The ontological difference was opened up in Aristotle, in beings (ὄντα) and being qua being (ὄν ᾗ ὄν). ('Opened up' here means that Aristotle's thinking took place in the light of the ontological difference. Whether Aristotle thought the difference as difference, i.e., explicitly, is another question entirely and an open question.) What we call a being (Seindes) Greek philosophy called das Answende (ὄν, ἐόν): what presences, emerges, comes forth. Greek philosophy called a being das Anwesende because. as Heidegger wrote in 1965, being spoke to the Greeks as das Anwesenheit - names in Aristotle as οὐσια - which got reduced in the history of metaphysics to 'being,' 'beingness,' and even substance. This reduction is clearly demonstrated in F.E. Peter's Greek philosophical Terms, where he writes, 'Aristotle is further convinced that the problem posed by metaphysics, and indeed by all philosophy, i.e., "what is being [ὄν]?" really comes down to "what is ousia?" since being is, first and foremost substance.'
The difficulty here is that, when philosophy thinks being, over against beings and what is, it takes its queue from beings. Thus, whereas the difference is indeed thought, it is determined as what it is over against beings. The danger in this way of thinking is that it thinks being as the being of a being from out of and unto a being.
To release the thinking of being from the shape given to it in terms of beings - over against beings (as beingness, as presence) - thinking as enaction needs to 'return' the 'ontological difference' to its 'own' place within the question 'that historically decides metaphysics and decides about metaphysics and its inquiry'. Therefore, the thinking of ontological difference is an unavoidable transitional moment, from within which the inquiry into being takes place. Thinking must pass through the ontological difference, so that the necessity of asking the grounding question of be-ing can be manifest.
Pp. 26-7