The purpose of this paper is to investigate composition models of incarnation, according to which Christ is a compound of qualitatively and numerically different constituents: a divine mind, a human body, and, on some models, a human mind as well. The examination focuses upon the metaphysical accounts of unity of the constituents that those who hold such composition models have offered.
Our investigation may be thought of as presupposing what is often called a ‘concretist’ approach to the incarnation, according to which the terms ‘human nature’ and ‘divine nature’ in Christ denote concrete parts, rather than sets of properties, as they do on an ‘abstractist’ approach.Footnote 1 Other kinds of models of the incarnation have also been defended.Footnote 2 In what follows, we do not relate the compositional models we examine to other doctrinal elements of Christianity, except where that is necessary to explain the models themselves. Our aim is not to consider the theological significance of the composition models, but the more modest one of examining their philosophical coherence.
Composition models of incarnation are commonly divided into two classes – two-part and three-part models – depending upon the number of constituents which are held to make up Christ.Footnote 3 If Christ is a composite of a number of constituents, how can he be genuinely one person? The challenge is especially pressing for those composition models according to which Christ's constituents include a human mind in addition to a divine mind and a human body. This position clashes directly with well-entrenched intuitions about what individuates persons – namely their minds or souls.Footnote 4 This is why, in this paper, we focus in particular on such three-part composition models, and examine a number of metaphysical relations that proponents of these models have suggested for unifying the composite into one person, Christ.
Two-part composition models
Two-part models conceive of the relation between the human body and God the Son as qualitatively identical to the relation that holds between a human body and a human mind in an ordinary human being. The classic statement of a two-part model is that of the fourth-century theologian Apollinarius of Laodicea, who wrote: ‘[Christ's] body lives by the sanctification of the Godhead and not by the provision of a human soul, and the whole is completely joined in one.’Footnote 5 Apollinarius adopts the (Aristotelian) view that in an ordinary case a human body is alive because it has a (human) soul. But in Christ's case his body is alive because it is ‘united’ to a divine mind, instead of a human soul. Apollinarius does not provide a metaphysical account of the relation of union that he holds obtains between a soul or mind and its body. But his key idea is clear: in Christ the divine mind supplants the human soul that would normally be there.
A prima facie strength of two-part models is that they give an intuitively appealing explanation of why Christ is a single person: because any of us is. But a weakness with these models, which their ancient critics did not identify but which may carry more weight in current debates, is that they presuppose mind–body substance dualism. Substance dualism holds – among other claims – that the mind and the body each have the capacity (at least in principle) to exist without the other.Footnote 6 Two-part models of the incarnation are committed to this view. According to any orthodox model of the incarnation, the Son (the divine mind in Christ) exists before his incarnation; he can therefore certainly exist independently of the body which he acquires in the incarnation. But according to two-part models, the Son has the same type of relation to the body that any human mind has to its body. It follows, then, that a two-part theorist is committed to the claim that any ordinary human mind could, in principle, exist independently of its body.
This commitment to a central tenet of substance dualism, which most philosophers nowadays regard as highly implausible as a theory of the mind–body relation, is a serious problem for two-part models.
Three-part composition models
Three-part models of incarnation conceive of Christ as consisting of a human body, a human mind, and the divine mind, that is, God the Son. On such models, the relation between the human body and the human mind in Christ is of the same type as the relation that holds between body and mind in any ordinary human being. But in Christ the human mind and the human body are, in addition, related to God the Son in some way as well.
Models of this kind go back at least to Origen of Alexandria. Origen believed that, before the creation of the physical world, created intellects were united to God; but they all at some point fell away, apart from one, and this is the soul of Christ. In the incarnation, this soul acquired a human body. So Christ consisted of a human body, animated by a human soul, which itself was united to a divine person. This union of the three constituents in Christ was ‘inseparable and indissoluble’.Footnote 7 Origen thought that the pre-existing soul that was united to God was identical with Christ's human soul. So Christ's human soul was always united to God, even before the incarnation. The incarnation was simply a matter of this soul acquiring a body.
