This very useful monograph has been prompted by the joint statements issued by the Eastern Orthodox and Oriental Orthodox theologians who have been meeting, at first unofficially and then officially, since the early 1960s. In examining these statements Hans van Loon was struck by their miaphysite presentation of Cyril of Alexandria's Christology, which was at variance with his own reading of Cyril. He therefore set himself the task of investigating the key terms physis, hypostasis, ousia and prosopon in Cyril's writings up to the end of 430, not only as an exercise in historical theology but also with the aim of contributing to current ecumenical dialogue.

He begins with a chapter on Cyril's use of logic which establishes some helpful facts. Since the work of Labelle, Siddals and Boulnois, there has been general agreement that Cyril was familiar with Aristotelian methods of argument. He was certainly more skilful in his handling of logic than de Durand, for example, would allow. He is able to apply Aristotle's teaching on predicables to the doctrine of God, and discusses categories and their characteristics with confidence. But he does make mistakes, believing, for example, that ‘like’ and ‘unlike’ are relatives. He appears to be not quite the master of logic that some have claimed. However, as van Loon shows, he did advance in knowledge between the Thesaurus and Contra Julianum.

In the next chapter van Loon rapidly surveys the terms ousia, physis, hypostasis and prosopon. Ousia is equivalent to a reality which is a secondary substance, the ousia of God indicating what is common to the Father, Son and Holy Spirit. Physis also means a secondary substance, but in a less precise way. Kata Physin has two meanings: ‘substantially’ (i.e. belonging to an entity in virtue of being a differentia, proprium or inseparable attribute of the substance); and ‘naturally’ (i.e. being the result of a ‘natural’ process). Hypostasis (a Porphyrian term) is equivalent to a primary substance, something that exists by itself. Prosopon, finally, is primarily a grammatical person, but is also used to mean a rational ontological being.

Having established a basic outline of Cyril's use of these terms, van Loon devises a number of expressions to translate them into modern notions. These expressions are printed throughout the book in small capitals and are referred to as small-capital terms. This is an excellent device which gives visual clarity to the discussion. One is never in any doubt whether it is Cyril's terms that are being discussed or the ideas that they refer to. There is a further advantage, too. Modern writers each have their own preferred vocabulary which makes it difficult to compare their views with any precision. Van Loon translates their vocabulary into his small-capital terms to ‘reduce the equivocality’ of their discussions. He is thus able to correlate the views of all the major Cyril scholars who have concerned themselves with the technical aspects of Cyril's Christology. What emerges is a complete lack of consensus among them. There are two main groups, one beginning with Lebon, who treats physis, hypostasis and prosopon as equivalents (‘separate realities’ in small-capital terms), the other with Jugie, who disagreed with Lebon, seeing physis as equivalent sometimes to ousia and sometimes to prosopon (an ‘essence’ or ‘separate reality/individual nature’). Van den Dries builds on Jugie and is followed by Grillmeier. Lebon is followed by de Halleux and the theologians of the Eastern Orthodox/Oriental Orthodox dialogue. Other theologians, such as McGuckin, Meunier, McKinion and Weinandy, are plotted in relation to them according to their interpretations of the key terms as reduced to their small-capital equivalents.

The main body of the book consists of three chapters analysing Cyril's terminology in his writings from the beginning of the controversy to the eve of the Council of Ephesus. Van Loon does not find that the Nestorian controversy brings about any change in Cyril's Christology, but the language he uses does undergo some development. The word hypostasis, for example, comes to the fore during this period. In the earlier works it is not applied to the incarnate Son, but by the later phase of the controversy ‘one hypostasis’ comes to replace ‘one prosopon’ because Nestorius interprets the latter as implying a union of two prosopa. For Cyril a prosopon is ‘a rational being capable of communication with other such beings’. It does not denote a metaphysical ‘bearer’ already bearing the divine nature who also becomes the bearer of a human nature. But Nestorius' use of the word has made its meaning equivocal, so hypostasis suits Cyril's purposes better.

An important finding of van Loon's is how insignificant miaphysite language is in Cyril's writings. In the first two years of the Nestorian controversy Cyril uses the mia physis formula only twice, whereas he speaks of a union ‘according to hypostasis’ seventeen times. Even with the mia physis formula he thought he was quoting Athanasius. But the ‘self-perpetuating myth that the mia physis formula was his favourite formula’ goes back to the fifth century. In van Loon's view, people of Cyril's own party put such emphasis on mia physis because ‘they were more influenced by pseudepigraphic Apollinarian writings than by those of Cyril’.

Van Loon is convinced that ‘Cyril's dyophysitism is not just notional but real’. He is therefore opposed to Lebon and de Halleux who, in his view, see Cyril through a miaphysite lens. Nor does Grillmeier do justice to Cyril's fundamental dyophysitism. The modern author van Loon seems most drawn to is Keating, whose conclusions, he says, ‘are vindicated’. It follows that van Loon is against saying that Cyril made ‘concessions’ to the Orientals (as McGuckin believes) at the time of the Formula of Reunion. Cyril's terminology was already dyophysite.

This is an important book which sets a benchmark for future work on Cyril's Christology. Nobody has previously studied the terminology that Cyril uses in such detail or with such thoroughness and clarity. Van Loon has succeeded admirably in filling the lacuna that he identified at the outset of his studies. It will be interesting to see what influence his compelling defence of Cyril's dyophysitism will have on ongoing ecumenical discussions.