angels from the slime of the earth, so neither was the body of the woman formed by them from the man's rib.
Reply to Objection 3: As Augustine says (Gen. ad lit. ix, 18): "The first creation of things did not demand that woman should be made thus; it made it possible for her to be thus made." Therefore the body of the woman did indeed pre-exist in these causal virtues, in the things first created; not as regards active potentiality, but as regards a potentiality passive in relation to the active potentiality of the Creator.