Later theologians maintained the tri-partite constitution of Christ, like Origen. But they rejected the notion of the pre-existence of souls, and instead assumed that Christ's soul and body both came into existence at the same moment that they were united to the Son. Peter Lombard, for example, reported such a view thus:
[Supporters of this theory] say that that human being consists not only of a rational soul and of flesh, but of a human and of a divine nature, that is of three substances: divinity, flesh, and soul. This [human being] they confess to be Christ, and [they further confess] that he is only one person, who was simple only before the incarnation, but in the incarnation came to be composed of divinity and humanity.Footnote 8
Models of the incarnation of this kind are most associated with the so-called Antiochene theologians. But many accused the Antiochenes of reducing the incarnation to the assumption, or (even more weakly) the inspiration, of a human being by the divine mind.Footnote 9 Indeed, one of the reasons Apollinarius developed his two-part model was to preclude this doctrinally unwelcome consequence. He claimed that any model that did not make Christ's body uniquely the body of the Son, but allowed for a human mind in Christ, effectively denied the incarnation. The reason was that in such a case, the body would be the body of that human mind and not of the Son:
If the Lord is not incarnate intellect, he must be Wisdom enlightening the intellect of a human being. But this happens in the case of all human persons, and if this is the way of it, then the coming of Christ is not a visit from God but the birth of a human being. If the Word did not become intellect incarnate but was Wisdom within the intellect, the Lord did not come down.Footnote 10
However, at least in the existing fragments, Apollinarius does not justify his claim that if there were a human mind in Christ alongside a divine mind, the human body would be of the human mind but not of the divine mind. Despite Apollinarius' arguments, three-part models were eventually favoured over two-part ones. The most important reason for this was that three-part models appeared (at least prima facie) better at accounting for the genuine humanity of Christ, by allowing that Christ had a human mind.
Varieties of three-part composition models
Three-part models are much more diverse than two-part models, because of the different relations that one may posit between the three constituents of Christ in trying to account for the unity of his person. Although the distinction between two-part models and three-part models is well established in modern literature on philosophical approaches to the incarnation, the distinction between different kinds of three-part models is not. We therefore turn to presenting the main varieties of three-part models that have been defended, and to examining which ones appear philosophically more promising.
For ease of reference, we will use D for the divine mind, or God the Son; H for the human mind or soul; and B for the human body. The diagrams below indicate some of the ways in which they could be related, with the lines indicating the direct relations between the constituents. Setting out the structure of the different models in this abstract way allows us to distinguish between two questions: first, which ‘parts’ are directly related or united to which? And second, what are the relations in virtue of which they are united? Four different kinds of models can be distinguished, illustrating different answers to the first question:
Other structures are theoretically possible. The ones we consider here are those which have been most prominent in the history of three-part models. They are arranged, moreover, in a sequence that expresses increasing degrees of unity in the composite substance that is Christ.
Uncoordinated dual mentality
The first model holds that Christ's human mind bears to his body the same type of relation that human minds normally bear to their bodies. In addition to this, the divine mind also bears a direct relation to the same body. But there is no direct relation between the two minds; the two minds are related to each other only in virtue of each being directly related to the same body.
Cyril of Alexandria sometimes implies a model of this kind:
We worship one Christ and Lord … one and the same because the body of the Word, with which he shares the Father's throne, was not alien to him … . [T]his does not mean two sons were sharing the throne, but one, because of the union with the flesh … [Christ] is said to have been born according to the flesh in so far as the Word was hypostatically united to that holy body which was born from her, endowed with a rational soul.Footnote 11
In passages such as this, we are told that the human body of Jesus was ‘endowed with a rational soul’ – it had a normal human mind or soul, and was related to it in the ordinary way. But this same body was also, and directly, united to God the Son. What kind of union was this? Cyril tells us that the body was united in such a way that it became the Son's own body. For example, he speaks of the Son ‘taking flesh from the holy Virgin and making it his very own’. He also describes the relation between the Son and the body as ‘that kind of indwelling which the soul of man can be said to have with its own body’.Footnote 12
So the relation between D and B is the same relation that normally holds between the human mind and the human body. This allows Cyril to make the same basic claim as Apollinarius, that Christ's body is really the body of the Son.Footnote 13 And yet Cyril diverges from Apollinarius in holding that there is also a human mind, which is also related to the body in the ordinary way. The one body has two minds: it is in a state of dual mentality.
This is scarcely adequate as a model of incarnation. It certainly assumes substance dualism, for exactly the same reasons that we gave earlier in the case of two-part models: it conceives of the pre-existent divine mind as having the same relation to the human body that minds ordinarily have to their bodies. More worrying, though, is the prospect of two minds operating upon the same body. What if the divine mind moves the body one way and the human mind moves it the other? If there is no direct relation between the two minds, there is nothing to prevent such an occurrence. But this is hardly an appealing prospect if there is to be any personal unity to Christ. Such a Christ would be two people sharing a body, not one person with two minds.
Models of this kind, then, are inadequate. A direct relation between the two minds is required, to avoid clashes between them. The other models we consider below all seek to address this problem, with varying degrees of success.
Derivative dual mentality
On the second model, the divine and the human minds are directly related to each other, and also related by two distinct relations to the same human body.
