Objection 1. It would seem that the Son of God assumed the whole human nature through the medium of its parts. For Augustine says (De Agone Christ. xviii) that "the invisible and unchangeable Truth assumed the soul through the medium of the spirit, and the body through the medium of the soul, and in this way the whole man." But the spirit, soul, and body are parts of the whole man. Therefore He assumed all, through the medium of the parts.
Objection 2. Further, the Son of God assumed flesh through the medium of the soul because the soul is more like to God than the body. But the parts of human nature, since they are simpler than the body, would seem to be more like to God, Who is most simple, than the whole. Therefore He assumed the whole through the medium of the parts.
Objection 3. Further, the whole results from the union of parts. But the union is taken to be the term of the assumption, and the parts are presupposed to the assumption. Therefore He assumed the whole by the parts.
On the contrary, Damascene says (De Fide Orth. iii, 16): "In our Lord Jesus Christ we do not behold parts of parts, but such as are immediately joined, i.e. the Godhead and the manhood." Now the humanity is a whole, which is composed of soul and body, as parts. Therefore the Son of God assumed the parts through the medium of the whole.
I answer that, When anything is said to be a medium in the assumption of Incarnation, we do not signify order of time, because the assumption of the whole and the parts was simultaneous. For it has been shown (3],4) that the soul and body were mutually united at the same time in order to constitute the human nature of the Word. But it is order of nature that is signified. Hence by what is prior in nature, that is assumed which is posterior in nature. Now a thing is prior in nature in two ways: First on the part of the agent, secondly on the part of the matter; for these two causes precede the thing. On the part of the agent--that is simply first, which is first included in his intention; but that is relatively first, with which his operation begins--and this because the intention is prior to the operation. On the part of the matter--that is first which exists first in the transmutation of the matter. Now in the Incarnation the order depending on the agent must be particularly considered, because, as Augustine says (Ep. ad Volusianum cxxxvii), "in such things the whole reason of the deed is the power of the doer." But it is manifest that, according to the intention of the doer, what is complete is prior to what is incomplete, and, consequently, the whole to the parts. Hence it must be said that the Word of God assumed the parts of human nature, through the medium of the whole; for even as He assumed the body on account of its relation to the rational soul, so likewise He assumed a body and soul on account of their relation to human nature.
Reply to Objection 1. From these words nothing may be gathered, except that the Word, by assuming the parts of human nature, assumed the whole human nature. And thus the assumption of parts is prior in the order of the intellect, if we consider the operation, but not in order of time; whereas the assumption of the nature is prior if we consider the intention: and this is to be simply first, as was said above.
Reply to Objection 2. God is so simple that He is also most perfect; and hence the whole is more like to God than the parts, inasmuch as it is more perfect.
Reply to Objection 3. It is a personal union wherein the assumption is terminated, not a union of nature, which springs from a conjunction of parts.
Source: Summa Theologica (New Advent)