This book is the fifth in a series of monographs Richard Cross is writing on western Christology. Now spanning the years 1050 to 1700, I know of no better resources for a philosophically sophisticated curation of the theological discussions.

Cross’s book focuses primarily on three answers to the question, ‘What was assumed in the incarnation?’, charting the trajectory of the arguments for and against those views. The views can be divided, as Lombard did, between:

1. 1. The assumptus homo theory, according to which the thing assumed is itself a man; the man and the Word are not strictly identical, but they are still the same person, by some manner of relative identity.

2. 2. The subsistence theory, which Cross designates as the view of conciliar Christology. This view claims that the thing assumed was a human nature, a flesh-and-blood composite. The man and the Word are strictly identical. It isn’t that a man was assumed; rather, the Word of God became a man. The words ‘God’ and ‘man’ both refer to exactly the same thing – a divine person.

3. 3. The habitus theory, which Cross calls the non-aliquid theory. This theory claims that the Word assumed a human body and soul, but that, importantly, these two created things were not united into some third thing (hence non-aliquid). Since there is no further thing there composed of body and soul, there’s no worry of that further thing being itself a man or a person.

In the first six chapters of the book, Cross walks the reader through the twists and turns of the early scholastic discussions of these views. The last four chapters, which comprise Part 2, ‘Conciliar Christology, 1180–1250’, primarily present the advances of the subsistence theory.

Much of the work of the book is showcasing the trajectory of the arguments for or against these three views, teasing out the view of sameness (strict identity; relative identity) assumed on the part of the proponents of the views, and analysing the success of the arguments. Allow me to say just a little about these discussions, highlighting what I found most interesting.

The homo assumptus view, Cross notes, was ‘widely regarded as a weak form of Nestorianism’ (p. 54). Simon of Tournai (1130–1201) argued against the homo assumptus view by noting that if the Word assumed a man, then he assumed either the same man as himself or another man. If another, then we have a form of Nestorianism – there are two men there. If the man assumed is the Word himself, then the assumer is the assumed, which is absurd. Thus, the Word did not assume a man, contrary to the homo assumptus view (pp. 65–66).

Another interesting argument to the same conclusion comes from Richard Fishacre’s (1208–1248) ‘devastating impugnation of the orthodoxy of the homo assumptus theory’ (p. 214). If, as the homo assumptus view has it, ‘man’ supposits for the assumed thing, the composite of body and soul, then ‘man’ cannot be predicated of the Creator. But then you cannot say, ‘this man is God’, or ‘this man created the stars’, things one must say according to traditional Christian teaching.

Turn now to arguments concerning the non-aliquid view. Peter of Poitiers (1125/30–1205), himself a proponent of the view, considers an argument against it and ‘admits defeat’ (pp. 126–7). If there is nothing there that is Christ’s composite of body and soul, then there is nothing there in which certain properties can inhere (for instance, to be able to laugh, to run, to cry, etc.). But if there is no human nature there, in what would such features inhere? Not just the soul. Not just the body. Not just the divine nature. And so, there’s an essential ontological component missing from the non-aliquid view. Cross traces this argument down to one of its earliest proponents, Magister Vacarius (1115/20–1200).

Another variety of argument, again from Vacarius, claims that if the non-aliquid view were true, ‘there would be no subject for the abstract form or essence humanity’ to inhere in (p. 138). See also Cross’s listing of reasons why the non-aliquid theory is ‘theologically inadequate from the perspective of Conciliar Christology’ (p. 145).

The reader might wonder: why stop the narrative at 1250? Wouldn’t including Aquinas, whose arguments against the first and third views are well known among those interested in Christology, be most germane to this volume? I offer two replies for Cross.

First, this book is one in a series on western Christology, and there’s already a volume that focuses heavily on Aquinas (The Metaphysics of the Incarnation: Thomas Aquinas to Duns Scotus, 2002; Cross is preparing the second edition now). So, the work is already done and done well elsewhere for the reader.

Second, I think that part of Cross’s project in this book, and across the others as well, is to broaden the contemporary narrative of mediaeval philosophical thought. Aquinas is, of course, a discipline-changing thinker. But someone so towering casts a large shadow. Cross’s thinkers, many of whom I’d never heard of, work out careful and clever arguments for or against these three theories. Indeed, much of the reasoning that Aquinas uses in his celebrated criticisms of the rival views was already deployed by previous thinkers, as Cross points out. He’s telling their story in this excellent book.