What kind of relation might hold between the divine and human minds in Christ? One possibility is that they are generated by ‘fission’, i.e. the splitting of an original single mind into two distinct ones. The viability of this suggestion depends, in part, upon whether one also accepts (as we do for the sake of argument here) an account of personal identity of the kind that goes back to John Locke.Footnote 14 To put it extremely briefly, on views like Locke's, a person at time t 1 is the same person as a person at time t 2 if and only if there is some specified kind of connection, e.g. causation, between their mental states.Footnote 15
In the context of ‘derivative dual mentality’ models of incarnation, the concept of fission may be used to suppose that the human mind in Christ ‘branches off’ from the divine mind (the Son's). So the divine and the human minds in Christ might be thought to be related to each other by psychological continuity. This could provide a basis for ascribing personal unity to Christ. The concepts employed in sketching such a present model are theoretical acquisitions that were not available in antiquity, and have entered philosophical discussions only relatively recently via psychology and medicine. But even if none of the classical theologians appealed to such concepts, it is worth considering whether they can be used to construct a three-part composition model of incarnation. Brian Leftow, for example, describes, but does not endorse, such a model:
… the orthodox who also hold psychological continuity theories of personal identity … must hold that the S-series [i.e. the series of mental states that constitute Christ's human mind] branches out of the Son's mind in some way sufficient to count as a second mind, but not with such independence as to constitute a second person. But this seems possible in principle. One person's having first one mind and then two, the second branching off from the first psychologically, is in fact a legitimate description of what goes on in cases of cerebral commissurotomy.Footnote 16
However, such an account of the relation between the human and the divine minds in Christ suffers from all the objections that have been raised against such an understanding of personal identity, which is considered by many highly counterintuitive and, arguably, requires a series of stipulations to be made plausible.Footnote 17
More specifically, as an account of incarnation, this is only an account of the origin of the human mind, in relation to the divine mind. But something stronger is required for backing up the compositionalist claim that an individual with two minds can be a single person. It would be quite consistent with the claim that H branches off D to suppose that, thereafter, H and D have no contact with each other and behave quite differently from each other – a situation very similar to models of type (1), with the attendant problems. So there needs to be some kind of ongoing relation between the two minds after the human mind has originated to avoid such a situation. What could such a relation be like? If the two minds are to be genuinely coordinated, and not conflict with each other, there must be communication of some kind between them. This brings us to the third type of compositionalist model.
Mental hierarchy
Models of the third group hold that Christ's divine mind is directly related to his human mind (but not to his body), and his human mind in turn is directly related to his human body in the ordinary way. These models aim to guarantee even greater unity between the constituents of Christ than models of the second type do, by allowing the divine mind to have direct control over the human mind.
Like other versions of a three-part Christology, models of this third kind go back to Origen.Footnote 18 Origen held that Christ's human soul was united to the divine mind with such an intimate dependence that it could be considered divine in a derivative way, just as an iron bar in the fire becomes so ‘fiery’ that it is qualitatively indistinguishable from the fire itself.Footnote 19 But Origen also held that the human mind is directly related to its body, in a way that the divine mind is not. He saw the human mind as a necessary mediator between the divine mind and the human body:
This soul, then, acting as a medium between God and the flesh (for it was not possible for the nature of God to mingle with a body apart from some medium), there is born … the God-man, the medium being that existence to whose nature it was not contrary to assume a body.Footnote 20
Later theologians retained the idea that Christ's human mind acted as mediator between his divine mind and human body (although most of them dropped Origen's notion of the pre-existence of Christ's human mind). For example, Maximus the Confessor writes of the Son ‘taking up flesh through the mediation of a rational soul’.Footnote 21 Later, John of Damascus presents the same theory more fully:
The Word of God is united to the flesh by the intermediary of mind which stands midway between the purity of God and the grossness of the flesh. Now, the mind has authority over both soul and body, but, whereas mind is the purest part of the soul, God is the purest part of mind. And when the mind of Christ is permitted by the stronger, then it displays its own authority. However, it is under the control of the stronger and follows it, doing those things which the divine will desires.Footnote 22
All of these theologians think of the human mind in Christ as naturally a sort of medium between the divine and the physical. It is literally half way between them in nature. This is a very common view among ancient and medieval theologians,Footnote 23 and derives from a Neoplatonic conception according to which mind is superior to body, and God is superior to mind. But its supporters do not provide an argument for it. It is indeed hard to make it intelligible, let alone palatable, to modern thinkers. How could any entity, or kind of entity, be halfway between the infinity of divinity and the limitedness of creatures?
But suppose that we grant this possibility for the sake of argument: a further question arises. Why does the divine mind need such a mediator, given the assumption that God is omnipotent? Even if the human mind is more similar to God than the human body is, why does that mean that the divine mind can be directly related only to the human mind? Must things be similar to each other to a sufficient degree before they can be united? What is that degree anyway? The defenders of this model do not address such questions.
There are, then, fundamental general weaknesses with the ‘hierarchy of minds’ model. However, it is worth considering some of the forms it has taken. In particular, what relations have its defenders believed to hold between the components of Christ?
