The doctrine of incarnation has been a central topic of Christian analytic theology ever since 1986, when Thomas Morris's groundbreaking book The Logic of God Incarnate landed – if not quite like a bomb on the playground of the theologians – at least like a fair-sized grenade. In the face of Morris's arguments, it was no longer possible for liberal and sceptical theologians to assert without argument that the Christian doctrine of incarnation, as traditionally understood, was incoherent and must be replaced. Analytic theologians have since built upon (or disagreed with) Morris's arguments in a discussion whose size and scope are rivalled only by its complexity. (Key volumes from the last twenty years include Marylin McCord Adams's Christ and Horrors, Oliver Crisp's three works Divinity and Humanity, God Incarnate, and The Word Enfleshed, Timothy Pawl's In Defense of Conciliar Christology and In Defense of Extended Conciliar Christology, and Jc Beall's The Contradictory Christ.) Much of this work has helped to establish consistent terminology, as well as to classify the different approaches. Most (not all) contributors to the discussion share Morris's view that a traditional doctrine of incarnation is coherent: that is, Christians can affirm, without inconsistency, that Jesus was a single person who was fully divine and fully human. But there is sharp disagreement over the truth conditions of this claim. What is the metaphysical story that could make such a claim true? Different answers to this question – or ‘models’ of the incarnation – have proliferated.

Robin Le Poidevin's new monograph is a welcome addition to this literature. He brings to the topic two key strengths: a weighty background in metaphysics and a lightness of style, both of which are deployed effectively throughout. Le Poidevin proves a sure-footed guide through topics such as essentialism, dualism and monism, conditional and absolute necessity, the nature of time, three- and four-dimensionalism, and more, with his elegant and often witty writing enhancing the clarity. I would single out the discussion of persons and substances (140–141) as especially clear and helpful.

The book divides neatly into two main sections. In the first, Le Poidevin surveys the available models of the incarnation, which he sorts into three main categories (which themselves can be further subdivided). The first group is compositional models, according to which the divine Son became human by acquiring a human body (and soul, if there is such a thing). So Christ is a composite with a divine part (the pre-incarnational Son) and a human part, which are distinct from each other. Models of this kind have dominated in classical Christian theology and are probably the most popular today. The second group is divided mind models, according to which the Son became human by (a) acquiring a human body and (b) dividing his mind in some way, so that it hosts both divine and human streams of consciousness. And the third group comprises kenotic models, according to which the divine Son became human by losing certain properties.

Le Poidevin argues that both compositionalism and the divided mind models are irredeemably flawed. In the case of compositionalism, there are two main sub-categories, for which Le Poidevin borrows Thomas Flint's names of ‘Model A’ and ‘Model T’. On Model A, the Son is a proper part of Christ, along with his human part, but for Le Poidevin this leaves the model unable to explain how the Son can legitimately be called human. On Model T, the Son is identical with the composite Christ, and has his human nature as a proper part of himself, but Le Poidevin thinks this falls victim to the paradox of growth (the Son is somehow identical to both the pre-incarnate Son and the post-incarnate Christ, but the former is a proper part of the latter). The divided mind fares little better. Le Poidevin argues that proponents of this view must show that the two mental streams in Christ's mind are distinct enough to represent genuinely divine and human consciousnesses, but at the same time are not distinct enough to represent two persons. If this is possible at all, he thinks, it can be done only by collapsing the model into compositionalism.

That leaves only kenotic models. The final chapter of the first part is devoted to categorizing kenotic models and considering some of the key arguments against them, which clears the way for the second part of the book. The purpose of this part is to defend kenotic models against different versions of a single objection. The objection is that, according to kenotic models, the Son loses certain properties in becoming human; but some of these properties are essential to the Son's identity. If he were to lose them, he would simply cease to exist. These ‘kenotically recalcitrant properties’ (118) are immateriality, necessary existence, being the ground of goodness, and atemporality. The worry is that the gulf between (say) an immaterial object and a material one is so vast that one cannot simply change into the other while remaining the same individual. Even worse, to suppose that a necessary object can transform into a contingent one (or a timeless object into a temporal one) is itself incoherent, since if it did, it would show that it was never necessary (or timeless) to start with. (Note: this objection is distinct from the more common objection that there are some properties, such as omnipotence and omniscience, that are essential to divinity, and if the Son were to lose them he would cease to be divine. In response to that objection, Le Poidevin follows Morris's suggestion, adopted widely among kenoticists, that the really essential properties of divinity are not ‘omnipotence’ etc. but ‘omnipotence unless choosing not to be omnipotent’. The Son could retain the latter property while divesting himself of the former.)

