How Would a Thomist Respond to the Buddhist Doctrine of Emptiness?
The ideas that nothing is real (but only appears to be) & that substances don't exist depend on the assumption that conditioned (what thomist would call "caused") things are unreal. They lack an absolute, self-sufficient ontological core, the reasoning goes.
Though there is no doubt variation to the concept, DT Suzuki explains it thus: "emptiness does not mean nothingness... Mahayana Buddhism has another term with an affirmative connotation [for the same concept:] 'suchness' or 'thusness'... the Buddha nature in everything. To regard existence as this or that, eternal or transient, is our thought-construction... they are maya, have no self-substance, there is no real distinction between one thing and another."
We see here how a lack of ontological independence is taken as evidence of unreality of finite things.
A thomist would respond that finite things, (the vast reaches "maya") are only unreal in relation to unconditioned being-in-itself [however we want to call it, God, emptiness, etc] if being is univocal or essentially-homogenous. If being means "absolute or unconditioned being" to the exclusion of finite things, then obviously the appearence of finite things will be illusory. We started with the assumption that they are impossible!
But this assumption of being's univocity isn't something to gloss over; the thomist would argue such univocity is both an unfounded assumption & that it compromises the very framework of illusion-vs-"emptiness" the whole scheme relies on.
Being is not univocal but analogous; it is not essentially consistent, but essentially diverse, divided (as Plato/Aristotle discovered) into act & potency [& the various special modes of being that arise when differing ratios of potency are actuated]. This information (that being is analogous or internally-diverse, not univocal) is "prior to any semantic or logical issue whatsoever" as Steven Long says, indeed prior (in the order of principles or nature) to any other epistemic or metaphysical concerns: it allows us to explain/defend the reality of *finite, limited & changing things* against parmenidean monism.
Armed with this insight, we can ask the buddhist: "why would a conditioned thing's lack of ontological self-sufficiency indicate its unreality?" It rather appears that lack of self-sufficiency is the very reason there can be finite things at all (which come to be in causal relation to/in dependence upon the first cause, being-itself).
Moreover the buddhist appeals to this very act-potency distinction & thereby a correct reading of finite vs infinite being in articulating his doctrine of emptiness & maya. We might say he is as much an ontotheologian as anyone: he has elaborated a conception of being-in-itself vs caused beings, Being vs beings. The distinction between unconditioned emptiness & the illusory strata of finitude, if operative at all (and it is; Suzuki elaborates on the "formal causality" of maya as thomists would call it, its laws, compulsions & internal structures) simply IS, so far as the thomist should be concerned, a distinction between caused and uncaused beings, pure act vs the "unreal" or relative modes that participate in it. There are no simpler terms to reduce it to, we have a straight-forward acknowledgement of self-identical vs dependent being. This being the case, the doctrine of emptiness either only successfully applies, singles out as different-than-itself, weird liberal conceptions of autonomous subjectivities. (Or it undoes itself by denying the framework of its own analysis: without [something like] the act-potency distinction, there is no illusion, all-there-'is', even maya, is pure unconditioned 'suchness'.)