Probably old hat to some, but reading this today in the translator’s note to Gregory of Nyssa’s “An Answer to Ablabius” really helped me with Trinitarian metaphysics.
In the Cappadocians, the divine ousia (essence) is not strictly speaking a “universal” of which the three Persons are individual instances. This would entail tritheism, because the three Persons (hypostases) would be particularizations of a generic essence, in the same way that individual human beings are particularizations of generic human nature. Rather, in the Cappadocians the divine ousia is a “particular, concrete existence”–it is identical in each of the three Persons. Whereas the ousia expressed by individual men is only generic, in the Trinity it is identical.
Correspondingly, the three Persons are “modes of being” in the one God, existing perichoretically (in interpenetration with each other). The divine ousia is not ontologically divided into three hypostases; rather, the three hypostases are causal distinctions within the one ousia. The Father is uncaused, but the Son and Spirit are caused (of course, understanding from other Cappadocian works that “caused” does not mean “finite with a beginning”).
This is Gregory’s answer to Ablabius’ incipient tritheism. Quite intriguing, and it helps me understand Trinitarian metaphysics much better.