The relation of control
As the passage quoted above makes clear, John of Damascus identifies the key relation between the human mind and the body as one of control. John conceives of a hierarchy in every human being, from the mind (that is, the rational part of the soul) to the lower parts of the soul, and finally to the body. Each member of the hierarchy controls what is directly below it, and is controlled by what is directly above it. In the incarnation, John thinks that the hierarchy simply acquires an extra element at the top, the divine mind. This controls the human mind, which controls the lower part of the soul, which controls the body. But such a model faces obvious problems. The relation of control between the divine and the human minds appears inadequate for capturing the metaphysical uniqueness of the hypostatic union. God may, after all, act upon any human being to control her. Yet such an event would not count as an incarnation.
There are two possible responses to this. One might say that, in the case of the incarnation, there are additional relations involved as well, which do the metaphysical job of bringing about hypostatic union. But then one is committed to finding such other relations – the relation of control does not explain the incarnation. The second possible answer is that control is the only relation involved, but that it is different from other instances of divine control of creaturely minds. We can think of at least two ways in which it could differ. The first is that God's direct control over Christ is a constant and permanent state of affairs. This seems Origen's position: ‘Suppose … a lump of iron … [is] placed for some time in a fire. It receives the fire in all its pores and all its veins, and becomes completely changed into fire, provided the fire is never removed from it and itself is not separated from the fire.’Footnote 24 Yet this is inadequate too. If a relation is insufficient to support a claim of incarnation, making it last for longer will not change that insufficiency.
The second way in which the relation of control in the case of Christ could differ from other instances of divine control of creaturely minds is by degree. It could be a more direct and intimate kind of control. But if Christ's human mind is being controlled by something else to that degree, it loses all human agency in its own right. If Christ's human mind is simply a tool of the divine mind, to be directed as the divine mind sees fit, then his human mind does not really have a will of its own at all. It was precisely to guard against such a notion, where the divine nature of Christ overrules his human nature, that the third council of Constantinople proscribed monotheletism (as we shall see in more detail below).
We conclude, then, that the relation of control is not a good candidate for the relation between the divine and the human mind in three-part models. Either it is too weak to support a claim of incarnation, or, if it is strong enough, the human mind loses all agency.
The relation of containment
Contemporary studies in psychology have made available new ways of conceiving of the relation between the divine and the human mind in Christ. They suggest that we can make sense of a single person having a ‘split mind’ even without involving the theories of fission touched upon above. Where someone has a single mind that is ‘split’ into more than one mental stream, there remains the possibility of direct communication between the mental streams.
This is a key difference between the ‘split-mind’ scenario and the sort of ‘splitting’ that we considered earlier in cases of ‘fission’, devised as problems for psychological continuity theories of personal identity. With ‘fission’ cases, the focus is upon the origin and identity of each of the minds or persons, not upon their subsequent relation to each other. But with ‘split-mind’ cases, the focus is upon the current relation between the mental streams. In a normal mind, for example, the conscious and subconscious streams are not completely parcelled off from each other: ideas may pop into the conscious stream from the subconscious, and the subconscious may work away on problems that have been presented to the conscious stream. And at least in principle, such communication could be either two-way or one-way. One can imagine a mind that is split into two mental streams, but where one stream has access to the contents of the other, but not vice versa.
This model was not available in late antiquity and the Middle Ages, but more recent writers have appealed to it as part of a model of the incarnation. Thomas Morris, for example, suggests that the relation between the two minds of Christ is to be conceived as similar to the relation between two mental streams in a ‘split mind’:
The two minds of Christ should be thought of as standing in something like an asymmetric accessing relation: the human mind was contained by but did not itself contain the divine mind, or, to portray it from the other side, the divine mind contained, but was not contained by, the human mind. Everything present to the human mind of Christ was thereby present to the divine mind as well, but not vice versa.Footnote 25
Here, Morris appeals to ‘containment’ as the relation between the two mental streams. What does this mean?
