Aquinas and the unity of Christ: a defence of

compositionalism

Abstract

Thomas Aquinas is often thought to present a

compositionalist model of the incarnation, according to

which Christ is a composite of a divine nature and a human

nature, understood as concrete particulars. But he sometimes

seems to hedge away from this model when insisting on the

unity of Christ. I argue that if we interpret some of his

texts on the assumption of straightforward compositionalism,

we can construct a defence of Christ’s unity within that

context. This defence involves the claim that the divine

unity is so great, and the relation between Christ’s two

natures so unusual, that the divine unity can be transferred

to the composite Christ as a “borrowed property”.

Key words

1

Aquinas; incarnation; unity; union compositionalism;

christology

Thomas Aquinas is generally recognised as teaching a

composite model of the incarnation.1 According to such

models, the two natures of Christ (his human nature, or

humanity, and his divine nature, or divinity) are concrete

particulars, which should be conceived as proper parts of

Christ.2 Christ is thus a composite, constituted of these

1 English translations of the Summa theologiae are taken from the edition

of the Fathers of the English Dominican Province (1981) (FEDP).

Translations of the Commentary on the metaphysics are taken from Rowan

(1995) (R). Translations from De unione Verbi are my own.

2 In the terminology of modern discussions of the metaphysics of

incarnation, Aquinas is thus a “concretist”, taking “nature” to refer to

a concrete particular, rather than an “abstractist” (which would involve

taking “nature” to refer to a set of properties). The distinction goes

back to Alvin Plantinga’s analysis (1999: 183-84). It has been argued

that the distinction is really one between different ways of describing

models of the incarnation, rather than between different models

2

two parts, just as a hat is constituted of a crown and a

brim:3

The Person or hypostasis of Christ may be viewed in two ways. First as

it is in itself, and thus it is altogether simple, even as the Nature of

the Word. Secondly, in the aspect of person or hypostasis to which it

belongs to subsist in a nature; and thus the Person of Christ subsists

in two natures. Hence though there is one subsisting being in Him, yet

there are different aspects of subsistence, and hence He is said to be a

composite person, insomuch as one being subsists in two. (ST III q. 2 a.

4 in FEDP IV 2031)

themselves – see Leftow (2002: 278-79) and Marmodoro and Hill (2009:

101). It is perhaps simpler to categorise Aquinas as a

“compositionalist”. See Hill (2011: 12-14).

3 For Aquinas, Christ’s human nature is itself composite, since it

consists of a body and a soul, and for this reason he sometimes speaks

as if Christ is composed of three constituents. See, for example, De

unione Verbi, a. 2 arg. 13. However, Aquinas also notes that, in the human

nature, body and soul are not distinct substances – rather, the soul

informs the body, so that they constitute a single substance. It is thus

inaccurate to speak of Christ as composed of three substances – he is

composed of only two, one of which is metaphysically complex.

3

Although this passage is often taken as Aquinas’ classic

statement of a composite model of the incarnation, it raises

as many questions as it answers. For one thing, in it

Aquinas seems to state that Christ both is and is not

composite at the same time, something that not only seems

impossible but suggests that the model of the incarnation he

endorses is not as straightforwardly compositional as is

sometimes supposed. There is thus a problem at the very

heart of his understanding of the incarnation.

Commentators have suggested different ways of interpreting

Aquinas.4 One possible interpretation is that when Aquinas 4 It is important to note that on some interpretations, Aquinas does not

think that, in the incarnate Christ, the Son is one proper part of

Christ, alongside the human nature; rather, he thinks that the Son

expands, as it were, to contain within himself the human nature. On this

model, the whole of Christ is identical with the Son, and what has

happened is that the human nature has become a proper part of the Son,

not that the Son and the human nature have jointly become proper parts

of Christ. See, for example Thomas Flint (2011), who dubs this the

“Model T” theory and refers to the theory on which the Son and human

nature are both proper parts of Christ as the “Model A” theory. Brian

4

says Christ is composite, he means it quite literally, with

the implication that when he says Christ is not composite,

he either does not mean it literally or he means it in some

sense which is compatible with Christ’s really being

literally composite. Michael Gorman, for example, notes that

to say that Christ is composite could mean one of two

claims. The first, weaker claim is simply that Christ is not

merely divine. The second, stronger claim is that Christ

literally has a divine part and a human part. Gorman goes on

Leftow (2002 and 2011) and Oliver Crisp (2007 and 2011) have both

defended the “Model A” theory. It is questionable whether the “Model T”

theory can be called a version of compositionalism at all, depending on

how one defines “compositionalism”. In this paper, I am going to assume

that “compositionalism” means “Model A”, and I am going to interpret

Aquinas’ texts in the context of that model (in line with what I take

Eleonore Stump’s interpretation of Aquinas to be – see Stump 2003 ch.

14). As I note below, my intention is not to expound Aquinas’ theory,

but to construct a possible model of the incarnation and defence of that

model using ideas taken from Aquinas, so the strictly exegetical

question of which model Aquinas actually favoured does not affect this

aim. However, the defence I construct here can in any case probably be

restated in the context of “Model T”.

5

to argue that Aquinas’ way of defending the weaker claim

commits him to hold the stronger as well, in a consistent

and explicit way (Gorman 2000: 145-46). And Gorman argues

that because of this, Aquinas has straightforward answers to

key problems associated with the incarnation. He can say

that although God became man, divinity itself did not

change, because the divine part of Christ did not change

when it became incarnate – it simply entered into a certain

relation with a human part, and this did not involve it in

any real change itself (Gorman 2000: 146-52.).5 Such a view

depends in part upon an understanding of relations which

allows for something to enter into a relation without

undergoing change. So a composition model of the incarnation

is not, by itself, sufficient for Aquinas to avoid the

charge of imputing change to divinity. But it is necessary,

at least if he is to use a defence of this kind.

5 A similar way of explaining how the Son could become incarnate without

undergoing change can be found in Peter Abelard. See Marmodoro and Hill

(2010: 34-36).

6

We may call the model attributed to Aquinas on this view

“strict” compositionalism. However, there is an obvious

problem to overcome with such an interpretation: why does

Aquinas say in the passage with which we began that Christ

can be viewed as altogether simple? In what way can Christ

be considered a single individual? Aquinas admits, after

all, that “the human nature in Christ… is a particular

substance” (ST III q. 2 a. 3 ad 2, in FEDP IV 2030). If that

is so, there are two substances in Christ, the divine nature

and the human nature, which may be considered his

constituents. That would mean that he is not simply a

composite substance, but a composite of substances, which in

turn would imply that he is not a single individual at all.

