Objection 1. It would seem that the Son of God did not assume a human mind or intellect. For where a thing is present, its image is not required. But man is made to God's image, as regards his mind, as Augustine says (De Trin. xiv, 3,6). Hence, since in Christ there was the presence of the Divine Word itself, there was no need of a human mind.
Objection 2. Further, the greater light dims the lesser. But the Word of God, Who is "the light, which enlighteneth every man that cometh into this world," as is written John 1:9, is compared to the mind as the greater light to the lesser; since our mind is a light, being as it were a lamp enkindled by the First Light (Proverbs 20:27): "The spirit of a man is the lamp of the Lord." Therefore in Christ Who is the Word of God, there is no need of a human mind.
Objection 3. Further, the assumption of human nature by the Word of God is called His Incarnation. But the intellect or human mind is nothing carnal, either in its substance or in its act. for it is not the act of a body, as is proved De Anima iii, 6. Hence it would seem that the Son of God did not assume a human mind.
On the contrary, Augustine [Fulgentius] says (De Fide ad Petrum xiv): "Firmly hold and nowise doubt that Christ the Son of God has true flesh and a rational soul of the same kind as ours, since of His flesh He says (Luke 24:39): 'Handle, and see; for a spirit hath not flesh and bones, as you see Me to have.' And He proves that He has a soul, saying (John 10:17): 'I lay down My soul [Douay: 'life'] that I may take it again.' And He proves that He has an intellect, saying (Matthew 11:29): 'Learn of Me, because I am meek and humble of heart.' And God says of Him by the prophet (Isaiah 52:13): 'Behold my servant shall understand.'"
I answer that, As Augustine says (De Haeres. 49,50), "the Apollinarists thought differently from the Catholic Church concerning the soul of Christ, saying with the Arians, that Christ took flesh alone, without a soul; and on being overcome on this point by the Gospel witness, they went on to say that the mind was wanting to Christ's soul, but that the Word supplied its place." But this position is refuted by the same arguments as the preceding. First, because it runs counter to the Gospel story, which relates how He marveled (as is plain from Matthew 8:10). Now marveling cannot be without reason, since it implies the collation of effect and cause, i.e. inasmuch as when we see an effect and are ignorant of its cause, we seek to know it, as is said Metaph. i, 2. Secondly, it is inconsistent with the purpose of Incarnation, which is the justification of man from sin. For the human soul is not capable of sin nor of justifying grace except through the mind. Hence it was especially necessary for the mind to be assumed. Hence Damascene says (De Fide Orth. iii, 6) that "the Word of God assumed a body and an intellectual and rational soul," and adds afterwards: "The whole was united to the whole, that He might bestow salvation on me wholly; for what was not assumed is not curable." Thirdly, it is against the truth of Incarnation. For since the body is proportioned to the soul as matter to its proper form, it is not truly human flesh if it is not perfected by human, i.e. a rational soul. And hence if Christ had had a soul without a mind, He would not have had true human flesh, but irrational flesh, since our soul differs from an animal soul by the mind alone. Hence Augustine says (Qq. lxxxiii, qu. 80) that from this error it would have followed that the Son of God "took an animal with the form of a human body," which, again, is against the Divine truth, which cannot suffer any fictitious untruth.
Reply to Objection 1. Where a thing is by its presence, its image is not required to supply the place of the thing, as where the emperor is the soldiers do not pay homage to his image. Yet the image of a thing is required together with its presence, that it may be perfected by the presence of the thing, just as the image in the wax is perfected by the impression of the seal, and as the image of man is reflected in the mirror by his presence. Hence in order to perfect the human mind it was necessary that the Word should unite it to Himself.
Reply to Objection 2. The greater light dims the lesser light of another luminous body; but it does not dim, rather it perfects the light of the body illuminated--at the presence of the sun the light of the stars is put out, but the light of the air is perfected. Now the intellect or mind of man is, as it were, a light lit up by the light of the Divine Word; and hence by the presence of the Word the mind of man is perfected rather than overshadowed.
Reply to Objection 3. Although the intellective power is not the act of a body, nevertheless the essence of the human soul, which is the form of the body, requires that it should be more noble, in order that it may have the power of understanding; and hence it is necessary that a better disposed body should correspond to it.
Source: Summa Theologica (New Advent)