Post on 29-Apr-2023
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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