Let’s take Heidegger’s fundamental metaphysical question as being about what makes the being of entities possible. Whether one considers the being of beings (the entia, in St. Thomas’s Latin, or the Heideggerian ontic) or the very being of beings (St. Thomas’s esse) Heidegger wants to figure out something else, namely the condition of the possibility of ens/esse. Seen from this angle, Heidegger has ground to critique the Thomistic claim that the condition for the possibility of ens or esse is ipsum esse subsistens, for he appears to ask a more fundamental question that is simply not addressed by speaking of God as the cause of ens/esse. “Ontotheology” stops short of seeing the fundamental metaphysical question of how being as being can be given at all. That said, the mind completely fails out in the face of what could satisfy such a condition, for the critique of ontotheology cannot be contained to mere theology: any reality that might allow for the possibility of entities would fall by the same critique. And so being becomes an absolute abgrund of nothingness, dynamically giving birth to things from an unthinkable oblivion.
There are two obscure responses. The more familiar one – which is almost a reflex to contemporary Thomists – is to invoke analogous names. God is not some member of a set called “being” (whether ens or esse) just as the idea of a house is not some member of a class called “houses” (which might include, say, ramblers, colonials, split-levels, etc.) We can speak of a “house” in the mind without placing it among the class of houses. In fact, the closer an actual house gets to completion – the closer it gets to being an actual house – the further it gets from the sort of being that characterizes the house in the mind, for it gets closer and closer to existing by itself in matter. The fullness of its being thus consists in a separation in being from its source. At the limit of such a division, the very notion of being itself must admit of a division.
A better obscurity – and I say that it is better because St. Thomas considers it at greater length – is to point to focus on the notion of possibility. On the Thomistic account, the condition for the possibility of ens/esse reduces not just to the ipsum esse subsistens but also to the divine potentia, and St Thomas wrote a massive treatise on the power of God. The first question God asks is whether God had power at all. This is a much thornier question than it first appears: power is a sort of indetermination (at the very least, it is an indetermination to the exercise of the power) but indetermination involves non-being, and so it seems that by making power intrinsic to God we make non-being intrinsic to ipsum esse subsistens- which is a pretty clear contradiction. St. Thomas here speaks in words that makes it clear that he appreciates the force of the difficulty:
It must be said that our intellect strains to express God as some most perfect being, because it cannot attain to him except from a similitude to his effects, and there is not found in creatures some highest perfection wholly lacking imperfection… and so we attribute power by reason of what remains [through the action] and what is its source, and not by reason of that which is perfected by the exercise of power [ed. or by the thing made].
Sed et sciendum, quod intellectus noster Deum exprimere nititur sicut aliquid perfectissimum. Et quia in ipsum devenire non potest nisi ex effectuum similitudine; neque in creaturis invenit aliquid summe perfectum quod omnino imperfectione careat… [and so] Potentiam vero attribuimus ratione eius quod permanet et quod est principium eius, non ratione eius quod per operationem completur.
To read the whole response one is struck by the uncharacteristic language of the intellect straining (nititur) to express itself. But note that the key thing it is trying to express is perfection. This is Aristotle’s great contribution to the question of being- he identifies act and perfection (to the extent of making them synonyms) and makes act knowable only by a pure intuition (though an intuition that is facilitated by comparisons and examples), and not in light of any other prior known or even more knowable thought. There is nothing more one can say about act than that it is what seeing is to the power sight, what the finished building is to the pile of building materials, what the oak is to the seed, what a meaning is to the sound of the word, and what God is to the universe. Act cannot be reduced any more than being can, since being is act.
And so a Thomist sees – at least claims to see – that the reduction of the question of being to the ground of its possibility can only be meaningful in the sense of a possibility which is active power. Given the status of our intellect, we conceive power as being both a source and fountainhead, and an incomplete and indeterminate sort of being, at least with respect to its exercise. It is precisely the notion of perfection that allows us to make a division in these two elements, and if we did not reduce being itself to perfection it would be absolutely impossible for us to dissociate these two elements. But it is better to drop this last counterfactual and put the claim positively: it is because we understand being as a perfection, though in a hazy way, that, when we reduce being to the ground of its possibility, we are befuddled by whether this ground is absolute nothingness or the supreme existence. We haven’t yet purified the notion of possibility in light of being as perfection, which illuminates that our very notion of possibility is divided against itself and needs to be distinguished in light of the first and irreducible concept of actuality or perfection.