Aquinas addresses a number of concerns along these lines

throughout the Summa theologiae III q. 2 and the short work De

unione Verbi incarnati. One of the most pressing is the subject

of ST III q. 2 a. 9, especially the third objection and its

response. The objection states:

7

… from union there results one [per unionem aliquid fit unum]. But from the

union of soul and body in us there arises what is one in person and

nature; whereas from the union of the Divine and human nature there

results what is one in person only. Therefore the union of soul and body

is greater than that of divine and human natures; and hence the union

[unio] of which we speak does not imply the greatest unity [maximam

unitatem]. (ST III q. 2 a. 9 obj. 3, in FEDP IV 2035)

The objection comes in the context of the question “Whether

the union of the two natures in Christ is the greatest of

all unions?” and clearly it is intended to support the claim

that the union is not the greatest of all unions. But it is

more problematic than that. For this objection, if it is

successful, does not merely show that the union of Christ is

not the greatest of all unions – it shows that it is

inferior to the union of any human being. In other words,

Christ lacks the unity that any human being has. But if that

is so, then serious doubt is cast over whether Christ has

sufficient unity to count as a substance at all, given that,

in the Aristotelian framework within which Aquinas works,

human beings are substances par excellence.

8

We may consider the objection which Aquinas articulates a

version of a much more general argument against composition

models. Let us call this the Plurality Argument. In its

simplest form, the Plurality Argument can be stated like

this:

(1) Where there is plurality, there is no (true)

unity.

(2) But there is plurality in Christ.

(3) Therefore there is no (true) unity in Christ.

The Plurality Argument, then, poses a serious threat to

Aquinas’ model of the incarnation. To salvage his model,

perhaps he does not have to show that the union of natures

in Christ is the greatest of all unions, as he attempts to

do in the article in question6 – but he does have to show 6 There are, of course, other reasons for Aquinas to want this union to

be the greatest of all unions – for example, if it were not, then there

would be a lack of perfection in God the Son. My point here is simply

that this is not required to answer the particular problem discussed

9

that it is at least as great as the union of body and soul in

normal human beings, or (which is the same thing, in an

Aristotelian context) the union of form and matter in normal

substances.

This is, on the face of it, an enormously difficult

challenge. The power of the Plurality Argument lies in the

fact that it attacks composition models of the incarnation

of the kind that Aquinas presents at precisely their weak

point – their difficulty in accounting for the unity of

Christ. It exposes the fact that Aquinas wants to have it

both ways. On the one hand, he wants Christ to be composite,

to safeguard the reality of his divinity and his humanity.

But on the other, he wants Christ to be a perfect unity, to

safeguard his substantial nature. And the first premise of

the Plurality Argument states that precisely these two

concerns are fundamentally irreconcilable.

here.

10

Indeed, Aquinas does deny some of the consequences that

appear to follow from the composite model that he offers.

For example, if Christ is a composite of a divine nature and

a human nature, then one would think that Christ cannot be

identical with either of these natures. But Aquinas rejects

this conclusion. He holds that when the divine nature and

the human nature are united, what results is not a new thing

– a composite of both – but something that is identical to

the Son, the divine person who is identical with the divine

nature (ST III q. 2 a. 1 ad 2).7 It would follow that, in

this case, a composite whole is identical with one of its

proper parts, which seems absurd.

Given this apparent backtracking on Aquinas’ part, some

commentators have argued that there is a fundamental

inconsistency to Aquinas’ understanding of Christ: he wants

him to be both composite and non-composite at the same time.

For example, Richard Cross portrays Aquinas’ account as

fundamentally incoherent. On this interpretation, Aquinas

7 See Weinandy (2004: 72-73).

11

goes so far as to suggest that neither of the two parts is

really a part at all. He presents the divine nature – the

Son – as complete even before he assumes the human nature,

which is tantamount not merely to identifying the divine

nature with the whole but also to denying that the human

nature is as much as a part (Cross 2002: 58-59). As if that

were not problematic enough, he also denies that Christ is

really composite anyway, suggesting that if we call him

composite that is only because he has two natures, and not

because those natures are really parts (Cross 2002: 59).8

On this interpretation, then, Aquinas resolves the problem

by abandoning the model that raises it in the first place.

He presents Christ as a composite – or like a composite in

some significant but unexplained way – when he wants to

stress the reality of his two natures. But he denies that

Christ is a composite – or like a composite in some other

way – when he wants to stress his unity.

8 See ST III q. 2 a. 4 ad 2, in FEDP IV 2031and also De unione Verbi art. 2

ad 18

12

Other commentators argue that Aquinas is not really

inconsistent, because he intends his composition account to

be taken merely as a descriptive model. It provides us with

a set of rules governing how we are to speak about Christ,

without the implication that this is what Christ is actually

like in himself. On this view, Aquinas does not believe that

Christ is really a composite, at least in the sense that his

divine and human natures are proper parts. Eleanor Stump,

for example, regards Aquinas as more consistent than Cross

does. On her interpretation, Aquinas does not backtrack from

his composition account, but he does make it clear that it

is limited. He is consistently a compositionalist, but not a

strict compositionalist. In the last analysis, Aquinas leaves

the union of divine and human a mystery (Stump 2003: 425).

The model must be interpreted as just that – a mere model,

and not an accurate description of Christ’s metaphysical

make-up. But as Cross points out, the more that is the case,

the less any sense can be made of the account (Cross 2002:

59). The less seriously Aquinas wants us to take his

13

statements about the incarnation, the less meaningful they

become.

As we saw at the beginning of this paper, even when Aquinas

is stating his composition view as clearly as he can, he

tempers it: there is a sense in which Christ is composite,

but another sense in which he is not. Both Stump and Cross

thus seem justified in thinking that Aquinas is not a

consistent strict compositionalist as Gorman suggests. If

this is so, then Aquinas is effectively answering the

Plurality argument by denying its premise (2): he denies

that there is really plurality in Christ. He does so either

by, on occasion, rejecting the composition model (in which

case he is inconsistent, since on other occasions he upholds

that same model) or by presenting it as only a descriptive

model, not to be taken literally (in which case there is

little metaphysical content to his account: it is not really

an account of the incarnation at all but a set of linguistic

rules for speaking about it). Either way, the strategy which

Aquinas apparently uses in the face of the Plurality

14

Argument involves distancing himself from the composition

model. The implication is that the composition model does

not have any resources of its own with which to construct a

reply to the argument.

But there is an alternative strategy, one that does not

involve abandoning or qualifying strict compositionalism at

all. This strategy instead involves rejecting premise (1) of

the Plurality Argument, not premise (2). In what follows, I

argue that this strategy is open to Aquinas. I do not claim

that this is necessarily the correct interpretation of what

Aquinas says. At points in what follows, the interpretation

I offer is one that is consistent with Aquinas’ texts,

interpreted in line with strict compositionalism, rather

than clearly suggested by them. My purpose is to construct a

possible defence of a composition account like the one that

Gorman attributes to Aquinas, using materials found in

Aquinas’ writings, rather than to expound Aquinas’ own

theory. I do not seek to show that Aquinas himself did hold

a strict composition account, as Gorman argues, or that,

15

even if he did, he would agree with the defence outlined in

this paper. But I do suggest that it can be devised using

Thomist ideas, and is of philosophical interest in its own

right.

