Behr (282-324), Ayres (188-221)
Behr helpfully situates Basil against the theology of Aetius and Eunomius. Their theology was characterised by certain key elements: that speech of God must be accurate, otherwise it was fantasy; that words applied to God are not done so analogically, but homonymically: ie., 'Father' is a different word altogether when spoken about God than when it is spoken of humans. It is important to distinguish this from the Athanasian position, that 'Father' takes its meaning from the nature of the thing spoken of, so that a certain analogy is preserved. Eunomius' position is far more radical with regards to language. For Eunomius, 'unbegotten' is the primary and exact designation for God, which by terminology excludes the Son from proper deity. In Eunomius' theology, the Son is the image of God's activity, not of his essence. (see Behr, p267-82).
When we come to Basil then, we see him emerging from a broad Homoiousian context, but rapidly moving to the development of a pro-Nicene theology with strong resonances (though not dependencies) to Athanasius, and even the western pro-Nicenes.
On the question of theological language, Basil makes 'Father' the primary category, not 'unbegotten'. This is grounded in the revelation of the scriptures, that it is only through the Son that God is known. Speculation about 'the unbegotten' by itself is pure speculation, not theology grounded in the Apostolic proclamation. Basil develops an account of human reflection, ἐπίνοια, in which the mind stands between words and essence, and words do not reach the essence itself (Ayres traces some lineage of Stoic thought behind epinoia). Indeed, for Basil as for the pro-Nicenes as a whole, one cannot know the 'what' of God, but rather the 'how': we know God, as he is, through his activities, not in his essence. So 'unbegotten' tells us of the 'how' of the Father's existence, not 'what' he is.
In response to Eunomius' exegesis of the Son as 'offspring' and 'created thing' ( γέννημα and ποίημα), Basil follows the same partitive exegetical method we have encountered already, speaking of things spoken theologically, and economically. This is key for his interpretation of the disputed Acts 2:36.
One of the key contributions of Basil, and the Cappadocians as a group, is work on the concept of identity and difference, and how the One and the Three should be related. Basil's basic solution is to ascribe “whatever properties are held to characterize divinity” (Behr, p294) to the principle of the essence, but whatever is peculiar to each person, to the Person. Basil neither considers the divinity as some kind of substratum, nor does treat the essence as a kind of 'genus' or class of being, of which the Three are instances.
Basil also affirms the doctrine of inseparable operations, pointing in De Spiritu Sancto to their common work as affirming their common divinity. Although Basil is engaged in the terminological development of one ousia, three hypostaseis, that is not the be all and end all of his trinitarian thought. Importantly, Basil's account of hypostaseis means that, since they are distinguished according to what is particular to each of them, there is nothing 'common' to all three as 'hypostasis': there is no general account of what a 'hypostasis' is, much as there is no definition of the 'what' of the ousia.
Ayres offers a number of key points to keep in mind when considering Basil. Firstly, Basil expresses some initial concern over homoousios, though seems to affirm the meaning, if not the term. Neither Nicaea, homoousios, ousia/hypostasis, nor Athanasius, are the starting points for Basil's theological work. Analogies of the human person are relatively absent from the core of Basil's theology, and he makes no sustained use of human nature in 3 instances. In treating the Spirit, Basil offers one of the first full-fledged integrations of the Spirit into the Godhead, both by aligning the Spirit's work with the Father's and the Son's, and then by “ placing all discussion of ἐνέργεια language within the context of a causal sequence in which activity is caused by and reveals a power (δύναμις) which in turn is an inherent and constituting aspect of a substance” (Ayres, p216).
Part I: Introduction to Basil of Caesarea
Part III: De Spiritu Sancto, and selected Letters