Two main ways of understanding this relation have been proposed in the context of three-part models of incarnation. The first, and possibly the simplest, is that to say that one stream of consciousness is ‘contained’ by another is to say that it is a proper part of it. So if Christ's divine consciousness contains his human consciousness, that means that the mental contents of his human consciousness are a subset of the mental contents of his divine consciousness. This is how Tim Bayne interprets both Morris and Richard Swinburne. But Bayne argues persuasively that such a view has very counterintuitive consequences. In particular, it means that some of Christ's mental or phenomenal events occur within both his divine and human consciousnesses, and others occur within only his divine consciousness. It follows from that that some of his experiences are co-conscious from the point of view of one consciousness, but not from the point of view of the other. But this is highly implausible, since it violates the very attractive principle that two token experiences should be either co-conscious or not.Footnote 26
We may add that this model conceives of the two mental streams as existing within a single mind – they must do, if one is a proper part of the other. So Christ's human consciousness is not a ‘part’ of Christ distinct from the divine mind. Such a model thus collapses into a two-part model of the incarnation, with the attendant difficulties that we noted above.Footnote 27
An alternative conception of ‘containment’, which avoids objections of this kind, explains it in terms of accessibility: all the mental contents of Christ's human mind are accessible to his divine mind, in the sense that the divine mind can perceive or know them, but not all of the mental contents of his divine mind are accessible to his human mind. One difficulty with this as it stands, as Morris himself points out, is that it fails to account adequately for the metaphysical uniqueness of the hypostatic union, because if the divine mind is omniscient then the contents of everyone's mind are accessible to it.Footnote 28
Some defenders of the accessibility model have therefore modified the account, to add an element to the divine mind's knowledge of Christ's human mind that is unique. Richard Sturch, for example, has argued that ‘containment’ should be understood in the sense given above, but with the additional feature that it has access to them as its own.Footnote 29 Suppose the human consciousness thinks ‘I am lost’. The divine consciousness does not merely observe this and have an awareness that Jesus thinks he is lost, as it would if it were observing any other human mind. Rather, its awareness is ‘I am thinking I am lost’. And this, contends Sturch, is what guarantees the personal unity of the two consciousnesses.
But this view is problematic. First, how can the phenomenal feeling of one subject be appropriated by another subject, when phenomenal feelings are by definition the way something feels like to the subject? And second, it begs the question. If the divine consciousness accesses the contents of the human consciousness as its own, there must be some reason for this. In virtue of what can the divine consciousness say that it is ‘I’ who am thinking what the human consciousness is thinking? One cannot appeal to the ability of the divine consciousness to access the thoughts of the human consciousness in the first person as the grounds for the unity of the two. Rather, it is because the two are united (supposing that they are) that the divine consciousness can do this. But that means that we must seek the explanation of the union of the two consciousnesses somewhere else.
We thus conclude that no version of the containment relation is without difficulties as a relation between the divine and the human mind.
Hierarchy of natures
Our final model regards the divine mind as related to the compound of a human mind and a human body. The relation between the human mind and body in Christ is of the same kind as the one that normally holds between the mind and the body; and the two of them are, jointly, directly related to the divine mind. We call this a ‘hierarchy of natures’ model, taking ‘nature’ here, in the concretist sense, to refer to the divine mind on the one hand and the human mind and body on the other.
What might the relation be between the divine mind on the one hand, and the human mind and body on the other? One of the most common analogies which Christian theologians have used to answer this question is that of the relation between mind and body in an ordinary human being. The use of this analogy goes back, ultimately, to the fourth-century Christian philosopher Nemesius of Emesa.Footnote 30 On Nemesius' view, the normal relation between mind and body is a causal one. In the case of Christ, then, his body is in this causal relationship to his human mind, and the two of them hold jointly a qualitatively identical relationship to the Son as well. Later theologians have fleshed this relation out in a variety of ways.
The relation of instrumentality
Many theologians, such as Thomas Aquinas, think of the relation between the divine and the human mind as one that we can call instrumentality. In a normal human being, on this view, the mind acts upon the world through the body: the body is its instrument. In Christ's case, similarly, D acts upon the world through H and B. But some critics have noted that it is not the instrument that acts – it is the person who uses the instrument. The instrument remains passive, because it does not really contribute to agency. (One way to cash this out is to say that it does not play a role in setting the goal of the action.)Footnote 31 Furthermore, the relation of instrumentality is simply too weak to support genuine unity. If a person uses an instrument, she does not thereby become one with that instrument.
Attempts have been made to address these difficulties by qualifying the relation of instrumentality. Thomas Aquinas, for example, distinguishes between what it is to be merely an instrument for the agent and what it is to be an organ of the agent, thereby distinguishing between two kinds of instrumentality. He writes:
[In an ordinary human being] the body and its parts [e.g. hands, legs] are the organ of the soul in one fashion; external instruments [are a means for the soul] in quite another [fashion]. For this axe is not the soul's very own instrument, as this hand is, for by an axe many can operate, but this hand is deputy to this soul in its very own operation … . This is the way, then, in which even the union of God and man can be considered. For all men are related to God as instruments of a sort … . But other men are related to God as extrinsic and separated instruments, so to say; for God does not move them only to operations which are his very own, but to the operations common to every rational nature … . But the human nature in Christ is assumed with the result that instrumentally he performs the things which are the proper operation of God alone … . The human nature of Christ, then, is compared to God as a proper and conjoined instrument is compared, as the hand is compared to the soul.Footnote 32
Aquinas's distinction between an organ and an instrument is intuitively clear. But even assuming we accept it, he does not provide arguments for the claim that Christ's human mind and body should be considered God's organs rather than instruments. What makes Jesus analogous to hands and not to axes? Aquinas does not tell us.