The ninth article of q. 2, which contains the formulation of

the Plurality Argument which we saw earlier, also contains

the seeds of this defence. In the main section of that

article, Aquinas writes:

…this union [unio] has a pre-eminence over other unions; for the unity

[unitas] of the Divine Person, in which the two natures are united, is

the greatest. (ST III q. 2 a. 9, in FEDP IV 2035)

Here, Aquinas tells us that the two natures of Christ are

united in the Divine Person, that is, the Son. It may seem

that this is one of those passages in which he modifies or

even abandons his compositionalism: if the natures are

united in the Son, then the Son cannot be merely a proper

part of Christ. To determine whether Aquinas did mean this

16

passage in a way consistent with compositionalism lies

outside the scope of this paper: as stated above, my purpose

is not to expound his theory, but to construct a defence

against the Plurality Argument using key ideas in his texts.

On this basis, I suggest that we interpret this passage

within a strictly compositionalist framework, and see what

follows. Can it be made consistent with that framework?

If we interpret it in this way, the Divine Person of which

Aquinas speaks here must be understood as one of the

components of Christ, namely his divine nature. And on this

interpretation, Aquinas is stating that not only is there

union in Christ, but it derives from the unity of this

components. In other words, the reality of Christ’s unity as

a whole can be guaranteed in a way that does not involve

retreating from the claim that Christ is composite. On the

contrary, Christ’s unity can be affirmed in conjunction with his

composite nature. The reason is that one of Christ’s

components has a property which is such that having a

component with that property confers unity upon the whole.

17

Unity, in other words, can be achieved through composition

(under certain conditions), without having to abandon

composition.

How does this work? To construct the defence, we first need

to understand the two key terms that feature in Aquinas’

text, unio (union) and unitas (unity).

Aquinas and unity

Aquinas’ understanding of unity takes its cue from

Aristotle’s in the Metaphysics. There are two sections of

that work which are particularly relevant; Aquinas takes

ideas from both and combines them to create his own account

of identity.

The first relevant section is Metaphysics 3.2, where

Aristotle argues that being and unity are different concepts

but the same thing. Christopher Kirwan suggests that

Aristotle is arguing that being and unity have the same

18

truth conditions: a thing is one thing precisely when it is

one thing, and vice versa (Kirwan 1971: 82). Aquinas

interprets Aristotle as arguing for the stronger thesis that

being and unity are simply the same thing, although they are

19

understood in different ways.9 So unitas is not an accident.10

It is not as if one can take a substance and add unitas to

it, as one could add colour or size.11 To the extent that a

thing is a substance, it has unitas already.

9 “…things can be one in two ways; for some things are one which are

associated as interchangeable things, like principle and cause

[principium et causa]; and some are interchangeable not only in the sense

that they are one and the same numerically [or in subject] but also in

the sense that they are one and the same conceptually, like garment

[vestis] and clothing [idumentum]. Now the terms one [unum] and being

[ens] signify one nature according to different concepts [secundum diversas

rationes], and therefore they are like the terms principle and cause, and

not like the terms tunic and garment, which are wholly synonymous.”

(Commentary on the Metaphysics book 4, lesson 2 548-49 in R 203)

The example of “principle” and “cause” is Aristotle’s (1003b24, in

Kirwan 1971: 2), but the contrasting example of “garment” and “clothing”

is Aquinas’. It is clear from the way he contrasts the first example

with the second that, in both, he is thinking of cases where different

words refer to the same thing. The difference is that with “garment” and

“clothing” there are merely different words, whereas with “principle”

and “cause” there are different concepts as well. To call something a

“principle” and to call it a “cause” are to think of it in two different

20

The second relevant passage of Aristotle is Metaphysics 5.6.

In that chapter, Aristotle distinguishes between a number of

different senses in which something may be said to be “one”.

Does this mean that “one” is used equivocally? Aquinas

thinks not. Citing 1016b3-1017a6 (Kirwan 1971: 37-38),

Aquinas tells us that Aristotle’s aim is to identity a

feature which all the different kinds of unity have in

common. This feature is indivisibility.12

ways. Nevertheless, the same thing is thought of in each case. A person

who speaks of a “principle” and one who speaks of a “cause” are speaking

about the same object, although they apply different concepts to it as

well as different words. This is much more explicit in Aquinas’

discussion than it is in Aristotle’s.

10 It is worth noting that, for Aquinas, unum and unitas seem to be very

closely connected: something is unum, or one, to the degree that it has

(or is a) unitas, or unity. When discussing the meaning of unum, for

example, he says: “Si autem sit omnino indivisibile et secundum

quantitatem et non habeat positionem, dicitur unitas” (Commentary on the

Metaphysics book 5, lesson 8, 874, in R 316).

11 Aquinas attributes that view to Avicenna (Commentary on the metaphysics

book 4, lesson 2 556 in R 204; see also ST I q. 11 a. 1 ad .1).

21

Aquinas goes on to combine the two ideas, drawn from

different parts of the Metaphysics, about unity. From

Metaphysics 3.2 he takes the idea that unity is being, and

from Metaphysics 5.6 he takes the idea that unity is

indivisibility, and he concludes that unity is ens indivisum,

indivisible being. To have unity is precisely the same thing

as to exist indivisibly. Moreover, he asserts that this is a

12 “[Aristotle] accordingly says, first, that it is evident from what

precedes that things which are indivisible in every way are said to be

one in the highest degree [illa quae sunt penitus indivisibilia, maxime dicuntur unum].

For all the other senses in which things are said to be one are

reducible [reducuntur] to this sense, because it is universally true that

those things which do not admit of division are said to be one insofar

as they do not admit of division [quaecumque non habent divisionem, secundum

hoc dicuntur unum, inquantum divisionem non habent].” (Commentary on the Metaphysics

book 5, lesson 8, 866, in R 315)

This certainly goes beyond what Aristotle says explicitly in the passage

under consideration. In particular, Aquinas not only states that unity

is a matter of being indivisible, but introduces a sort of hierarchy of

unity: the more divisible something is, the more unity it has, something

that Aristotle does not say.

