Hypostatic union

Thomas argues that “if the human nature is not united to God the Word in person, it is not united to him in any way, and thus belief in the incarnation is altogether done away with, which subverts the entire Christian faith.” But since there is a union, it must be a union that took place in the “person of the Word, and not in the nature.”

This is standard, but the reasoning that Thomas employs to reach this conclusion is delightfully medieval. If an individual of a particular kind had nothing in him than the nature of the kind, then there would be no room for distinction between the suppositum and the nature (the suppositum being that about which something can be predicated). This is the case for God, in whom essence and existence are identical.

But for all other things, composite things composed of form and matter, the suppositum must be distinguished from the nature. The two are not separate, but they are really different (and not merely difference in concept). We don’t predicate the nature of the suppositum: “this human being (suppositum) is his humanity (nature).” The suppositum instead consists of the nature of the species, and also all the other things, the accidents and the individuating principles, that appear in the individual thing. When I predicate about “George Bush,” the suppositum is not “human nature” but “human nature individuated by all the accidents of George Bush – Texan, President, Yalie, etc. In short, “whatever is in any person is united to him in person, whether it belongs to his nature or not.” If therefore the human nature of Jesus is united to the Word, it must be united in person, to form a single suppositum, a single subject about whom the gospel narratives speak.