Aquinas on the Metaphysics of the Hypostatic Union

by Michael Gorman

For him, a “part” is by definition something imperfect, i.e., incomplete.

So while Christ is composite, he is not (in Aquinas’s terminology) a whole made up of parts. This sounds strange to us, but again, it is merely a verbal matter. Aquinas gives a technical meaning to the word “part” that makes it (for him) a poor word for describing Christ’s natures, but when he denies on that basis that these natures are “parts” of Christ, he is not denying what twenty-first-century English speakers would be affirming in calling them “parts.”

Although, as we have just seen, Aquinas denies that Christ’s human nature is literally a part of him, nonetheless he considers the relationship between Christ and his human nature to have a certain similarity to the relationship between a whole and a part.

It is clear enough that Aquinas uses these models – and others as well, such as the model of the union of soul and body.40 But he always holds them lightly. In his view, each has advantages and disadvantages, and none of them can ever match the incarnation precisely: As he says, citing John of Damascus, if something was exactly like the incarnation, it would be the incarnation itself, rather than a model for it.

there is more to Socrates than his human nature. He has many accidents: he is Greek-speaking, for example. He meets not only the requirements for being human but also the requirements for being a Greek-speaker. Similar things can be said of Christ: he is Aramaic-speaking, for example. He meets the requirements for that as well as the requirements for being human.

“human reality.”

And then with this terminology I will express Aquinas’s view as follows: what the Word assumes in the incarnation – the assumptum – is a full human reality, and not merely a human nature in the strict sense.

The basic notion of “union in person” is fairly straightforward. If two or more things are united in such a way that they all go to make up just one person, then the union involved is a union in person. For example, Socrates’s body and soul are united in person, and so are his soul and his left hand. By contrast, Socrates and his wife Xanthippe are not united in person, because they do not make up one person.

Could there be a theory that says that Christ is one person but two supposits?

two complete natures are brought together in such a way that they are transformed into a third nature.

First, the divine nature is altogether incapable of being transformed. Second, because such unions involve a transformation of the united elements, the result would be that Christ was neither divine nor human, which is clearly heretical. Third, this sort of mutually transforming union cannot take place when one of the items is divinity: somewhat as putting a drop of water into a large vessel of wine results (Aquinas says) in just plain wine, so too would mixing humanity with divinity result in something that was entirely divine, again giving us a heretical result.

what if perfect natures were brought together, in a non-transforming way, to make up a third nature?

But Aquinas rejects it too, for three reasons. Such a union would be merely accidental, and (as we shall see in Chapters 4 and 5) the hypostatic union cannot be an accidental union.

Or, to put it differently, the union would not be a union in person. Finally, this sort of union is really the kind of union we have in artifacts – the kind of union whereby slate and wood and metal are brought together to make a house. This, says, Aquinas, is not a union in nature at all! It is just a way of bringing two natures together, not a way of uniting them into a nature.

By denying that the two natures are united in nature, while affirming that they are united in person, Aquinas arrives at the central claim of orthodox Christology: Christ is one person who really does have two different natures, divinity and humanity.

Reflection on what is true about the case of Socrates and his whiteness creates some intellectual space for accepting that Christ has a humanity that lies outside of his divine nature,

2. It is part of Aquinas’s understanding of God that for any divine person, person and nature are identical.

So when Aquinas points out that “person” and “nature” have different significations, he is telling us that “union in person” does not mean the same as “union in nature,”

Aquinas says that Christ is God and that God is simple. He also says, as noted briefly in Chapter 2 of this book, that Christ is composite. This sounds problematic: If Aquinas thinks that simplicity is a divine attribute, how can he think that the divine Christ is composite?

The text to focus on is ST I, q. 3. Aquinas does not ask, in a generic and undifferentiated way, “Is God simple?” and then answer “Yes.” Instead, he develops a specific notion of divine simplicity that consists in the negation of certain specific kinds of composition. This is important, because if we assume that Aquinas holds that God is simple in just any sense we ourselves can imagine, then we will be likely to jump too quickly to the conclusion that he is contradicting himself when he says that Christ is composite.