22

matter of degree: the more indivisibly something exists, or

the more indivisible its being is, the more unity it has. We

find all of these ideas in ST I q. 11:

One [unum] does not add any reality to being [ens]; but is only a

negation of division; for one means undivided being [ens indivisum] This is

the very reason why one is the same as being. Now every being is either

simple or compound. But what is simple is undivided, both actually and

potentially. Whereas what is compound, has not being whilst its parts

are divided, but after they make up and compose it. Hence it is manifest

that the being of anything consists in undivision [indivisione]; and hence

it is that everything guards its unity [unitatem] as it guards its being

[esse]. (ST I q. 11 a. 1, in FEDP I 45)

Aquinas goes on to note that God has unity to a supreme

degree, something which follows quite closely from his

conception of what unity is (ST I q. 3 a. 7, in FEDP I 19).

Aquinas thinks not only that something has unity in virtue

of being indivisible, but that the more indivisible

something is, the more unity it has. It follows from this

that if God is perfectly indivisible, he must have perfect

23

unity. He also seems to think that the more being something

has, the more unity it has. This premise is not explicit in

Aquinas but it is required for the following argument to

work:

Since one [unum] is an undivided being [ens], if anything is supremely

one it must be supremely being, and supremely undivided [indivisum]. Now

both of these belong to God… Hence it is manifest that God is one in the

supreme degree [maxime unus]. (ST I q. 11 a. 4, in FEDP I 48)

This argument can be restated like this. Unity has two

components: being, and indivisibility. Unity comes in degrees, in

virtue of both of these components: the more being something

has, the more unity it has, and the more indivisible

something is, the more unity it has. Therefore, anything

that has more being than anything else and is more

indivisible than anything else must have more unity than

anything else. But God has more being, and is more

indivisible, than anything else, so it follows that he has

more unity than anything else.

24

Aquinas and union

That, then, is what unitas, or “unity”, means. What about

unio, or “union”? Aquinas indicates what he understands by

this in the course of his discussion of the Trinity. He

remarks, first, that “unity is said absolutely, as it does

not presuppose anything else; and for this reason it is

appropriated to the Father” (ST I q. 39 a. 8, in FEDP I

202). It is perhaps trite to observe that to have one thing

you do not need any other things, but it does reflect what

we have already seen. To meet the two criteria of unity – to

exist, and to be indivisible – a thing needs nothing outside

itself. These are entirely intrinsic, non-relational

properties.

Aquinas then moves on to “union”:

25

Union [connexio] implies the unity [unitatem] of two; and is therefore

appropriated to the Holy Ghost, inasmuch as He proceeds from two. (ST I

q. 39 a. 8, in FEDP I 202) 13

Unio, then, is unitas when applied to more than one thing.

Unio is to be understood as a sort of unitas. In some ways,

this assimilation of unio to unitas makes sense. It explains

the two features of Aquinas’ use of these terms in ST III q.

2 a. 9: their close association (there is unio to Christ

because there is unitas to his divine nature) and also their

distinction (unitas is a feature of the divine nature, while

unio is a feature of Christ as a whole).

13 The word here translated as “union” is not unio but connexio. This word

is closely linked to nexus, which Aquinas uses frequently in this

question to refer to the Holy Spirit, who is the nexus or bond between

the Father and the Son (see, for example, ST I q. 39 a. 8: “spiritu

sancto… est duorum nexus”). In his commentary on the Sententiae, Aquinas

also uses the word unio in the same context. The implication that unio

and nexus are interchangeable is confirmed by his frequent use there of

the phrase “nexus vel unio” (Super Sent., lib. 1 d. 10 q. 1 a. 3). So

according to this passage, unio, connexio, and nexus all refer to the same

thing: “unitas of two”.

26

But in other ways, Aquinas’ assertion that unio is a kind of

unitas is extraordinary. For one thing, we have just seen how

in the passage from ST I q. 39 a. 8 Aquinas specifies that

an object has unitas absolutely, in virtue of its own

properties, without requiring anything else. Yet he then

goes on to tell us that unio is unitas of two things! Surely

there is a contradiction here. If unitas requires nothing

else, how can it involve more than one thing?

We can resolve this problem if we note Aquinas’ phrasing:

“Connexio autem importat unitatem aliquorum duorum.”

“Unitatem” is singular. Union does not come about simply when

two things, each with unitas, are brought together. There has

to be a single unitas between them. In the case of Christ,

Aquinas coins the term “double unity” (duplica unitate) to

express the union of his two natures, the point being that

although two natures are united, the union itself is

singular:

27

From the fact that a created unity is not an uncreated unity, one cannot

conclude that Christ is two; but that he subsists in a double unity [in

duplici unitate], just as he subsists in a double nature [in duplici natura]. (De

unione Verbi art. 3 ad 7)

This resolves the apparent contradiction. Whether or not a

thing has unitas is a matter of its own, non-relational

properties – external objects are not involved. And the same

is true in the case of a union. Two (or perhaps more)

objects are involved a union, but the unitas in virtue of

which they constitute a union is a shared unitas – a unitas that

they have jointly, considered as a single subject. And,

again, the existence of this unitas is a matter of the

properties that the united objects have jointly – their

existence and their indivisibility. Nothing outside the union

is implicated.14

14 We may note another problem. Aquinas wants there to be two kinds of

unity: there is unity by simplicity, which is unitas, and unity by

composition, which is unio. As he puts it elsewhere:

“Something is properly called ‘united’ [unitum] according to a union

[unionem], just as something is called ‘one’ [unum] according to a unity

28

Unity and union in the incarnation

How, then, are these concepts applied to the incarnation?

Aquinas distinguishes three different ways in which unio can

[unitatem]”. (De unione Verbi art. 1 ad 5)

But unio is defined as a kind of unitas. And unitas is defined as depending

upon indivisibility. But a composite thing is, by definition, not

indivisible. So it seems that although the component parts of a

composite thing may themselves have their own unitas, the composite thing

itself cannot, because it lacks indivisibility. In which case, a

composite thing cannot have unio, if that is a kind of unitas. But unio is

supposed to be a feature of composite things. Clearly there is a problem

here. Indeed, Aquinas’ definition of union as “unitas of two” seems,

given his definition of unitas, to be paradoxical to the point of

incoherence. Something that is composed of two things cannot be

indivisible.

Aquinas does not offer a clear solution to this problem. But we have

seen that Aquinas thinks that indivisibility comes in degrees.

Presumably, then, one might say that two things are united to the extent that

they are indivisible. That would imply that the union of two things can

29

come about in the normal run of things, but rejects them all

as viable models for the unio of the incarnation.15 Little

wonder that he insists that “to explain this union perfectly

is beyond man’s strength” (Contra Gentiles, lib. 4 cap. 41 n. 9

in O’Neil 1975: 195). Eleonore Stump argues accordingly that

Aquinas’ metaphysics, which offers only these possibilities

never be a perfect unity, because as long as there are two things, they

are not perfectly indivisible. This solution may work for ordinary kinds

of union, but it will not work for the incarnation, because Aquinas

holds that the incarnation has perfect union and perfect unity. We shall

see in the next sections, however, that a way is open for Aquinas to

attribute perfect union and perfect unity to Christ.

