Objection 1. It would seem that the Union of the Word Incarnate took place in the nature. For Cyril says (he is quoted in the acts of the Council of Chalcedon, part ii, act. 1): "We must understand not two natures, but one incarnate nature of the Word of God"; and this could not be unless the union took place in the nature. Therefore the union of the Word Incarnate took place in the nature.
Objection 2. Further, Athanasius says that, as the rational soul and the flesh together form the human nature, so God and man together form a certain one nature; therefore the union took place in the nature.
Objection 3. Further, of two natures one is not denominated by the other unless they are to some extent mutually transmuted. But the Divine and human natures in Christ are denominated one by the other; for Cyril says (quoted in the acts of the Council of Chalcedon, part ii, act. 1) that the Divine nature "is incarnate"; and Gregory Nazianzen says (Ep. i ad Cledon.) that the human nature is "deified," as appears from Damascene (De Fide Orth. iii, 6,11). Therefore from two natures one seems to have resulted.
On the contrary, It is said in the declaration of the Council of Chalcedon: "We confess that in these latter times the only-begotten Son of God appeared in two natures, without confusion, without change, without division, without separation--the distinction of natures not having been taken away by the union." Therefore the union did not take place in the nature.
I answer that, To make this question clear we must consider what is "nature." Now it is to be observed that the word "nature" comes from nativity. Hence this word was used first of all to signify the begetting of living beings, which is called "birth" or "sprouting forth," the word "natura" meaning, as it were, "nascitura." Afterwards this word "nature" was taken to signify the principle of this begetting; and because in living things the principle of generation is an intrinsic principle, this word "nature" was further employed to signify any intrinsic principle of motion: thus the Philosopher says (Phys. ii) that "nature is the principle of motion in that in which it is essentially and not accidentally." Now this principle is either form or matter. Hence sometimes form is called nature, and sometimes matter. And because the end of natural generation, in that which is generated, is the essence of the species, which the definition signifies, this essence of the species is called the "nature." And thus Boethius defines nature (De Duab. Nat.): "Nature is what informs a thing with its specific difference,"--i.e. which perfects the specific definition. But we are now speaking of nature as it signifies the essence, or the "what-it-is," or the quiddity of the species.
Now, if we take nature in this way, it is impossible that the union of Incarnate Word took place in the nature. For one thing is made of two or more in three ways. First, from two complete things which remain in their perfection. This can only happen to those whose form is composition, order, or figure, as a heap is made up of many stones brought together without any order, but solely with juxtaposition; and a house is made of stones and beams arranged in order, and fashioned to a figure. And in this way some said the union was by manner of confusion (which is without order) or by manner of commensuration (which is with order). But this cannot be. First, because neither composition nor order nor figure is a substantial form, but accidental; and hence it would follow that the union of Incarnation was not essential, but accidental, which will be disproved later on (6). Secondly, because thereby we should not have an absolute unity, but relative only, for there remain several things actually. Thirdly, because the form of such is not a nature, but an art, as the form of a house; and thus one nature would not be constituted in Christ, as they wish.
Secondly, one thing is made up of several things, perfect but changed, as a mixture is made up of its elements; and in this way some have said that the union of Incarnation was brought about by manner of combination. But this cannot be. First, because the Divine Nature is altogether immutable, as has been said (I, 9, 1; I, 9, 2), hence neither can it be changed into something else, since it is incorruptible; nor can anything else be changed into it, for it cannot be generated. Secondly, because what is mixed is of the same species with none of the elements; for flesh differs in species from any of its elements. And thus Christ would be of the same nature neither with His Father nor with His Mother. Thirdly, because there can be no mingling of things widely apart; for the species of one of them is absorbed, e.g. if we were to put a drop of water in a flagon of wine. And hence, since the Divine Nature infinitely exceeds the human nature, there could be no mixture, but the Divine Nature alone would remain.
Thirdly, a thing is made up of things not mixed nor changed, but imperfect; as man is made up of soul and body, and likewise of divers members. But this cannot be said of the mystery of Incarnation. First, because each nature, i.e. the Divine and the human, has its specific perfection. Secondly, because the Divine and human natures cannot constitute anything after the manner of quantitative parts, as the members make up the body; for the Divine Nature is incorporeal; nor after the manner of form and matter, for the Divine Nature cannot be the form of anything, especially of anything corporeal, since it would follow that the species resulting therefrom would be communicable to several, and thus there would be several Christs. Thirdly, because Christ would exist neither in human nature nor in the Divine Nature: since any difference varies the species, as unity varies number, as is said (Metaph. viii, text. 10).
Reply to Objection 1. This authority of Cyril is expounded in the Fifth Synod (i.e. Constantinople II, coll. viii, can. 8) thus: "If anyone proclaiming one nature of the Word of God to be incarnate does not receive it as the Fathers taught, viz. that from the Divine and human natures (a union in subsistence having taken place) one Christ results, but endeavors from these words to introduce one nature or substance of the Divinity and flesh of Christ, let such a one be anathema." Hence the sense is not that from two natures one results; but that the Nature of the Word of God united flesh to Itself in Person.
Reply to Objection 2. From the soul and body a double unity, viz. of nature and person--results in each individual--of nature inasmuch as the soul is united to the body, and formally perfects it, so that one nature springs from the two as from act and potentiality or from matter and form. But the comparison is not in this sense, for the Divine Nature cannot be the form of a body, as was proved (I, 3, 8). Unity of person results from them, however, inasmuch as there is an individual subsisting in flesh and soul; and herein lies the likeness, for the one Christ subsists in the Divine and human natures.
Reply to Objection 3. As Damascene says (De Fide Orth. iii, 6,11), the Divine Nature is said to be incarnate because It is united to flesh personally, and not that It is changed into flesh. So likewise the flesh is said to be deified, as he also says (De Fide Orth. 15,17), not by change, but by union with the Word, its natural properties still remaining, and hence it may be considered as deified, inasmuch as it becomes the flesh of the Word of God, but not that it becomes God.
Source: Summa Theologica (New Advent)