Divine simplicity, understood in this way, does not exclude substantial composition.

anything with an accident

The article asks whether God is the same as his nature, and Aquinas says that he is.

that a divine person cannot have a human nature: Humanity is clearly something distinct from divinity, so it would seem that if a divine person is just the same as his own divinity, then a divine person could not have a human nature. But proper attention to the argumentation that leads up to Aquinas’s conclusion shows that this would be an over-reading.

Aquinas focuses explicitly on the contrast between natures that involve matter and natures that do not. Supposits with material natures, he says, will necessarily possess non-essential prin...

such as color;

immaterial natures, like the divine nature, are not like that. They do not give rise to or require any non-essential constituents.

ST I, q. 3, a. 3: when he says that a divine supposit is the same as its nature, he means only to deny that a divine person must necessarily, in virtue of being divine, have non-essential principles.

To make that denial is not to take any stand at all on whether a divine person can have a second substantial nature – an issue that is far from Aquinas’s mind at this point. As long as Christ’s human nature does not arise from his divinity – which, of course, it does not – that human nature’s being unit...

Based on his way of understanding divine simplicity, therefore, there is no opposition between divine simplicity and having substantial composition.

If we keep our eye on precisely what he means by divine simplicity, we will see that for him, divine simplicity is consistent with a divine person’s having two natures.

It leaves open the possibility that Christ is non-substantially composite on account of his human nature. And indeed, we know that for Aquinas, Christ most definitely is non-substantially composite on account of his humanity:

Divine simplicity does, of course, exclude non-substantial compositeness arising from divinity, but it does not exclude non-substantial compositeness arising from humanity.

On Aquinas’s way of thinking, to ascribe divine simplicity to Christ is to say that Christ has no multiplicity of constituents derived from his divine nature. That in no way excludes his having a second nature, nor does it exclude his having composition within that nature or composition of that nature with something extra-essential.

For Aquinas, a mutatio is a temporal event that meets at least the following three necessary conditions: (1) there is one entity that endures from beginning to end; (2) a potency in that entity is actualized; and (3) the entity is composite both before and after.

someone who comes to know something is perfected but not changed, strictly speaking.

This shows clearly the truth of something mentioned earlier, viz., that Aquinas’s definition of mutatio is rather technical, and definitely stricter than our everyday English notion of “change.”

I take passibility to be the same thing as mutatio but without the aspect of before-and-after, i.e., without the aspect of first-this-way-then-that-way.

For example: Aquinas holds that God could in principle have created the universe in such a way that it had always existed, i.e., such that no matter how far back we go in time, it was always there.

The planet is being illuminated by the sun, but ex hypothesi it has always been that way, i.e., it never changed so as to come to be that way. The planet’s capacity for being illumined would be actualized and would always have been actualized. This would be passio without mutatio.

it is easy to see why, according to Aquinas, God is immutable and impassible. For Aquinas, God has no potencies that can be actualized, and he is simple.

Is it not obvious that in the incarnation, the assuming person changes from not being human to being human – that he undergoes a mutation?

seems, then, that from Aquinas’s perspective, God cannot become or be incarnate.

At the same time, we should not forget something noted earlier, namely, that for Aquinas, only a divine person can have or acquire a second nature.

The key move for Aquinas has to do with relations.

But things that are said relationally can be predicated newly of something without any change in it.

Whenever the predication in question is an “absolute” one, i.e., one that is not “relational,” the becoming will indeed be a change or mutation. But when the new predication is relational, then there is at least the possibility that the becoming is not a mutation, i.e., not a change in Aquinas’s sense.

Further, we can guess that his strategy will involve an appeal to relations, because relations for him are where we can have becoming without change.

But before looking at how he applies it to the incarnation, let us look at how he applies it to a different case, namely, the case of the relations that hold between each creature and God.

When there is a relation between God and a creature, he says, the creature is the subject of a relational accident with God as its term, but there is no corresponding relational accident with God as its subject and the creature as its term.