15 “…one thing [unum] is made of two or more in three ways. First, from

two complete things [perfectis] which remain in their perfection. This can

only happen to those whose form is composition [compositio], order, or

figure, as a heap is made up of many stones brought together without any

order, but solely with juxtaposition; and a house is made of stones and

beams arranged in order, and fashioned to a figure… Secondly, one thing

is made up of several things, perfect but changed, as a mixture is made

up of its elements… Thirdly, a thing is made up of things not mixed nor

changed, but imperfect; as man is made up of soul and body, and likewise

of divers members.” (ST III q. 2 a. 1, in FEDP IV 2027)

30

of unio, is intrinsically incapable of explaining how the

divine and human natures in Christ can be united (Stump

2003: 424-25). She defends Aquinas on the grounds that, in

his view, the unio of the two natures in Christ is unique,

and so one could hardly expect it to be explicable in terms

of any other union.

We may add that Aquinas’ analysis implies a fourth kind, a union where

one thing is perfect (that is, whole and complete) and remains perfect,

but where the thing to which it is united is imperfect. An example of

this would be the union of a substance and an accident, since a

substance is a perfect thing in this sense (it is whole and complete

even before it receives the accident) but an accident is not (it is not

a whole thing in its own right). A union of this kind is distinct from

the three listed above, because it does not change the nature of the

substance that acquires the accident (ST III q. 2 a. 6 ad 2). The thing

which has unitas after this union has taken place is the same thing that

there was before. This is not the case with the three kinds of union

listed above, in which the thing which has unitas after the union did not

exist before the union – whether it be a heap, a mixture, or a substance

composed of form and matter. This is why Aquinas considers the union of

substance and accident in a separate discussion, in article 6, later in

the same question.

31

But to do that is really to leave the union entirely

unexplained. It is simply to assert that there exists

perfect union between Christ’s two natures, without doing

anything to address the difficulties that such a claim

Aquinas’ reasons for rejecting the idea that any of these four kinds of

union apply in the incarnation are well known. The first kind of unio

that he considers in is not an absolute unio, and is in any case

accidental in nature. The second is impossible, since it involves mixing

two elements together and changing them to create a new one – but the

divine nature cannot be changed. And besides, the result of such a unio

is a new nature which is not identical to either of the originals – but

Christ is supposed to retain both the divine and human natures. The same

consideration applies to the third kind of unio, which is also

impossible since both the divine and human nature are complete things in

their own right, unlike form and matter. In short, the attempt to

explain the unio of Christ as any kind of unio of nature is the error of

Eutyches, who, in Aquinas’ interpretation, advocated a unio by mixing,

or Apollinarius, who advocated a unio of form and matter (De unione Verbi,

a. 1; Contra Gentiles, lib. 4 cap. 41 n. 4).

As for accidental unio, that is also disposed of quickly. The relation

of Christ’s human nature to his divine nature cannot be the relation of

32

brings up. I suggest that we can draw from Aquinas’ texts

another way of securing unio in a composite – through the

unitas of one of its components. As we have seen, the third

objection in ST III q. 2 a. 9 is a version of the Plurality

Argument. Aquinas’ response to that objection is brief:

The unity [unitas] of the Divine Person is greater than the unity of

person and nature in us; and hence the union [unio] of Incarnation is

greater than the union of soul and body in us. (ST III q. 2 a. 9 ad 3,

in FEDP IV 2036)

As we saw earlier, Aquinas’ comments in this article may be

seen as a retreat from compositionalism. But we may choose

to read them within a strictly compositional framework, in

which case the Divine Person referred to here would be a

proper part of Christ, his divine nature. On such an

accident to substance, because his human nature is a substance itself

(De unione Verbi, a. 1; Contra Gentiles, lib. 4 cap. 41 n. 5-8; ST III q. 2

a. 6 con.). And it cannot be an accidental relation between two

substances, because that is not a true union at all: such a Christ would

really be two people, and this is the error of Nestorius (Contra Gentiles,

lib. 4 cap. 41 n. 6).

33

interpretation, the “hence” (et ideo) would indicate that the

greatness of the unio of the two components derives from the

greatness of the unitas of one of these components, namely

the divine nature. In other words, the divine nature has

supreme unitas, and because of this, the composite of which

that divine nature forms a part has supreme unio. The unio of

the composite must, in this case, derive in some way from

the unitas of its part. As we have seen, there is good reason

for thinking that Aquinas holds that unio is a species of

unitas. So on a strictly compositionalist reading of this

passage, Aquinas is saying that the unity that is already a feature

of the divine nature is transferred to the composite Christ, of

which the divine nature is just a part.

As I have indicated, I do not claim that this is Aquinas’

meaning – merely a possible meaning based on giving this

text a strictly compositionalist reading. It is important to

recognise that such a reading offers a means of defending

the unity of Christ which is based upon the claim that he is

genuinely composite. Rather than backtracking from the

34

composite model, and claiming that Christ has unity because

he is not really composite in some way, the compositionalist

could affirm Christ’s unity through his composite nature.

Christ’s unity derives from the unity of one of his parts.

In the case of Christ, a thing with very strong unity, plus

another thing, results in a composite that has very strong

unity (which we call union, because it is the unity of a

composite thing). It seems, then, that for the defence to

work, unity would have to be a “borrowed” property, that is,

a property that the whole has in virtue of the fact that one

of its parts does. My property of having a nose is a

borrowed property, because my head has the property of

having a nose, and my head is part of me. Similarly, the

proponent of this defence would hold that the divine nature

has the property of being a (supreme) unity; the divine

nature is part of Christ; Christ therefore has the property

of being a (supreme) unity.16

16 See Stump (2003: 412-14) on borrowed properties. Stump, like most

authors who consider borrowed properties in the context of the

incarnation, appeals to them as part of a “reduplicative strategy” of

attributing certain properties to Christ qua man and other, apparently

35

We can summarise this argument like this:

(1) In Christ, if his divine nature has unitas to a

superlative degree, the whole has unio to a superlative

degree.

(2) Christ’s divine nature does have unitas to a superlative

degree.

(3) Therefore, Christ has unio to a superlative degree.

We can call this the Borrowed Property Argument. Its premise

(1) is a rejection of premise (1) of the Plurality Argument.

There is plurality in a composite whole, but this premise

claims that there are cases where a composite whole can

nevertheless have unio – which is what the first premise of

the Plurality Argument denies. This, then, is the key claim

around which the defence revolves.

contradictory, properties to him qua God. For more on this, see Leftow

(2002: 288-91). Here, however, Aquinas is putting the same idea to a

quite different purpose, even though it is still within the context of

the incarnation.

