Thomas Aquinas, Compendium of Theology

We can gather together the various points established in the foregoing chapters and assert that, according to the true teaching of Catholic faith, Christ had a real body of the same nature as ours, a true rational soul, and, together with these, perfect deity. These three substances are united in one person, but do not combine to form one nature.

In undertaking to explain this truth, some theologians have taken the wrong path. Persuaded that every perfection accruing to a being subsequent to its complete existence is joined to it accidentally, as a garment is joined to a man, certain theologians taught that humanity was joined to divinity in the person of the Son by an accidental union, in such a way that the assumed nature would be related to the person of God's Son as clothing is related to a man. To bolster up this view, they brought forward what the Apostle says of Christ in Philippians 2:7, that He was "in habit found as a man." Likewise, they reflected that from the union of soul and body an individual possessed of rational nature is formed, and that such an individual is called a person. If, therefore, the soul was united to the body in Christ, they were unable to see how they could escape the conclusion that a person would be constituted by such a union. In this event there would be two persons in Christ, the person who assumes and the person who is assumed. On the other hand, there are not two persons in a man who is clothed, because clothing does not possess what is required for the notion of a person. If, however, the clothes were a person, there would be two persons in a clothed man. To avoid this conclusion, therefore, some proposed that Christ's soul was never united to His body, but that the person of God's Son assumed soul and body separately.

This view, while trying to escape one absurdity, falls into a greater, for it entails the necessary consequence that Christ would not be true man. Surely true human nature requires the union of soul and body; a man is a being made up of both. A further consequence is that Christ would not be true flesh, and that none of His members would be a true member. For if the soul is taken away, there is no eye or hand or flesh and bone, except in an equivocal sense, as when these parts of the body are depicted in paint or fashioned in stone. Further, it would follow that Christ did not really die. Death is the privation of life. Obviously the divinity could not be deprived of life by death, and the body could not be alive if a soul were not united to it. A final consequence would be that Christ's body could not experience sensation; for the body has no sensation except through the soul united to it.

This theory falls back into the heresy of Nestorius, which it set out to overthrow. The error of Nestorius consisted in holding that the Word of God was united to Christ the man by the indwelling of grace, so that the Word of God would reside in that man as in His temple. It makes no difference, with regard to the doctrine proposed, whether we say that the Word is in the man as in a temple, or whether we say that human nature is joined to the Word as a garment to the person wearing it, except that the second opinion is the worse, inasmuch as it cannot admit that Christ was true man. Accordingly this view is condemned, and deservedly so. Moreover, the man who is clothed cannot be the person of the clothes or garment, nor can he in any way be said to be in the species of clothing. If, therefore, the Son of God took human nature to Himself, He cannot in any sense be called the person of the human nature, nor can He be said to pertain to the same species as the rest of men. Yet the Apostle says of Him that He was "made in the likeness of men" (Phil. 2:7). Clearly, therefore, this theory is to be utterly rejected.