The relation of co-action
An alternative relation might be called ‘co-action’. John of Damascus puts forward the clearest account of it, in an extended explanation of the term ‘operation’ or ‘activity’ (energeia) as it applies to Christ. John's account, and the theory of ‘co-action’ which we draw from it, had its roots in the convoluted debates over the appropriate terminology to apply to Christ that occurred in the sixth and seventh centuries.
In the seventh century the term energeia had become the subject of sustained theological debate. In this debate, some Aristotelian principles (and terminology) were still in play, although they had developed beyond their original metaphysical contexts.Footnote 33 Very roughly put, the common view was that entities have ousiai (or essences) that make them what they are; the ousiai are realized, and hence known, through the entities' energeiai (or activities) which manifest them. It was also often assumed that a single ousia can be manifested only by a single energeia, although no justification was offered for this assumption. This is despite the fact that the alternative metaphysical option that a single ousia is manifested by multiple energeiai was certainly discussed in a theological context.Footnote 34
Within this broadly Aristotelian framework, a number of theologians before the seventh century took the position that there is only one energeia in Christ.Footnote 35 Severus of Antioch, for example, held this view.Footnote 36 But ultimately the doctrine that Christ has only one energeia was condemned at the third Council of Constantinople, on the basis that a nature that is not manifested in an energeia is not a complete (i.e. fully real) nature. Since Christ has a divine and a human nature, he has two energeiai, not one.
The Council added that each nature has its own will or thelema. On this view, Christ's human mind has a will of its own, distinct (but not independent) from the will of his divine mind. The divine mind acts upon the human mind to ensure that the human will is in accordance with the divine will. Here, then, we already have an advance on the relation of instrumentality. Christ's human mind actively contributes to Christ's thoughts and actions, although it is guided in its aims, and in this sense used, by the divine mind. Because the human part of Christ has a will of its own, and is not simply dragged along by the divine will, it can properly be called an agent – just as a servant remains an agent even when obeying a master. But at the same time, it remains subject to the divine will. It is, we might say, an active instrument towards the accomplishment of God's plans.
But a servant and a master do not constitute a single person. How, on this picture, can the two minds, with their two wills, be a single person? John of Damascus picks his way through the unpromising terminological thicket to offer part of a possible answer: they jointly perform a single action. Like his predecessors, John appeals to the relation between mind and body in an ordinary human being as an illustration of the relation that holds in Christ between the divine mind on the one hand, and the human mind and human body on the other hand. He writes:
The mind first considers the thing to be done and then acts accordingly through the body. So, it is to the soul that the control belongs, since it uses the body as an instrument which it guides and directs. The operation of the body as guided and moved by the soul, however, is a different one. And as to the effect, while that of the body is, as it were, the touching, holding, and clasping of the thing made, that of the soul is the thing's formation and configuration. It was also the same with our Lord Jesus Christ. While the power of working miracles was an operation of his divinity, the work of his hands, his willing, and his saying: ‘I will. Be thou made clean,’ were operations belonging to his humanity.Footnote 37
In this passage, John effectively distinguishes between operation and action (although he does not name the latter). He recognizes that when an ordinary human being performs a single action, the mind and the body accomplish different things: the mind plans, and the body moves; two operations are involved. With Christ, similarly, every action he performs is a single action, but it involves two operations – one of his divine mind, and one of his human mind and body.
The relation of co-action avoids the problem that we saw with the relation of instrumentality. We argued that if Christ's human mind and body are viewed simply as the instruments of his divine mind, they lose all agency of their own. With co-action, this is not the case: the human elements of Christ retain their agency. Thomas White has recently developed John of Damascus's suggestion in this direction. White argues for something like co-action as fundamental to the relation between the two minds in Christ (although he calls it ‘instrumentality’). But he stresses that this relation is not the basis for their union, but a consequence of it.Footnote 38 It is because Christ is a single person that the two minds co-act, not vice versa.
But assuming that each mind in Christ has its own faculty of willing, how do the two wills co-ordinate? Different theologians have given different answers to this question. Maximus the Confessor, for example, wrote:
The human will of the Saviour, like all that is human in him, although it was natural, was nevertheless not that of a mere man like us, since, in a superior fashion to us, it was totally divinized by the union: the sinlessness depends principally on that. On the contrary, being merely human, our will is not in any way sinless, because of its inclination which moves it sometimes one way, sometimes another. This inclination does not change its nature, but it determines its movement, or, to speak more accurately, it changes its tropos.Footnote 39
The implication here is that the human mind is transformed by the union in such a way that it will never choose to sin, which means that it will always choose in accordance with the divine mind. So on this view, the agreement between the two minds is guaranteed by their sinlessness.