36

Relations in the incarnation

How can unity be a borrowed property? It seems, on the face

of it, an implausible supposition. If we accept the

principle that a whole can have unity simply in virtue of

containing a part which has unity, then any mereological sum

that contains God will have unity, which is absurd. Simply

having a part that has unity, then, is not enough for the

whole to borrow that unity. So what is special about Christ

that he can borrow the unity of his divine nature?

We can construct a possible answer to this question using

two claims to which Aquinas is committed: the first

concerning the relation that holds between Christ’s two

natures, and the second concerning the nature of the unity

that the divine nature has. In Aquinas’ view, both of these

features of the incarnation are unique to Christ, and are

not replicated in any other human person or indeed in any

created thing. I argue that these two features provide

37

conceptual space to show why there is a unique circumstance

to the incarnation which could allow unity, in this case, to

become a borrowed property.

We can begin with the most obvious point, which is that

Christ, on Aquinas’ account, is not a mere mereological sum.

The divine and human natures are related to each other in a

certain way which makes them a composite. This compositeness

is a necessary condition for the property of unity to be

transferred from the part to the whole, in a way that does

not occur in a mereological sum, a heap, or an accidental

union. And this compositeness is based upon a mixed

relation. For Aquinas, as for most scholastics, relations

are properties (Henninger 1989: 4-6). That is, to say that a

certain relation exists between A and B is to say that A has

a relational property whose term is B, and B has a

relational property whose term is A. For example, “is the

father of Isaac” is a predicate that expresses a relational

property whose term is Isaac, while “is the son of Abraham”

is a predicate that expresses a relational property whose

38

term is Abraham. Abraham instantiates the first of these

properties and Isaac instantiates the second. Aquinas notes

that relations that involve such properties are real, while

those that do not are mere beings of reason, having no

extra-mental reality – for example, if somebody compares one

thing to another, the things being compared do not really

have relational properties, and the relation of comparison

exists only in the mind of the person doing the comparing

(ST I q. 28 a. 1).

However, Aquinas also believes that there are relations

where A has a relational property whose term is B, but B has

no corresponding relational property whose term is A. The

prime example is relations of knowledge or perception: if I

think about or perceive an object, I have a relational

property whose term is the object of my knowledge or

perception, but there is no relational property in the

object whose term is me. Aquinas believes that all relations

between God and creatures are “mixed” relations of this

kind: creatures can have relational properties whose term is

39

God, but God has no relational properties whose term is a

creature (ST I q. 13 a. 7).

This holds in the incarnation as well. When the two natures

of Christ are united, the relation between them is based

upon a relational property in the human nature, not in the

divine nature (ST III q. 2 a. 7 ad 1, in FEDP IV 2034).17 So

for Aquinas, the union between the divine and human natures

in Christ is based upon a property of the human nature, not

a property of the divine nature. But the Borrowed Property

Argument reverses this. According to that argument, the pre-

eminence of the union in Christ is based upon a property of

the divine nature, not a property of the human nature. Is

this a contradiction?

On the contrary, it sheds light on the problem. Thomas

Weinandy points out that Aquinas has an answer to a common

criticism of his understanding of the relation between the

two natures as a mixed one. The criticism is that if the

17 On this, see Gorman (2000: 147-52).

40

humanity has a relational property whose term is the

divinity, but the divinity has no corresponding relational

property whose term is the humanity, then the divinity is

not really related to the humanity at all; the relation is

“outside” the divine nature. The response is that the

divinity really is related to the humanity, in virtue of the

relational property that is in the humanity. Weinandy

writes:

Aquinas neither means that the Son is not actually related to the

humanity and thus is not actually man, nor that he is only conceived to

be related by our way of thinking, but actually is not… What Aquinas

does mean is that the Son is understood to be related, and is actually

and authentically related, not by some effect or change in him, but

because the humanity is really related to him as he actually exists as

God. It is because the humanity is really related to the Son as the Son

is that the Son becomes and is man, and we thus understand him in a new

way, that is, as man. (Weinandy 2004: 78)

The point is that, for Aquinas, a mixed relation is a real

relation. Each of the related things is really related to the

41

other, even though only one of them has a relational

property. It is enough for A to have a relational property

whose term is B for A and B to be genuinely united to each

other, and for it to be really true that B is related to A,

even if B has no relational property whose term is A. This

means that a unifying relation can hold between the divine

nature and the human nature, in virtue of which the divine

nature is really united to the human nature, even though

only the human nature has a relational property.

We may reason in a parallel fashion with the Borrowed

Property Argument. Just as A and B may be said to be related

to each other in virtue of a relational property in just one of

them, even in the absence of a corresponding relational

property in the other, so too the composite AB may be said

to have the property of unio in virtue of just one of them

having the property of unitas.

There is a neat complementary structure to the argument. If

we focus solely on the question of the relation between the

42

divine and human natures, then it seems that all the work is

being done by the human nature. It is because the human

nature has a relational property that the two of them are

related at all. But the Borrowed Property Argument redresses

the balance. If we accept this argument, it is because the

divine nature has such unity that there is such a union

between the two of them. If the human nature brings

relatedness to the table, the divine nature contributes unity.

They both do metaphysical work to support the reality of the

hypostatic union.

The supposit and the whole

So there is symmetry in the union of Christ, according to

the defence we are constructing. It is in virtue of a

property of the human nature that the two natures are

united, and it is in virtue of a property of the divine

nature that they jointly have unity. The former is a

relational property and the latter is a non-relational

property. But there is more: both of these properties are

43

unique. As we have seen, the relational property that the

human nature has is a relation of “being united to” unlike

any relation found in the created order. This provides the

conceptual space to allow for this relation to have a unique

feature, namely the ability to allow the property of unity

which the divine nature has to become borrowed.

We can construct an argument for this on the basis of what

Aquinas says in De unione Verbi art. 3. Here, Aquinas argues

that Christ is one in the supposit and two in the natures.

That is, Christ is not simply one person, but one supposit.

This supposit is identical with his divine nature. Aquinas

states that the term suppositum is equivalent to the Greek

hypostasis, and means simply an individual substance (De unione

Verbi art. 2).18 Usually, the supposit of an individual

substance is that individual substance itself, or to put it

another way, every individual substance is a supposit.

Little wonder, then, that on Aquinas’ account Christ turns

18 See Stump (2003: 50-51).

44

out to be a single supposit – if he were not, he would not

be a single individual.

