Objection 1. It would seem that in Christ there was no union of soul and body. For from the union of soul and body in us a person or a human hypostasis is caused. Hence if the soul and body were united in Christ, it follows that a hypostasis resulted from their union. But this was not the hypostasis of God the Word, for It is eternal. Therefore in Christ there would be a person or hypostasis besides the hypostasis of the Word, which is contrary to 2,3.
Objection 2. Further, from the union of soul and body results the nature of the human species. But Damascene says (De Fide Orth. iii, 3), that "we must not conceive a common species in the Lord Jesus Christ." Therefore there was no union of soul and body in Him.
Objection 3. Further, the soul is united to the body for the sole purpose of quickening it. But the body of Christ could be quickened by the Word of God Himself, seeing He is the fount and principle of life. Therefore in Christ there was no union of soul and body.
On the contrary, The body is not said to be animated save from its union with the soul. Now the body of Christ is said to be animated, as the Church chants: "Taking an animate body, He deigned to be born of a Virgin" [Feast of the Circumcision, Ant. ii, Lauds]. Therefore in Christ there was a union of soul and body.
I answer that, Christ is called a man univocally with other men, as being of the same species, according to the Apostle (Philippians 2:7), "being made in the likeness of a man." Now it belongs essentially to the human species that the soul be united to the body, for the form does not constitute the species, except inasmuch as it becomes the act of matter, and this is the terminus of generation through which nature intends the species. Hence it must be said that in Christ the soul was united to the body; and the contrary is heretical, since it destroys the truth of Christ's humanity.
Reply to Objection 1. This would seem to be the reason which was of weight with such as denied the union of the soul and body in Christ, viz. lest they should thereby be forced to admit a second person or hypostasis in Christ, since they saw that the union of soul and body in mere men resulted in a person. But this happens in mere men because the soul and body are so united in them as to exist by themselves. But in Christ they are united together, so as to be united to something higher, which subsists in the nature composed of them. And hence from the union of the soul and body in Christ a new hypostasis or person does not result, but what is composed of them is united to the already existing hypostasis or Person. Nor does it therefore follow that the union of the soul and body in Christ is of less effect than in us, for its union with something nobler does not lessen but increases its virtue and worth; just as the sensitive soul in animals constitutes the species, as being considered the ultimate form, yet it does not do so in man, although it is of greater effect and dignity, and this because of its union with a further and nobler perfection, viz. the rational soul, as has been said above (2, ad 2).
Reply to Objection 2. This saying of Damascene may be taken in two ways: First, as referring to human nature, which, as it is in one individual alone, has not the nature of a common species, but only inasmuch as either it is abstracted from every individual, and considered in itself by the mind, or according as it is in all individuals. Now the Son of God did not assume human nature as it exists in the pure thought of the intellect, since in this way He would not have assumed human nature in reality, unless it be said that human nature is a separate idea, just as the Platonists conceived of man without matter. But in this way the Son of God would not have assumed flesh, contrary to what is written (Luke 24:39), "A spirit hath not flesh and bones as you see Me to have." Neither can it be said that the Son of God assumed human nature as it is in all the individuals of the same species, otherwise He would have assumed all men. Therefore it remains, as Damascene says further on (De Fide Orth. iii, 11) that He assumed human nature "in atomo," i.e. in an individual; not, indeed, in another individual which is a suppositum or a person of that nature, but in the Person of the Son of God.
Secondly, this saying of Damascene may be taken not as referring to human nature, as if from the union of soul and body one common nature (viz. human) did not result, but as referring to the union of the two natures Divine and human: which do not combine so as to form a third something that becomes a common nature, for in this way it would become predicable of many, and this is what he is aiming at, since he adds: "For there was not generated, neither will there ever be generated, another Christ, Who from the Godhead and manhood, and in the Godhead and manhood, is perfect God and perfect man."
Reply to Objection 3. There are two principles of corporeal life: one the effective principle, and in this way the Word of God is the principle of all life; the other, the formal principle of life, for since "in living things to be is to live," as the Philosopher says (De Anima ii, 37), just as everything is formally by its form, so likewise the body lives by the soul: in this way a body could not live by the Word, Which cannot be the form of a body.
Source: Summa Theologica (New Advent)