Eunomius of Cyzicus' epistemological optimism led him to a seductive but fatal reductionism: to know ‘unbegottenness’ is to know the divine essence (ousia). According to Eunomius, the Son may be ‘divine’ in some sense, but his ‘begottenness’ means he cannot be divine in the same way, or to the same degree, that God the Unbegotten Father is divine. The Son must be ‘of another essence’ (heterousia). A particular understanding of divine simplicity motivated Eunomius' position. Andrew Radde-Gallwitz terms that understanding the ‘identity thesis’. Eunomius claims that all the ‘goods’ of the divine nature (goodness, wisdom, life and so on) should ultimately be identified with one chief theological datum. Radde-Gallwitz' monograph argues that Basil and Gregory's response to Eunomius transformed the theory of divine simplicity. The result was a reversal of working assumptions. Theologians shifted from assuming that divine simplicity entailed the ‘identity thesis’ (with its exclusion of multiple divine propria) towards viewing divine simplicity as entailing the existence of multiple divine characteristics (propria).

Radde-Gallwitz tracks major shifts in early Christian understandings of simplicity from Marcion to Eunomius. With Ptolemy's response to Marcion, simplicity functioned as a ‘grammatical rule’ in rendering the complex God of scripture as not self-contradictory, as consistent. Clement of Alexandria's take on the doctrine led him to a radical apophaticism before the Father. And, though Origen nuanced Clement's account by introducing the complex Son's many names (epinoiai), both theologians bequeathed to third-century Alexandria an ineffable Father. Early fourth-century debates over the Son's divinity pushed Athanasius to a more confident theological epistemology. For him, simplicity meant that ‘our names for [God] name nothing other than his essence’ (p. 83). In turn, Athanasius' version of simplicity stood behind that of Aetius and Eunomius, whose identity thesis lay at the other end of a spectrum from Clement's apophaticism.

Basil had to strike a balance ‘between direct correspondence and hence comprehensibility [of God] on the one hand, and pure agnosticism or equivocation on the other’ (p. 114). He refused to admit definition of the divine essence into his theological calculus, arguing instead that it is possible to know divine propria, which fall short of being definitional. An important theme running through Radde-Gallwitz' treatment of the difference between Eunomius and the Cappadocian brothers is their different conceptions of the theological task: one aspires to austere epistemological precision (Aetius and Eunomius), whereas the other is an ascetic quest for purity – moral as well as intellectual (Basil and Gregory). Modern scholars have tended to view Gregory of Nyssa's intellectual star as shining brighter than his brother's. Radde-Gallwitz resists this tendency by characterising Gregory as developing his brother's incipient insights. Gregory's riposte to Eunomius maintains that the divine goods, though reciprocal, are irreducible. Eunomius' epistemological reductionism ends up looking like moral hubris.

Some readers may be tempted to question Radde-Gallwitz' claim that Gregory maintains a distinction between divine ‘essence’ and ‘substance’ – a distinction that, on Radde-Gallwitz' account, saves Gregory from Eunomian criticism. On the face of it, one wonders whether the distinction actually belongs to Gregory, or whether Radde-Gallwitz' sympathies haven't got the better of him. Yet the textual case is strong: he goes to great lengths to defend his exegesis, well-researched and documented (e.g. p. 216). But if it is indeed Gregory's voice, hasn't Gregory made a distinction without a difference? After all, what does it matter that propria inhere in the divine substance if we have gained no knowledge of the divine ‘essence’? Doesn't ignorance of one amount to ignorance of the other? As tempting as that question may be, it turns on Eunomian commitments and concerns. And here we see the strength of the book's closing chapter, which forestalls this ‘Eunomian’ – or is it ‘modern’? – question. The distinction is not without a difference, Gregory could respond, just insofar as it grounds the important distinction between trustworthy seeking after God (theology) and ‘scientific’ definition – which ends theological conversation in a single blow.

Radde-Gallwitz is historically rigorous in his attention to intellectual influences running in several directions – both in the fourth and the twentieth centuries. His exegetical accounts are meticulously argued. When it comes to the big picture, he manages to characterise a trajectory of thought running from Basil to Gregory as anti-Eunomian, but also as non-Thomist, all without falling into the trap of portraying the ‘Cappadocians’ as ‘proto-Palamites’. Such a nuanced portrait is hardly a recipe for making fast friends in major theological camps, but it is a sure way to win the trust of scholars. The result is a tightly argued historical-theological account which historians of fourth-century doctrine will need to know.

A number of astute sidelong glances into modern philosophy of religion make the book a worthwhile read for specialists outside the discipline of ‘historical theology’ per se. Radde-Gallwitz is not too shy to transgress the chronological boundaries of the fourth century. Concerning divine agency, Basil and Gregory are supposed to have succeeded in rendering the world open to divine agency in a way foreign to both Eunomius and the late-modern ‘buffered self’ criticised by Charles Taylor. When it comes to theological epistemology, perhaps Radde-Gallwitz' narrative will hold the attention of Kevin Hector's readership (Theology without Metaphysics). Hector has tried to render optional (though not impossible) an ‘essentialist-correspondentist’ picture of truth, thereby obviating an apparent choice between apophaticism and (essentialist-correspondentist) metaphysics. Basil and Gregory, in Radde-Gallwitz' telling, remained committed to correspondentism even while staving off thoroughgoing apophaticism.