Now Aquinas specifies that this has important consequences

for the borrowing of properties (De unione Verbi art. 2). He

tells us that if something has a property in virtue of its

part having that property, it has it in only a qualified

way:

…if [one thing] belongs to [another thing] only according to the part,

it is said to belong to it according to something, and not simply. It is

as if an Ethiopian who has white teeth is [himself] called “white”. It

is different for something which is not said to be except according to a

part; as when someone who has curly hair is simply called “curly”. (De

unione Verbi art. 3)

So here Aquinas tells us that borrowed properties – that is,

properties which a composite is said to have because it has

a part which has them – are genuinely attributed to the

composite, but only in a qualified way. And he contrasts

properties which are attributed to a whole in virtue of its

45

parts with properties which are attributed to a whole in

virtue of itself:

We must know that a thing is said to be such-and-such simply and

properly when it is such-and-such according to itself [seipsum]. But

something is said to be such-and-such according to itself when it is

according to the whole [secundum totum], more than when it is according

to a part, because a part is not simply the same thing as a whole. (De

unione Verbi art. 3)

So far, all seems straightforward. But the key element in

Aquinas’ argument comes next, which is that in this context,

the supposit functions as a whole:

It is clear, however, that the supposit is signified in the way of a

whole [per modum totius], but the nature in the way of a formal part [per

modum partis formalis], as was shown above. (De unione Verbi art. 3)

Usually, “supposit” just means “individual substance”, so

the supposit of an individual substance, whether that

individual is simple or compound, will be the whole, and not

a part. So whatever property a substance has in virtue of

46

the fact that its supposit has it, it has simply and without

qualification. In Christ, however, things are rather

different. In his case, his divine nature is his supposit.

In the context of discussing whether Christ’s human nature

constitutes a supposit, Aquinas concludes:

So it follows, because the human nature in Christ does not exist through

itself, separately, but exists in something else, that is, in the

hypostasis of the Word of God… so the human nature in Christ certainly

can be called some individual or particular or singular, but it cannot

be called a hypostasis or supposit, just as it cannot be called a

person. So it follows that there is nothing but a single hypostasis or

supposit in Christ, and that is the divine Word. (De unione Verbi art. 2)

Aquinas’ main point there is that Christ’s human nature is

not a supposit. But his discussion makes it clear that the

divine nature is a supposit. The only candidates for the

rank of supposit that Aquinas considers are the divinity and

humanity themselves. Since the humanity cannot be a

supposit, Aquinas concludes that there is a single supposit

in Christ, which is identical with the divinity.

47

There are two ways we could interpret this. The first is to

suppose that Aquinas has simply abandoned the composite

model at this point. After all, if “supposit” means

“individual substance”, then to say that the divine nature –

the Word – is the supposit of Christ is simply to say that

the Word is Christ himself. The Word is not a part of Christ

– he is the whole Christ. If we adopt this interpretation,

then Aquinas’ account of the incarnation is fundamentally

inconsistent, given that, as we have seen, he also states

that Christ’s natures are to be understood as parts.

However, suppose that we treat these passages in the same

way that we have treated the passages from ST III q. 2 a. 9

that we considered before – namely, that we interpret them

within the framework of a strictly compositional model of

the incarnation. On such an interpretation, when Aquinas

identifies Christ’s supposit with his divine nature, he is

saying that in the case of Christ, his supposit is identical

with one of his proper parts. On such an interpretation,

48

Christ is radically different from other substances.

Normally, the supposit is the whole substance itself, which

is composed of matter, substantial form, and accidental

forms. In the case of Christ, his supposit is his divine

nature, which is only a part of him.

Now as we have seen, Aquinas holds that “something is said

to be such-and-such according to itself when it is according

to the whole”, and also that, in Christ, “the supposit is

signified in the manner of a whole”. If we combine these two

claims, still in the context of a compositional account, we

get the claim that although the divine nature is only a part

of Christ, it functions in some ways as if it were the whole

of Christ, because it is his supposit. In particular, any

property that Christ has in virtue of the fact that his

divine nature has it, he is said to have according to

himself, and in an unqualified way. Now his divine nature,

as we have seen, has superlative unity. It follows that

Christ himself has superlative unity – not in a qualified

way, but simply. In contrast, he has duality in virtue of

49

his parts: there are two parts and so there is duality, and

so he has duality in only a qualified way. The unqualified

nature of Christ’s unity is the main point of article 4 of

De unione Verbi:

…whatever is one simply is one according to being [esse]. But Christ is

one simply, as has been shown. Therefore, there is one being in him. (De

unione Verbi art. 4 con.)

But surely there is a contradiction here. “Supposit” is just

another word for the substance itself, considered as a whole

individual. If the divine nature in Christ is identical with

his supposit, then, it should be identical with the whole of

Christ – it cannot just be a part. Does this mean that

Aquinas really is retreating from his composition account

here, and denying that the divine nature is a part of

Christ? Not necessarily – at least, not if he were to use

the defence we are constructing. That defence, and the part

of it we have just articulated, is really an argument for

why the unity of the part can transfer to the whole in the

50

case of Christ. The proponent of this defence need not think

that unity can in general be a borrowed property. It is enough

for there to be a special reason why, in Christ, the unity

of the divine nature transfers to Christ as a whole. This

reason, which we have drawn from an interpretation of

Aquinas as a strict compositionalist, is that the divine

nature is also the single supposit of Christ. Even though,

on this interpretation, Christ’s supposit is only a part of

him, it is still the case that any property he has qua

supposit he has qua himself and therefore simply. So Christ

may be said to bear simply any property which his divine

nature bears. One of the properties which his divine nature

bears is the property of perfect unity; so Christ himself

has perfect unity too.

And that is in accordance with Aquinas’ own conclusion:

And so just as Christ is one simply because of the unity of the

supposit, and two according to the two natures, so he has one being

51

simply [unum esse simpliciter] because of the one eternal being of the

eternal supposit. (De unione Verbi art. 4)

The argument we have constructed is, as it stands,

undoubtedly peculiar. Even if we accept a model of the

incarnation according to which Christ’s divine nature is

both a proper part of him and his supposit – even though a

thing’s supposit is normally just identical with itself –

that does not explain why Christ may be said to bear simply

any property that his divine nature does. After all, if his

divine nature is only a part of him, then surely he doesn’t

bear simply any property that it does.