Although neat, this solution is somewhat problematic, since it assumes that two minds that are equally without sin will always choose in the same way. But it is implausible to suppose this. There could be situations where several different non-sinful options are available, in which case a sinless mind might, other things being equal, choose any one of them. John of Damascus presents a somewhat different picture:
He assumed a body animated by a rational and intellectual soul having dominion over the flesh, but itself being under the dominion of the divinity of the Word. Consequently, while he had … the power of willing both as God and as man, the human will followed after and was subordinated to his will, not being motivated by its own opinion, but willing what his divine will willed.Footnote 40
Here, the idea seems to be that the human will chooses in accordance with the divine will because the divine will determines it to do so. John does not specify how this works.
He does, however, address one problem with this picture, which is that the human will seems to have no choice. Like most ancient Christian authors, John of Damascus is an incompatibilist, and holds that having free will involves not being determined at all. But if Christ's human will is determined by his divine will, then he lacks human free will. In some passages, John suggests that the human will in Christ chooses to allow the divine will to determine its decisions.Footnote 41 So the human will retains its human freedom. This is an advantage of the theory over that of Maximus: Jesus must decide, as a human, that he is going to follow the divine will in all things. The outcome of his decisions is not a foregone conclusion as a result of his union with God, as Maximus implies.
A model like John's, then, offers real hope of establishing that Christ could have genuine unity of action even on a three-part model. However, unity of action is not, in itself, enough for ontological unity. Two people could co-operate perfectly in all matters, but it would not make them one and the same person. Indeed, Christian orthodoxy insists upon this point, since it holds that the members of the Trinity co-operate perfectly but are nevertheless distinct persons. In the case of three-part models of the incarnation, this conceptual distinction between unity of action and ontological unity is most starkly expressed in what we call the homunculus problem.
The homunculus problem
There is a problem which many critics have levelled at composite models of the incarnation. If Christ is a composite, containing (in addition to the divine mind) a human mind and body, which co-operate with the divine mind, then this human mind-body composite itself seems to be a person, a ‘smaller person’ or homunculus within Christ.Footnote 42 There are several assumptions underlying such an objection. One of the most important is the Aristotelian premise that a soul (the substantial form of a human being) and a body (the appropriate matter for a human being) together are immediately a person, with no further relation that unites them.Footnote 43
But the very same Aristotelian metaphysics offers resources to address it. Brian Leftow appeals to the following principle: ‘given a set of parts composing at time t a member of a natural kind (e.g. cat), no subset of that set composes at t a member of the same natural kind’.Footnote 44 If we accept this, then if the composite Christ is a person, no subset of his proper parts can count as a person. But Leftow does not give an explicit justification for adopting this principle (except that it allows one to solve puzzles of this kind). Robin Le Poidevin offers a slightly different principle in addressing the same problem:
… although, in the absence of the divine part annexed to it, the human part would constitute a person in its own right, where it exists merely as part of the composite Christ, it does not do so … . As we might put it: mere parts cannot be people, however accomplished or even divine those parts may be.Footnote 45
Le Poidevin appeals to Leftow's principle in defence of his own. The argument, then, is that a mere part cannot be a person, because a part of something that is F cannot itself be F. But what grounds that principle? Why can't a part be a member of the same natural kind as the whole of which it is a part?
We can provide a more systematic justification of the claim that the parts of Christ are not themselves persons if we regard it as based upon the Aristotelian so-called homonymy principle. We should stress that in so doing, we do not argue for the truth of this principle or of the Aristotelian metaphysics upon which it depends. Rather, we present it as a plausible position which the holder of a three-part composition model of incarnation could use to avoid the homunculus problem. The homonymy principle states that the proper part of a whole ceases to be what it is if it ceases to be a part of that whole. It might be called by the same name, but that name is used homonymously. As Aristotle puts it:
What a thing is is always determined by its function: a thing really is itself when it can perform its function; an eye, for instance, when it can see. When a thing cannot do so it is that thing only in name, like a dead eye or one made of stone, just as a wooden saw is no more a saw than one in a picture. The same, then, is true of flesh … . The parts of plants, and inanimate bodies like copper and silver, are in the same case. They all are what they are in virtue of a certain power of action or passion – just like flesh and sinew.Footnote 46
The homonymy principle is justified by Aristotle's functional analysis of natural kinds: something is identified on the basis of its function. Not just any function, however. Functions are closely linked to essences and definitions. Aristotle states that any given essence can be captured in only a single definition, because a successful definition will specify what makes that essence unique.Footnote 47 In Metaphysics VII.17, for example, Aristotle argues that the principle of unity of any substance is its form. Because a substance has a single form, it is one thing, no matter how many parts it may have. Where there is one substantial form, there is one function. So in Christopher Shields's words, ‘An individual x will belong to a kind or class F iff. x can perform the function of that kind or class’.Footnote 48
If this is so, then it follows straightforwardly that if something does not have the defining function of the kind it belongs to, then it is not a member of that kind. This has important consequences. On Aristotle's view, the function of a part in a whole is determined by its role as a part of the functional whole. It follows that if that part ceases to be a part, its role will change. And that means that its identity will change, too. For example, Aristotle tells us that part of what it is to be a finger at all is to be part of a living body.Footnote 49 Once a finger is severed, it does not simply become a dead finger – it is not a finger at all. We may still call it a ‘finger’, but we are using the word in a different sense. The attached finger acts with a purpose that is integral to the purpose of the living body of which it is a part: it is used to grasp things, for example. A detached finger lacks this property, and it would lack this even if it could move around by itself. The operation of the part is governed by the organizing principle of the whole; and the re-identification of the parts as parts secures the unity of the composite whole.
