Aquinas on the Metaphysics of the Hypostatic Union

by Michael Gorman

For example, God is said to be “Lord” because creatures are subject to him, but this being subject is a fact in the creature, not a fact in God.

What was just said shows how Aquinas reconciles relations between creatures and God with divine impassibility.

We use temporal language about God to speak truly of changes in the relations between God and creatures, and what makes such predications true is that creatures gain and lose relational accidents. At the same time, he says, there are no relational accidents in God, so a fortiori there are no changes in such accidents.12 Before going on,

object to Aquinas’s idea that creation is “not anything” in God. If creation is not a necessary emanation from God but instead, as Aquinas himself clearly holds, something that God contingently chooses to do,

the doctrine of mixed relations

similar to creation in a very important way, but also different from it in a very important way.

the relation between the Word and the human reality it assumes is a one-way or mixed relation, with a relational accident on the human reality’s side but not on the Word’s side.

But things that are said by way of relation can be predicated newly of something without any change in it: as

it is not necessary for everything that is said to become to change:

through the change of something else.

Now to be human applies to God by reason of a union, which is a certain relation. And therefore to be human is newly said of God without any change on his part, through the change [mutatio] of the human nature that is assumed to the divine person.

First, Aquinas uses the example of someone coming to be to the right of someone else in order to bring out that becoming does not always involve mutation; this, I believe, is the only point to be drawn from the comparison.

In other words, although the example Aquinas gives involves something accidental, this does not mean that he thinks of Christ’s humanity as being accidental.

If the Word’s becoming human were an absolute becoming – if it were, for example, the Word’s acquiring of a humanity-accident – then becoming human would be a mutatio on the part of the Word.

entering into a relation with a human nature, where the relation entered into was a standard, two-way relation, then again we would have a mutatio of the Word:

But if becoming human is an entering into a relation with a human nature where the relation entered into is a mixed relation, then there can be a becoming without a mutatio on the Word’s part: There will be a relational accident in the assumed humanity, and that humanity will be subject to a mutatio, but there will be no corres...

the union between the Word and the assumed humanity is something created,

But as was said in Part I [viz. of the Summa theologiae], every relation which is understood to be between God and creatures is really in the creature, through whose mutation such a relation comes to be; but it is not in God really, but only according to reason,

this union that we are speaking about is not in God really,

Aquinas’s idea is that the union, a relational accident, is a creature, because it exists not in the divine person but instead in the humanity.

Strictly speaking, as long as any one of the conditions of a mutatio is not fulfilled, Aquinas would be justified in saying that the becoming was not a mutatio.

Condition (2) is actualization. It should be noted right away that the question here is not whether the Word is actualized by the assumed nature itself; this nature is not an accident (see Chapters 4 and 5 of this book) and hence not an actualizer of the potency of any substance or person.

The question instead is whether the Word is actualized by an accident that relates the assumed nature to the Word.

and because he places the Word on the side that does not have a relating accident, he clearly would say that the incarnation does no...

This last point is worth reflecting on in a bit more detail. We ourselves might be tempted to say that since, in the incarnation, the simple divine Word becomes composite, then obviously the incarnation is a case of change: How could going from being simple to being composite not be a change? And yet the very fact that the pre-incarnate Word is simple is, on Aquinas’s way of thinking, sufficient all by itself to rule out the incarnation’s being a mutatio. It is a case of becoming, of course, but not – in Aquinas’s special sense – a case of mutatio.

I conclude that calling the incarnation a change or mutation on the part of the assumed nature is a bit of looseness on Aquinas’s part.

“change.” It sounds odd for us to say that when God becomes human, “no change occurs.”

What we mean by “change” is pretty much what Aquinas means by becoming, fieri; there is no obvious English word that corresponds to the precise concept that Aquinas is targeting with mutatio.

the Word is so constituted, by that very nature that had been joined to him in person; and that this “change,” or “modification,” or “becoming,” or whatever it might be called, happened without any actualization of a potency in the Word and without the Word’s having been composite.

As we saw above, passibility requires at least that an entity be actualized by some form or principle and that the entity be composite. It is clear that on Aquinas’s way of thinking, while the latter of these conditions is fulfilled, the former is not. So the incarnation from Aquinas’s perspective does not involve passibility on the Word’s part.

The relation between the assumed nature and the Word is a union in person

But when God creates a cow, God does not become bovine. So it seems that from Aquinas’s perspective, the hypostatic union is a mixed relation, but a mixed relation of a special sort, inasmuch as it is a mixed relation that is also a union in person, a union that results in the person’s coming to have a new nature.

The doctrine of mixed relations

To put the point perhaps too dramatically, the doctrine of mixed relations is a way of explaining why we say that an entity has “become something new” when at the same time “nothing has happened” to it.

The difficulty is that these two needs are seemingly in tension with one another. It is as if Aquinas wants to say both that something happens to the Word and that nothing happens to the Word.

It is not via mutatio and passio, but in some other way, that the union brings this about. But what is that other way? I think Aquinas would say that we do not know.

[God] was able to make a new mode of union … although no adequate exemplar for it can be found among creatures.

But having done so, we have for him reached a point beyond which further progress is blocked. Fortunately, an incomplete account is not for that reason false or entirely useless. We have discovered a lot, and in addition we have discovered what we cannot discover. To expect more, Aquinas would say, is to misunderstand what is involved in trying to shed light on something that is ultimately beyond our vision in this life.

a supposit is constituted by two kinds of principles: nature (in the strict sense) and accident.

virtue of which substances exist in some way or

Natures for Aquinas ground their supposits, they do not actualize any potentialities of those supposits, and they are not caused by any other principles of their supposits;

accidents, on the other hand, do not ground their supposits, they do actualize potentialities of their supposits, and they are caused by essential principles of their supposits.

Christ’s humanity is a substantial nature, Aquinas would seem to be committed to four things.

Christ’s human nature makes him human.

human nature does not actualize any potentiality in him.

Christ’s human nature does not actualize any potentiality in Christ because divine persons have no potentiality

Christ’s humanity is not caused by any of his essential principles: His being divine does not cause him to be human. That too seems

fourth implication, namely, that Christ’s humanity is a supposit-grounder, a principle in virtue of which Christ subsists.

it also seems that he cannot say it. If Aquinas were to say that Christ’s human nature is a supposit-grounder, then he would be saying either that Christ is one supposit twice, which makes no sense, or else that Christ is two supposits, which is Nestorianism or near enough.

the Son of God does not have existence simpliciter from his human nature… but only existence as human.