Objection 1. It would seem that it is not befitting to a Divine Person to assume a created nature. For a Divine Person signifies something most perfect. Now no addition can be made to what is perfect. Therefore, since to assume is to take to oneself, and consequently what is assumed is added to the one who assumes, it does not seem to be befitting to a Divine Person to assume a created nature.
Objection 2. Further, that to which anything is assumed is communicated in some degree to what is assumed to it, just as dignity is communicated to whosoever is assumed to a dignity. But it is of the nature of a person to be incommunicable, as was said above (I, 29, 1). Therefore it is not befitting to a Divine Person to assume, i.e. to take to Himself.
Objection 3. Further, person is constituted by nature. But it is repugnant that the thing constituted should assume the constituent, since the effect does not act on its cause. Hence it is not befitting to a Person to assume a nature.
On the contrary, Augustine [Fulgentius] says (De Fide ad Petrum ii): "This God, i.e. the only-Begotten one, took the form," i.e. the nature, "of a servant to His own Person." But the only-Begotten God is a Person. Therefore it is befitting to a Person to take, i.e. to assume a nature.
I answer that, In the word "assumption" are implied two things, viz. the principle and the term of the act, for to assume is to take something to oneself. Now of this assumption a Person is both the principle and the term. The principle--because it properly belongs to a person to act, and this assuming of flesh took place by the Divine action. Likewise a Person is the term of this assumption, because, as was said above (2, 1,2), the union took place in the Person, and not in the nature. Hence it is plain that to assume a nature is most properly befitting to a Person.
Reply to Objection 1. Since the Divine Person is infinite, no addition can be made to it: Hence Cyril says [Council of Ephesus, Part I, ch. 26]: "We do not conceive the mode of conjunction to be according to addition"; just as in the union of man with God, nothing is added to God by the grace of adoption, but what is Divine is united to man; hence, not God but man is perfected.
Reply to Objection 2. A Divine Person is said to be incommunicable inasmuch as It cannot be predicated of several supposita, but nothing prevents several things being predicated of the Person. Hence it is not contrary to the nature of person to be communicated so as to subsist in several natures, for even in a created person several natures may concur accidentally, as in the person of one man we find quantity and quality. But this is proper to a Divine Person, on account of its infinity, that there should be a concourse of natures in it, not accidentally, but in subsistence.
Reply to Objection 3. As was said above (Question 2, Article 1), the human nature constitutes a Divine Person, not simply, but forasmuch as the Person is denominated from such a nature. For human nature does not make the Son of Man to be simply, since He was from eternity, but only to be man. It is by the Divine Nature that a Divine Person is constituted simply. Hence the Divine Person is not said to assume the Divine Nature, but to assume the human nature.
Source: Summa Theologica (New Advent)