Russell Friedman states that his purpose in this volume is ‘to give a broad overview of some of the central aspects of and developments in the trinitarian theology written in the Latin West between roughly 1250 and 1350 ad’ (p. 1). The work of so many Western Christian theologians in the thirteenth and fourteenth centuries was not to formulate anew the doctrine of the Trinity, but to ‘explain precisely how three really distinct persons, the Father, the Son, and the Holy Spirit, can be essentially identical, i.e. identical in the one, simple divine essence’ (p. 2). The author restricts his focus to two major aspects of the late medieval trinitarian discussion: first, the metaphysics of identity and distinction in the Trinity (chapters 1 and 4); second, the use of the ‘psychological model’ which maintains that the Son is the Father's mental Word or Concept and the Spirit is the Gift or Love shared between Father and Son (chapters 2 and 3).

The basic contest Friedman addresses in chapter 1 is that between the Dominicans following Aquinas and the Franciscans following Bonaventure. The challenge for both parties was to explain the distinction between the divine essence and the personal relations by which each divine person was distinguished. Compared to the essence, the distinctions among the persons vanish since each person is the same as the divine essence (i.e. the persons are not accidents inhering in the essence); but, when compared to the other divine persons, each person is minimally distinct from the essence and really distinct from the other two persons. By affirming this twofold nature of the relations both parties could affirm real distinctions among the persons and a doctrine of divine simplicity in which God is not composed of essence and persons.

Though agreeing that the distinctions in the Godhead are based on the personal property unique to each person, the Dominicans and Franciscans disagreed on how to account for the personal properties. Did the relations precede persons and emanations, as the Dominicans held, or did they follow from the emanations and persons, as the Franciscans maintained? Friedman observes that Aquinas and his ilk could not abide the priority that Bonaventure granted to the Father's emanating activity in the production of the persons and relations because it presumed that the Father was a divine person before he was the Father of the Son. This ‘proto-Father’ was marked, in Bonaventure's scheme, by the personal property of primity or innascibility. In discussing the defence proffered by the Franciscan John Pecham, Friedman explains, ‘Primity is the starting point of the Father's distinction, since it gives the proto-Father enough being to allow the Father's full distinction to come about through the generation of the Son’ (p. 37). Aquinas’ metaphysical convictions regarding God's pure actuality undoubtedly fuelled his rejection of the Franciscan ‘proto-Father’ thesis. Unfortunately, Friedman's narrative omits any detailed discussion of this fact and will probably leave some readers confused as to the exact reasons for the Angelic Doctor's rejection of the Franciscan understanding of primity.

In chapter 2, Friedman considers the role of Henry of Ghent's ‘strong’ psychological model of the trinitarian relations as a key element in the development of the Franciscan tradition. He identifies two features of this model: (1) it stresses a tight link between the divine attributes and the divine emanations (understanding the Son as intellectual emanation and the Spirit as volitional emanation); (2) it attempts to make trinitarian explanations comport with concept theory and will theory or, as moderns would say, the theory of mind (p. 59). But the Dominicans insisted that this stronger psychological model undermines God's simplicity by making the distinctions between the divine persons depend too much upon the distinctions between the divine attributes (such as knowledge and will): the distinctions among the persons are real while those among the attributes are merely conceptual (each attribute being really identical to all the others in God). Friedman concludes the chapter by detailing how Duns Scotus advanced the Franciscan use of the psychological model beyond Henry (pp. 75–92). Given the subtlety of the issues involved, the author's brisk and clear style throughout the chapter is admirable.

Chapters 3 and 4 are shorter and focus primarily upon the development (or demise) of the Franciscan emanation account in the fourteenth century. Friedman observes that divine simplicity came to exert a more forceful influence upon some Franciscans, causing them to modify or abandon the psychological model of explanation. He remarks, ‘the more strictly a medieval theologian takes divine simplicity to be, the less room he is going to have for any kind of explanation in trinitarian theology, since the distinctions upon which analysis and explanation are built up will be ruled out’ (p. 100). In their quest for a stronger doctrine of simplicity many Franciscans began to tone down their use of the psychological model. Friedman denominates William Ockham's version of it as ‘psychological model lite’ (p. 130).

If Ockham advocated a ‘lite’ version of the psychological model some of his Franciscan heirs (Friedman considers Walter Chatton, Robert Holcot and Gregory of Rimini) fail even to mention the psychological explanation. ‘This in itself’, Friedman remarks, ‘is a manifestation of the search for simplicity’ (p. 138). It leaves these Franciscans fideistically asserting the orthodox trinitarian distinctions without a meaningful explanatory model. One deficiency in both chapters 3 and 4 is that Friedman fails to offer a working model or definition of divine simplicity so that readers can appreciate just what it was about the doctrine that made it so devastating to trinitarian explanation. After all, theologians such as Thomas Aquinas and, to a lesser extent, Augustine, were committed to strong accounts of divine simplicity and to an extensive use of the psychological model for explaining the trinitarian distinctions.

All told, Friedman's volume is a lively and informative contribution to our understanding of the dynamics of trinitarian debate and development in the thirteenth and fourteenth centuries. This volume should be welcomed not merely by the historically curious but by any Christian seeking to understand how to maintain God's unity of essence and diversity of persons.