How does all of this apply to models of incarnation? As we have seen, Leftow argues that there is no homunculus in Christ because Christ's human mind and body cannot constitute a whole human being and a person, because of their being parts of Christ. We suggest that this claim can most plausibly be justified by appealing to the Aristotelian claim that a part cannot retain its own identity in the whole, so a fortiori cannot be of the same kind as the whole – which itself is based upon the claim that identity is very closely connected to function. So on this view, Christ's human body and mind are re-identified as parts of Christ, to whom they lose their personhood. (They are not what they would have been if they weren't parts of Christ.) And this depends upon the principle of functional determination. That is, by being part of the whole, a part can perform a different function from the one it would perform if not part of the whole.
On the position just described, the case of a finger and the human organism it belongs to differs from e.g. the case of a married couple, in the following ways. Husband and wife are able to perform actions as a married couple that they could not do otherwise (e.g. celebrate their wedding anniversary). Yet they are still distinct individuals. Critically, they do not cease to be able to perform the functions they used to perform when not married – functions that qualify each as a person. The functions that the husband and wife are able to perform when married are not tied in with the acquisition of a new substantial form to which husband and wife lose their identity qua persons. (There might be extreme cases of mutual dependence between husband and wife, but no new substantial form is acquired by the married couple.) For the sorts of actions that only a married couple can perform (such as celebrating a wedding anniversary) do not require a single individual as agent. On the contrary, only a couple can perform them. Christ, by contrast, has a defining function which does require a single agent.Footnote 50
What could this function be? The Christian tradition has always held that the purpose of the incarnation was salvation. John 3.17 states that God sent His Son into the world in order to save the world, and Christians ever since have endorsed that this is the function of the incarnate Christ. How can that be cashed out? Mainstream Christian tradition has taught that, to be a saviour, Christ has to be human and also divine. Being only one of the two will not suffice to bring about salvation. Neither a mere human being nor a mere divine person could bring about salvation.Footnote 51 So in the case of Christ too, the constituents or parts of the whole cannot perform the same function as the whole.
Moreover, mainstream Christian tradition has also taught that the Saviour must be a genuine unity, a single person – for otherwise divinity and humanity would not be truly united to each other. That is the traditional rationale for the rejection of Nestorianism. So on the traditional view, the function of saving humanity can be performed only by a single individual who is both human and divine. For this to happen, then, the humanity and the divinity must be re-identified to such an extent that they constitute a single individual. That is a re-identification far more radical than that undergone by two people who marry each other: it involves the adoption of a new substantial form, one defined by a function that only such an individual can perform.
If we accept this functional definition of Christ, then we have a way forward for three-part models of the incarnation. The proponent of such a model needs to show that the constituents of Christ form a genuine unity, to the extent that they form a whole which itself functions in a unique way, that is, bringing about the salvation of humanity. Salvation is the single end to the action to which the divine and human constituents of Christ contribute their respective operations. This picture is directly analogous to Aristotle's example of the finger and its function as a part of the body. By operating as parts of the whole that is Christ, each constituent part of Christ has a function above and beyond the function it would have had if it were not part of Christ. And this function is integral to the function of Christ as a whole – without the contribution of each part, Christ would not save. The parts are thus re-identified as parts. Not only does the Homunculus problem not arise in this situation, but there is genuine unity to Christ as a whole.
In conclusion, we do not seek to present the co-action model just sketched here (or its Aristotelian version) as a full and watertight solution to the problem of the incarnation. A detailed defence of the model would require, for example, an examination of precisely why the function of salvation requires the union of divinity and humanity in a single individual in this way, such that it allows for the re-identification of divinity and humanity as a single individual. Our intention in this paper has been not to provide such a defence of any single composition model, but to distinguish between them and consider which one is the most philosophically promising. On the basis of what we have seen here, the co-action model offers the greatest potential to defenders of composition models of the incarnation.Footnote 52
