Aquinas on the Metaphysics of the Hypostatic Union

by Michael Gorman

To have existence simpliciter, as we know from Chapter 1, is to have subsistence, i.e., existence as a supposit.

There is normally a one-to-one correlation between essences and supposits, but the incarnation provides us with an exception.

At least in the normal case, when we hear that there is another human thing in the world, we assume there is another supposit in the world.

Ideally, we could arrive at such an alternative Thomistic understanding simply by discovering it spelled out in some texts that he wrote, but this is not possible.

The first would be to say that the human nature grounds a supposit, and then to say that this human supposit gets joined to the supposit of the Word in such a way that the result is only one person: union in person, but not union in supposit. Aquinas rejects this explicitly

is to treat Christ’s human nature as an accident. If it is an accident, then it is not a supposit-grounder, and the problem cannot arise.

God cannot have accidents; second, he holds that a human nature is a substantial principle and hence not the sort of principle that can be an accident.

Accidents actualize their subjects; since God is completely actual and therefore cannot be actualized, he cannot have accidents.

the second argument would still work, and Aquinas’s use of it helps bring out how seriously he takes the full metaphysical status of Christ’s humanity, even while admitting that Christ’s human nature is like an accident in certain respects.

necessary accidental feature – it is a contingent essential feature.

nature because human natures are never supposit-grounders. All humans are supposits, of course, but not on the basis of their being human.

A third way would avoid substrate-talk and hold that humanities do not ground supposits because other substantial natures do. Socrates, according to such an account, would be a supposit by virtue of (say) being material – his humanity would serve only to humanize an already-subsistent material object.

Socrates is not human by one nature and material by another – there is just one nature by which he is both of these.

Generally speaking, he thinks, they do. So the alternative understanding we are seeking will have to say that Christ’s human nature is somehow an exception to the general rule, without its exceptional status leading to its not truly being a human nature at all.

So, generalizing from the cases of whiteness and human nature, let us say that Aquinas would seemingly accept the idea that all accidents are capable of actualizing the potentialities of substances, and all substantial natures are capable of grounding supposits.

accidents as principles that are able to actualize, whether or not they actually

substantial natures as principles that are able to ground supposits,

alternative Thomistic understanding just proposed, it is not enough merely to deny that substantial natures must ground supposits. If one said only that, then there would be no way to differentiate substantial natures from accidents, of which one should also deny that they must ground supposits. The alternative understanding also affirms that substantial natures can ground supposits. This makes them different from accidents, which not only do not ground supposits but also cannot ground supposits.

The original understanding had said that the twin functions of a human nature – making something human and making something subsist

Everything that possesses a human nature must subsist.

It might, as in Christ’s case, subsist in virtue of possessing some other nature. But that does not mean that Christ’s human nature does nothing – before the incarnation, the Son was not human, and Christ’s human nature serves for him precisely as a humanizing principle, a principle in virtue of which he is human.

in the incarnation the Son does not come to be a supposit or person, then, he does come to be human,

Because Christ’s human nature can ground a supposit, it is legitimately a human nature. That helps us to avoid Monophysitism. At the same time, Christ’s human nature does not ground a supposit. That helps us avoid Nestorianism.

As I will show later in this chapter, Aquinas holds that the reason the assumed humanity does not ground a supposit is that it, the humanity, is united in person to the Word. The humanity is a non-grounder not because it lacks something, but because of something it has, viz., union with the Word.

It could become separated from the Word, at which point it would begin to ground its own supposit.

If a human nature starts its existence without being hypostatically united to the Word, but instead as grounding some distinct supposit, then according to Aquinas, it cannot later become hypostatically united to the Word.

Given that this particular human nature is, at some time or other, united in person to the Word, it must be the case (a) that it was so joined from the very first moment of its existence, and (b) that it cannot stop being so joined without passing out of existence.

Now, if Christ’s human nature is individuated prior to the union, then it could have existed apart from the union, in which case it, that particular human nature, could have grounded a supposit.

could have grounded a supposit if it had never been united in person to the Word in the first place (and in that case, it could never have become united to the Word later on).

Christ’s human nature for Aquinas meets the alternative requirement, because it can ground a supposit; the fact that it does not in fact ground a supposit does not count against its being a true human nature. Aquinas can avoid Nestorianism without falling into Monophysitism.

substantial natures,

In both cases, it seemed there was a requirement that something actually be the case, but in light of revelation, one weakens the requirement and says that what is needed is only that something possibly be the case.

They have a function or power that they do not exercise. Similarly, Christ’s human nature has a power that it does not exercise, viz., the power to ground a supposit.

well. In the incarnation, the human nature neither grounds a supposit nor actualizes a potency of one. In the Eucharist, the accidents neither ground a supposit nor actualize a potency of one.

this respect, Christ’s human nature and the Eucharistic species appear to be very close to one another – too close, one might worry.

There is: The human nature can ground a supposit, whereas the accidents cannot; likewise, the accidents can actualize a supposit, whereas the human nature cannot. So these metaphysical principles – Christ’s human nature and the accidents in the Eucharist – ...

Normally, a substantial nature is not united to a pre-existing person.

since the Eucharistic species are not united to anything, they do not perform the task of making anything white or round. (It is worth noting, however, that this does not make them altogether causally inert – they still, for example, cause visual experiences in those who see them.)

Aquinas’s views, even if he nowhere puts it just this way, to say that the reason why Christ’s humanity is a real substantial nature is that it is the sort of thing that can ground a supposit, despite the fact that it does not do so.

Christ’s human nature is unfulfilled.

It is more dignified for something to exist in something more dignified than itself than for it to exist through itself.

Grounding a supposit is a good and fulfilling thing for a substantial nature to do, but the nature can rightly give up this fulfillment for the sake of a higher fulfillment.

(for Aquinas) no human nature in the canonical sense can ever be a person:

a subsisting human person is always more than a human nature – it has accidents, for example.

Christ’s human reality

Because this is what a human reality is, Aquinas’s reason why a human nature cannot be a person, namely, that an existing human person necessarily involves extra-essential principles, does not apply to it.

the answer to the first one will be the answer to the second and the third.

Socrates’s human reality is made up of his human nature, his whiteness, his wisdom, and so on. Do all of these together really ground Socrates, i.e., constitute him as subsisting? Not from Aquinas’s point of view. For Aquinas, only Socrates’s human nature does that. The other principles are not principles in virtue of which he subsists.

Christ’s. The following would appear to be true of any human reality other than Christ’s: (a) it contains a human nature that grounds a person; (b) it contains a human nature – the same one, of course – that constitutes that person as human; and (c) it contains additional principles that constitute that person in all the various additional ways involved in the concrete existence of a human person. It seems that for Aquinas, Christ’s human reality does (b) and (c) but not (a).

that the question we really need to answer in this section is this: why, for Aquinas, does Christ’s human nature not ground a supposit?