Objection 1. It would seem that the Son of God did not assume a soul. For John has said, teaching the mystery of Incarnation (John 1:14): "The Word was made flesh"--no mention being made of a soul. Now it is not said that "the Word was made flesh" as if changed to flesh, but because He assumed flesh. Therefore He seems not to have assumed a soul.
Objection 2. Further, a soul is necessary to the body, in order to quicken it. But this was not necessary for the body of Christ, as it would seem, for of the Word of God it is written (Psalm 35:10): Lord, "with Thee is the fountain of life." Therefore it would seem altogether superfluous for the soul to be there, when the Word was present. But "God and nature do nothing uselessly," as the Philosopher says (De Coel. i, 32; ii, 56). Therefore the Word would seem not to have assumed a soul.
Objection 3. Further, by the union of soul and body is constituted the common nature, which is the human species. But "in the Lord Jesus Christ we are not to look for a common species," as Damascene says (De Fide Orth. iii, 3). Therefore He did not assume a soul.
On the contrary, Augustine says (De Agone Christ. xxi): "Let us not hearken to such as say that only a human body was assumed by the Word of God; and take 'the Word was made flesh' to mean that the man had no soul nor any other part of a man, save flesh."
I answer that, As Augustine says (De Haeres. 69,55), it was first of all the opinion of Arius and then of Apollinaris that the Son of God assumed only flesh, without a soul, holding that the Word took the place of a soul to the body. And consequently it followed that there were not two natures in Christ, but only one; for from a soul and body one human nature is constituted. But this opinion cannot hold, for three reasons. First, because it is counter to the authority of Scripture, in which our Lord makes mention of His soul, Matthew 26:38: "My soul is sorrowful even unto death"; and John 10:18: "I have power to lay down My soul [animam meam: Douay: 'My life']." But to this Apollinaris replied that in these words soul is taken metaphorically, in which way mention is made in the Old Testament of the soul of God (Isaiah 1:14): "My soul hateth your new moons and your solemnities." But, as Augustine says (Qq. lxxxiii, qu. 80), the Evangelists relate how Jesus wondered, was angered, sad, and hungry. Now these show that He had a true soul, just as that He ate, slept and was weary shows that He had a true human body: otherwise, if these things are a metaphor, because the like are said of God in the Old Testament, the trustworthiness of the Gospel story is undermined. For it is one thing that things were foretold in a figure, and another that historical events were related in very truth by the Evangelists. Secondly, this error lessens the utility of Incarnation, which is man's liberation. For Augustine [Vigilius Tapsensis] argues thus (Contra Felician. xiii): "If the Son of God in taking flesh passed over the soul, either He knew its sinlessness, and trusted it did not need a remedy; or He considered it unsuitable to Him, and did not bestow on it the boon of redemption; or He reckoned it altogether incurable, and was unable to heal it; or He cast it off as worthless and seemingly unfit for any use. Now two of these reasons imply a blasphemy against God. For how shall we call Him omnipotent, if He is unable to heal what is beyond hope? Or God of all, if He has not made our soul. And as regards the other two reasons, in one the cause of the soul is ignored, and in the other no place is given to merit. Is He to be considered to understand the cause of the soul, Who seeks to separate it from the sin of wilful transgression, enabled as it is to receive the law by the endowment of the habit of reason? Or how can His generosity be known to any one who says it was despised on account of its ignoble sinfulness? If you look at its origin, the substance of the soul is more precious than the body: but if at the sin of transgression, on account of its intelligence it is worse than the body. Now I know and declare that Christ is perfect wisdom, nor have I any doubt that He is most loving; and because of the first of these He did not despise what was better and more capable of prudence; and because of the second He protected what was most wounded." Thirdly, this position is against the truth of Incarnation. For flesh and the other parts of man receive their species through the soul. Hence, if the soul is absent, there are no bones nor flesh, except equivocally, as is plain from the Philosopher (De Anima ii, 9; Metaph. vii, 34).
Reply to Objection 1. When we say, "The Word was made flesh," "flesh" is taken for the whole man, as if we were to say, "The Word was made man," as Isaiah 40:5: "All flesh together shall see that the mouth of the Lord hath spoken." And the whole man is signified by flesh, because, as is said in the authority quoted, the Son of God became visible by flesh; hence it is subjoined: "And we saw His glory." Or because, as Augustine says (Qq. lxxxiii, qu. 80), "in all that union the Word is the highest, and flesh the last and lowest. Hence, wishing to commend the love of God's humility to us, the Evangelist mentioned the Word and flesh, leaving the soul on one side, since it is less than the Word and nobler than flesh." Again, it was reasonable to mention flesh, which, as being farther away from the Word, was less assumable, as it would seem.
Reply to Objection 2. The Word is the fountain of life, as the first effective cause of life; but the soul is the principle of the life of the body, as its form. Now the form is the effect of the agent. Hence from the presence of the Word it might rather have been concluded that the body was animated, just as from the presence of fire it may be concluded that the body, in which fire adheres, is warm.
Reply to Objection 3. It is not unfitting, indeed it is necessary to say that in Christ there was a nature which was constituted by the soul coming to the body. But Damascene denied that in Jesus Christ there was a common species, i.e. a third something resulting from the Godhead and the humanity.
Source: Summa Theologica (New Advent)