But we can identify a particular reason why Christ may be

said to bear simply the properties that his divine nature

bears – and it all comes down to the divine unity. Aquinas

states that the superlative unity of the divine nature is

what permits the existence of the extraordinary relation in

which it exists with the human nature in the first place:

52

Just as God is goodness itself and his being, so too he is unity itself

[ipsa unitas] by his essence. And so, just as his excellence is not

limited to those ways of goodness and being which are in creatures, but

he can make new ways of goodness and being that are unknown to us; so

too by the infinity of his excellence he could make a new way of union

[novum modum unionis], so that the human nature might be personally – but

not accidentally – united to the Word. (De unione Verbi art. 1)

So the unitas that the divine nature possesses is so

superlative that it allows a special kind of unio to occur

in the case of Christ. We have seen that this unio is matter

of the human nature having a relational property whose term

is the divine nature. So the unitas of the divine nature is

such that this relational property is possible. To put it in

another way, the unitas of the divine nature is such that

only the divine nature can be the term of the sort of

relational property that the human nature has. This is why

both of the properties involved in the incarnation are

unique. First, the property which the divine nature has (its

special and supreme kind of unitas) is unique, and this is

what allows the property which the human nature has (its

53

property of being united to the divine nature in a certain

way) to be unique as well. Second, it is the relational

property instantiated in the human nature, with its term in

the divine nature, which allows the supreme unitas of the

divine nature to be borrowed by the whole Christ.19

What is this relational property which allows the unitas of

the divine nature to be applied to the whole Christ? As we

have seen, Aquinas is coy when it comes to describing this

relational property, but it must involve the fact that the

divine nature is its supposit.20 In a normal human being,

soul and body constitute a supposit in their own right

19 “…the union [unio] is not understood to be terminated in the divine

person according to what is eternally one in him, but according to what

is temporally united to the human nature. And so as we understand it

[secundum modum intelligendi], the union comes before the person, not to the

extent that it is [prout] one, but to the extent that it is united.” (De

unione Verbi art. 1 ad 5)

20 “The union of soul and body in Christ is greater [dignior] than in us,

because it is terminated not in a created supposit, but in the eternal

supposit of the Word of God.” (De unione Verbi art. 2 ad 12)

54

(together with whatever accidental forms may be involved as

well). But according to the defence we have been

constructing, in Christ, uniquely, they do not, because

their supposit is something distinct from either soul or body

or their combination. That is, the relational property that

the human nature has is expressed (at least in part) by the

predicate “has the divine nature as its supposit”.21

21 There is clearly something very paradoxical about this. Being a

supposit is not normally a relational matter. Something just is a

supposit in virtue of its metaphysical components – matter and form. It

does not normally have a supposit external to itself. But on our

interpretation of Aquinas, this is the case with Christ’s human nature.

Yet this paradoxical situation mirrors the equally paradoxical situation

with Christ’s unity. Being a unity is normally simply a matter of being,

undividedly. A whole cannot normally borrow the property of being a

unity from one of its parts. Yet this is the case with Christ, who is a

unity because one of his parts is a unity. Both of these paradoxical

things can occur because one of Christ’s parts, the divine nature, is a

supremely undivided supposit – a supposit whose unity is so great that

it can act as the supposit of another thing, and it can pass on its

unity to the whole of which it is a part.

55

Conclusion

This, then, is the strategy for addressing the Plurality

Argument within the context of a composition model of the

As I have indicated, the defence I have outlined here is not to be found

explicitly in Aquinas. I have argued that it is a defence which can be

constructed from materials that Aquinas provides us with, if we choose

to read Aquinas as a strict compositionalist, and interpret his texts

within that framework. Whether this is a correct reading of Aquinas, and

whether Aquinas would accept this defence, are questions that lie beyond

the scope of this paper. He would, however, be willing to accept its

paradoxical nature. He writes:

“The union [unio] of the human nature to the person or the hypostasis of

the Word in Christ is not made in this way, that it is made equal to it

as if it were including it, or as though it were by some definite amount

that the person of the Word surpasses it; for the person of the Word

still remains infinitely surpassing the human nature. But this infinite

surpassing does not prevent the person of God, in some incomprehensible

way, from joining [copulaverit] the human nature to itself in the unity of

the hypostasis. On the contrary, the infinite excellence of the one that

assumes operates more effectively towards a greater union [maiorem

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incarnation. The composite Christ can have unity, as a

composite, because of two remarkable properties inhering in

his respective parts. His human nature has a relational

property whose term is the divine nature, such that the

divine nature is the supposit or hypostasis of the human

nature and of the composite whole. And his divine nature,

because of its radical simplicity, is perfectly undivided,

which means it has perfect unity. Since the divine nature is

the supposit of the composite Christ, the composite Christ

unionem].” (De unione Verbi art. 2 ad 15)

So what might seem to prevent the divine nature from being united to the

human nature – their infinite difference – actually allows them to be

united more closely than any other two things. Similarly, on the defence

we have constructed, it is not by retreating from a composition account

that we account for the unity of Christ. It is by embracing that account

– coupled with an emphasis on the extraordinary qualities of the divine

nature, which although it is a part of the whole discharges some of the

functions of the whole, such as acting as the supposit of the whole and

of the other parts of the whole, and of being the locus of the unity of

the whole.

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also has this property of perfect unity, as a borrowed

property. Yet he does not have it as composites normally

have borrowed properties, in a qualified way. Because he has

it in virtue of his supposit, he has it in an unqualified,

simple way. In this way, we can reject premise (1) of the

Plurality Argument: there is at least one case where

plurality does not compromise unity at all, the case of

Christ. Not only that, but we can defend premise (1) of the

Borrowed Property Argument. As a result, if one accepts this

defence, then not only does the argument against Christ’s

unity fail, but the argument for Christ’s supreme unity

succeeds.

Laying aside the question whether Aquinas would agree with

this defence, it does have one notable strength as an

account of unity within the context of a composition model

of the incarnation. It is certainly peculiar to suggest that

unity could be a borrowed property – that the supreme unity

of one of Christ’s parts could be transferred

straightforwardly to the whole of Christ. But as we have

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seen, part of the defence is that this is possible because

of the extraordinary nature of Christ’s divine part, and in

particular, the extraordinary nature of its unity. If we

accept that, then the rest of the account is not so

difficult; Christ is straightforwardly a composite of a

divine nature and a human nature. In other words, the

incarnation remains a mystery, but it is not a distinct

mystery from the mystery of the divine nature itself. The

relations between Christ’s natures are mysterious, but they

derive this mystery from the fact that they are made

possible by the divine nature, the ultimate mystery. On this

model, then, the incarnation is not a mystery in addition to

the mystery of God himself – it is part of that divine

mystery.

I conclude, then, that the problem of accounting for the

unity of Christ in the context of a composition model of the

incarnation – as articulated in the Multiplicity Argument –

need not force a defender of that model to abandon a literal

composition model in favour of either an inconsistent part-

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composition part-non-composition model or a merely

metaphorical composition model. Whichever of these options

Aquinas actually takes, he provides the materials for a way

of retaining a composition model, literally understood, and

still accounting for the genuine unity of Christ as part of

that composition model, by appealing to special features of

the parts that compose Christ, in the context of his theory

of unitas and unio.22

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22 Much of the research for this paper was undertaken while working on a

project on the incarnation at Oxford from 2008-2009, funded by the

Leverhulme Trust, and I would like to thank the Trust for that support.

An earlier version of the paper was presented at a conference on the

metaphysics of the incarnation held at Oxford in 2009, and I would like

to thank the participants at that conference, especially Anna Marmodoro,

for their helpful comments on it.

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