Accordingly, the chapters in the second half of the book focus on the kenotically recalcitrant properties, with Le Poidevin arguing that in each case the property in question is not recalcitrant after all. The Son could bear these properties in his pre-incarnational state and lose them when becoming incarnate without ceasing to exist, or to be himself, after all. Kenoticism therefore emerges as the most philosophically coherent model of incarnation.

Much of the material in the first half of the book follows already well-trodden routes. Many of the objections that Le Poidevin raises against compositionalist and divided mind models, for example, will be familiar to those already acquainted with the topic. But for those who are not, Le Poidevin gives a good overview of, and judicious commentary upon, the state of the debate. The second half is more original and interesting. Le Poidevin focuses on one of the less prominent objections to kenoticism, but one that plays to his strengths, as it proves metaphysically dense.

Le Poidevin offers different strategies for overcoming the problem of recalcitrant properties. For immateriality and ethical grounding, he argues that these properties need not be essential to the Son's identity, and that consequently he can give them up when becoming incarnate without ceasing to exist. But in the cases of necessity and atemporality, he redefines the properties in question. With atemporality, Le Poidevin sketches a causal relationist theory of time, according to which the temporality of any event is parasitic upon its forming part of a causal sequence. God's actions may, but need not, form part of the same causal sequence as our own, meaning that God may be part of our temporal structure without being bound by it. This means that, although God transcends time in a way that we do not, the transcendence is not absolute. So God the Son could become more enmeshed in our temporal sequence without a loss of essential properties. Although developed relatively briefly, this account of the divine relation to time is of inherent interest quite apart from its application to the incarnation. As is well known, theists are divided between those who view God as temporal and those who view God as atemporal. Le Poidevin's account here could easily be elaborated upon to provide the advantages of both views, and perhaps avoid the disadvantages.

On the question of divine necessity, Le Poidevin distinguishes between absolute and conditional necessity: the Father is absolutely necessary, but the Son is only conditionally necessary, which is to say that he is not self-sufficient, and this is enough for the Son to share in the human condition. Here, I would say that Le Poidevin under-sells his argument. He presents this distinction between the different kinds of necessity within the Trinity as a refinement of Trinitarian doctrine that cannot avoid ‘elements of Arianism’ (168), the implication being that kenoticists must simply put up with being, at best, Arian-adjacent. But the notion that the Son (and the Spirit) are ontologically dependent upon the Father, while the Father is dependent upon nothing, is clearly present within the Christian tradition even in the context of anti-Arian theology. Gregory of Nazianzus declares that the Son and the Spirit ‘are from’ the Father, ‘though not after him’ (Theological Orations III.3). For Gregory, they differ in causal status, but he strenuously rejects the Arian conclusion that this aetiological difference reflects an essential difference. A human father is the cause of his son, and not vice versa, yet they do not differ essentially. Le Poidevin's careful working out of the modal differences between Father and Son seems to me to fit well into Gregory's kind of Trinitarianism without needing to admit any Arian tendency.

Overall, Le Poidevin makes an impressive case for the viability of kenoticism – at least in response to the problem of recalcitrant properties. Just as Morris showed that anti-incarnational theologians could no longer assume the incoherence of the incarnation, Le Poidevin has now shown that anti-kenotic theologians can no longer assume the incoherence of (say) an immaterial God becoming material. He is perhaps a little too quick to reject other objections. For example, he answers the incoherence objection (how can the Son be genuinely limited in knowledge and yet also omniscient, for example) by distinguishing between different stages of the Son's career: the Son is omniscient prior to the incarnation, limited in knowledge during it, and omniscient again afterwards (99). But while this is coherent, it assumes that the incarnation is a temporary situation for the Son, who regains his divine powers at his glorification. If the kenoticist thinks that lacking these powers is essential to humanity, then she seems committed to thinking that the glorified Son is no longer human. Le Poidevin does not explicitly state such a view, but he does suggest that the Son's embodied state (123) and lack of grounding of moral values (188) are both temporary conditions. But such a view seems to me to be at odds with key elements of Christian spirituality. Christians pray to Jesus as God, certainly, but also regard him as a high priest now who ‘serves in the sanctuary’ (Hebrews 8:2), something that surely indicates his ongoing humanity. Le Poidevin does suggest that the Son's continued humanity is grounded in his memories of his human lifetime (116–118). This is a line of thought that could usefully be developed further. One potential issue is that if the Son's ongoing humanity consists solely in his memories of his lifetime, there seems no need for him to continue to be embodied after his exaltation. And that may cause problems for, among other things, sacramental theology.

