Objection 1. It would seem that the Son of God did not assume a soul through the medium of the spirit or mind. For nothing is a medium between itself and another. But the spirit is nothing else in essence but the soul itself, as was said above (I, 77, 1, ad 1). Therefore the Son of God did not assume a soul through the medium of the spirit or mind.
Objection 2. Further, what is the medium of the assumption is itself more assumable. But the spirit or mind is not more assumable than the soul; which is plain from the fact that angelic spirits are not assumable, as was said above (Question 4, Article 1). Hence it seems that the Son of God did not assume a soul through the medium of the spirit.
Objection 3. Further, that which comes later is assumed by the first through the medium of what comes before. But the soul implies the very essence, which naturally comes before its power--the mind. Therefore it would seem that the Son of God did not assume a soul through the medium of the spirit or mind.
On the contrary, Augustine says (De Agone Christ. xviii): "The invisible and unchangeable Truth took a soul by means of the spirit, and a body by means of the soul."
I answer that, As stated above (Article 1), the Son of God is said to have assumed flesh through the medium of the soul, on account of the order of dignity, and the congruity of the assumption. Now both these may be applied to the intellect, which is called the spirit, if we compare it with the other parts of the soul. For the soul is assumed congruously only inasmuch as it has a capacity for God, being in His likeness: which is in respect of the mind that is called the spirit, according to Ephesians 4:23: "Be renewed in the spirit of your mind." So, too, the intellect is the highest and noblest of the parts of the soul, and the most like to God, and hence Damascene says (De Fide Orth. iii, 6) that "the Word of God is united to flesh through the medium of the intellect; for the intellect is the purest part of the soul, God Himself being an intellect."
Reply to Objection 1. Although the intellect is not distinct from the soul in essence, it is distinct from the other parts of the soul as a power; and it is in this way that it has the nature of a medium.
Reply to Objection 2. Fitness for assumption is wanting to the angelic spirits, not from any lack of dignity, but because of the irremediableness of their fall, which cannot be said of the human spirit, as is clear from what has been said above (I, 62, 8; I, 64, 2).
Reply to Objection 3. The soul, between which and the Word of God the intellect is said to be a medium, does not stand for the essence of the soul, which is common to all the powers, but for the lower powers, which are common to every soul.
Source: Summa Theologica (New Advent)