Thomas Aquinas, Compendium of Theology

Other theologians, wishing to avoid these absurdities, proposed that in Christ the soul was indeed united to the body, and that this union constituted a certain man who, they maintained, was assumed by the Son of God in unity of person. By reason of this assumption they said that the man in question was the Son of God and that the Son of God was that man. Further, since this assumption had unity of person as its terminus, they admitted that in Christ there was one person of God and man. But since this man who, they maintain, is composed of soul and body, is a certain suppositum or hypostasis of human nature, they place two supposita and two hypostases in Christ: one of human nature, created and temporal; the other of divine nature, uncreated and eternal.

As far as words go, this view appears to recede from the error of Nestorius. But if we examine it a little more closely, we find that it slips into the heresy identified with Nestorius.

For a person, clearly, is nothing else than an individual substance possessed of rational nature. But human nature is rational. Therefore by the very fact that a hypostasis or suppositum of human nature, temporal and created, is admitted in Christ, a person that is temporal and created is also admitted in Him. This is precisely what the name of suppositum or hypostasis signifies, namely, an individual substance. Accordingly, if these people understand what they are saying, they must place two persons in Christ when they place two supposita or two hypostases in Him.

Another consideration is the following. Things that differ as supposita exist in such a way that what is proper to one cannot belong to another. Therefore, if the Son of God is not the same suppositum as the son of man, it follows that what belongs to the son of man cannot be attributed to the Son of God, and vice versa. Hence we could not say that God was crucified or born of the Virgin: which is characteristic of the Nestorian infamy.

If anyone should undertake to protest, in reply to this, that what pertains to the man in question is ascribed to the Son of God, and conversely, because of the unity of person, even though the supposita may be different, his answer simply cannot stand. Evidently the eternal suppositum of the Son of God is nothing else than His very person. Hence whatever is said of the Son of God by reason of His person, would also be said of Him by reason of His suppositum. But what pertains to the man is not said of Him by reason of His suppositum, for the Son of God is represented as differing from the son of man in suppositum. Therefore what is proper to the son of man, such as his birth from the Virgin, his death, and the like, cannot be said of the Son of God by reason of the person.

Furthermore, if the name of God is predicated of a temporal suppositum, this will be something recent and new. But any being that is recently and newly called God, is not God unless he has been made God. What is made God, however, is God not by nature, but only by adoption. Consequently the man in question would be God, not in fact and by nature, but merely by adoption: which, again, pertains to the error of Nestorius.