Most contemporary analytic theologians suppose that even if Jesus retained divine powers such as omnipotence and omniscience during his lifetime, he did not use them (and may have been unable to do so). Le Poidevin generally assumes the superiority of such a position. He states, for example, that ‘[o]ne of the advantages of kenoticism is that it allows us to say, without qualification, that Christ was not aware of his divine status’ (116). I suspect that most ordinary Christians would be startled by this! It is one thing to suppose that Jesus was humanly ignorant of relatively unimportant matters such as how to conjugate French verbs or the annual outflow of the river Amazon. It is quite another to suppose that he was unaware of vitally important information about his own identity. (If he was, then further unsettling thoughts are possible. What if I am divine without realizing it?) Perhaps there are good reasons for preferring a Christology which denies such knowledge to Jesus, but Le Poidevin does not give them – he merely takes it as read that such a Christology is to be preferred.

One section of Le Poidevin's book where this tendency shows especially strongly is in his treatment of divided mind models. One of his criticisms, taken from Tim Bayne, is that if (1) Christ had false beliefs, and if (2) Christ's mind is a subset of the divine mind, then (3) the divine mind contains false beliefs, which is inconsistent with divine omniscience. In support of (1), Le Poidevin writes (78, see also 100): ‘The Synoptic Gospels present Jesus as a figure who, like any other human being, is prone to error (at least of a factual, if not moral, kind). Surely this proneness would sometimes have resulted in false beliefs? (1), then, is highly plausible.’ But this is surely far too quick. Certainly it is true, as Le Poidevin and other analytic theologians remind us, that the Synoptic Gospels present Jesus, at least implicitly and in some places, as ignorant, tempted, limited in power, and so on. But it is also true that the Synoptic Gospels (even without mentioning John!) describe Jesus far more often as uniquely insightful, close to the Father, authoritative, and powerful. I would add that the claim that Jesus had a human belief system that was ‘prone to error’ and therefore must sometimes have actually been in error seems inconsistent with Le Poidevin's argument elsewhere (184–187) about Jesus’ temptations. There, he wants to say that Jesus was really tempted, in that he felt the desire to do wrong and was not ‘simply hard-wired to resist temptation’ (186). And yet he never did succumb. So, if it is consistent to hold that Jesus had ‘the desire for something other than the good’ (186) and also that Jesus nevertheless always chose the good, can it not also be consistent to hold that he was prone to factual errors but nevertheless never made any?

Again, perhaps theologians should maintain that Jesus held false beliefs – even false beliefs about his own identity. But if so, this is a position that needs to be robustly justified, in acknowledgement of both biblical material and the majority of Christian tradition that either implicitly suggests or explicitly proclaims otherwise. The notion that Jesus held false beliefs cannot, in the context of Christian theology, be a largely assumed premise in an argument for something else. Indeed, Le Poidevin himself elsewhere (155) wants to avoid attributing serious errors to Jesus. This seems to me inconsistent with supposing that (a) Jesus was divine and (b) Jesus was not aware that he was divine, since surely if Jesus was not aware that he was divine he must have thought that he was not divine. (Can anyone be neutral on the question of their own divinity?) And that, clearly, would have constituted a serious error indeed.

There is one other point I would like to comment on in the discussion of the divided mind, where Le Poidevin discusses the conciliar requirement that Christ be a single person. He notes that when classical theologians laid down this requirement, they were not working with the concept of ‘person’ that most of us assume today. They meant a substance (which is rational); they did not mean a psychological unity. Le Poidevin observes (83):

But if the conciliar fathers laid down that Christ is a single person, taking the word ‘person’ to mean X, and we today use the word ‘person’ to mean Y, it does not follow at all that we should take the fathers’ decree as requiring that Christ be a single person in the sense of Y. Not only do we have no pronouncement from the fathers stating that Christ was one ‘person’ in the modern psychological sense, but some of them – notably the Third Council of Constantinople's decree that Christ had two wills – at least strongly imply that he was two. Le Poidevin's point here is of course not intended as conciliar exegesis (he does not think that the conciliar fathers were trying to say that Christ is a single person in the psychological sense). Rather, he means that there is a theological case for the view that Christ was a single person in this sense. But then, if this case is not based on conciliar exegesis, how does it relate to Chalcedon? There may indeed be a good case for this view, but I think it needs to be spelled out more clearly.

If these are flaws in Le Poidevin's argument, they are common flaws in contemporary discussion of this subject. They are certainly outweighed by the strengths of Le Poidevin's original treatment of the objections to kenoticism and the extremely valuable discussions of how these objections – and his answers to them – reflect current issues in metaphysics more broadly. As such, this is an excellent volume that deserves to be widely read.