Another fine book I've just finished is Andrew Radde-Gallwitz's Basil of Caesarea, Gregory of Nyssa, and the Transformation of Divine Simplicity
I first became interested in the doctrine of simplicity a couple of months ago, in connexion with a number of other things. I've really been enjoying the books in the Oxford Early Christian Studies series - they are all top-notch. So I picked up this one to explore a bit deeper as to how simplicity functions for some of the Fathers.
First, let me say that RG writes with great clarity in a fairly tricky field. The arguments that occupy his book involve complex Greek debates over theology, language, and ontology. RG's main line of argument is to reject Christopher Stead's over-simplification (!) and dichotomisation between (a) the identity thesis, and (b) radical apophaticism.
The first couple of chapters look at figures such as Ptolemy, Marcion, Clement of Alexandria, Dionysius of Alexandria, etc.. I'm going to pass over them to focus on the later chapters which deal in depth with Aetius, Eunomius, Basil, and Gregory of Nyssa.
In picking up the argument with Aetius, the debate of the late 4th century with the Heterousians becomes very clear through RG's treatment. For Aetius, and Eunomius after him, 'ingenerate' names God's essence, and so we get a version of the identity thesis. The identity thesis is simply that God is his attributes, self-same and identical. For Eunomius, building on Aetius, this means that all God's attributes must be synonymous, since language and ontology must map 1-to-1. So, since ingenerate names God's essence, all other attributes of God are synonymous to ingeneracy. For Eunomius the alternatives are dire: an inability to say anything about God. To know God is to know his essence.
Basil's response is to chart a course between total apophaticism and Eunomius' identity thesis. Basil makes a number of moves. Firstly, he believes that language about God is fixed by common usage firstly, so that (contra Eunomius), 'father' is not a case of homonyms, two separate words in the case of God and humanity, but rather 'father' is applied with the same meaning to God and to humans, yet it also must be 'purified' of creaturely connotations. Further, Basil underlines a number of distinctions that preserve knowledge about God without requiring that we know the 'whatness', the essence of God.
Basil's distinctions destroy the ability of 'ingeneracy' to name God's essence, by showing that it is a relative term, which never names essences. Furthermore, he develops an account of propria, those things which properly belong and identify a thing, without being definitional, partitional, or complementary. That is, God's goodness does not define, nor constitute, nor go to making up, nor function as a predicate complement, to God's essence, but is a coextensive property not destructive of simplicity.
Moving into Gregory's defence of Basil, RG does a great job of teasing out the distinctives and the contributions. Gregory takes up Basil's account of conceptualisations, also key, which is a way of speaking about our understanding of God without either denoting the essence of God, or engaging in sheer fiction. That is, conceptualisations are second-order reflections on primary truths about God, primarily revealed through scripture, that allow us to make distinctions that exist conceptually, even where such distinctions do not exist ontologically in the simplicity of God.
Part of Gregory's work is to develop an account of Basil's propria as 'the goods', the unmixed and unlimited virtues that are God's idiomata, which are mutually entailing and so also co-extensive. This involves a more detailed treatment of true and false goods for humanity as well.
I fear I have not given a good account of RG's work. He is clearer than I am, by far. What is particularly enjoyable is that clarity, and to see how Gregory of Nyssa in particular develops Basil and provides a strong case for simplicity which involves a philosophical complexity that avoids total apophaticism, as well as the identity thesis of Eunomius, which is a hyper-rationalist account of Christianity that destroys orthodoxy. The Cappadocians' work is not always appreciated in its depth and nuance, and this fine volume goes someway to correcting this. It also helped me understand Eunomius a lot better, which is important for my thesis in other ways.