Consider Hirsch-type deflationary views on which many differences in ontology are simply verbal differences. A standard case is nihilism and universalism about composition: the nihilist says that multiple things can never compose a whole and the universalist says that every plurality must compose a whole. The deflationist sees the two views as notational variants. The universalist’s sentences describe the same facts as the nihilist’s. We can maybe even translate with little if any loss between the two idioms, replacing the nihilist’s quantifiers with quantifiers restricted to simples on the universalist’s side, and replacing the universalist’s quantifiers with plural quantification, or quantification over sets, or some other device acceptable to the nihilist.
Note, first, that in this particular case there is a bit of a problem. The universalist might allow for composed objects that have no simple parts—“gunk”. The claim that possibly there is gunk is one that cannot be translated into any statement in the nihilist’s language that has a hope of being true. The nihilist’s usual way of translating a universalist’s statement is to use plural quantification. So the statement that possibly there is gunk is going to get translated into something like the statement that possibly there is a plurality of things none of which is a simple. But that’s obviously false given nihilism, since the nihilist’s quantifiers can only quantify over simples, and so the statement basically says that there are simples none of which is a simple. Thus, we have a genuine, non-verbal disagreement.
So the only way we can take a nihilist-universalist disagreement to be merely verbal is if both theorists deny the possibility of gunk. I think they should deny the possibility of gunk.
Here is a second case, where disagreement on composition cannot be deflated. Consider a brutal composition view like Markosian’s. On this view, there will be possible worlds with the same simple objects standing in the same non-mereological relations but differing as to composition facts. For instance, in one world there might be three rocks that make up a whole and in the other world the very same three rocks do not make up a whole, even though they are arranged in exactly the same way. Any nihilist or universalist description of the two worlds will be unable to distinguish such worlds, but on a brutal composition view, there can be such pairs. Here we have a real disagreement, one that cannot be taken to be merely verbal. The brutal composition theorist has more possibilities than the nihilist and universalist. And the brutal composition theorist’s statement that the two worlds differ in composition facts but not in non-mereological facts either has no translation into either nihilist or universalist language or translates into something that is clearly false on the given theory.
The brutal compositionalist has an additional “degree of freedom”, as the scientist would say, on her theory than the nihilist or universalist does. The case here is similar to those dualists who believe in the possibility of zombies. While the disagreement between a dualist who thinks the mental supervenes on the physical and the pure physicalist could seem to be merely verbal to some (though I think it’s a mistake to see it that way), the disagreement between a dualist who thinks that the mental does not supervene on the physical and the pure physicalist is certainly not merely verbal.
In general, thus, the modal ramifications of theories can block deflationary moves. One theory may allow for a possibility that simply rules out the other theory (e.g., gunk ruling out nihilism), or one theory may posit contingent facts that do not supervene on reality as describable in the other theory (the brutal composition or zombie cases).
This leaves the possibility that there will be some ontological debates that are merely verbal. Perhaps the debate between the nihilist and the anti-gunk universalist is merely verbal. But that some pairs of ontological theories disagree merely verbally is not a very interesting deflationary thesis.
Moreover, I think that once we see that there are nearby debates that are clearly not merely verbal, the plausibility of the deflationary move in the cases that looks more verbal goes down. Once we realize that among the views under discussion there is a brutal composition view on which there is a possible world just like ours but where nihilism contingently holds and a possible world just like ours but where universalism contingently holds, it becomes pretty clear that holding nihilism to hold necessarily will also differ from holding universalism to hold necessarily. (That said, there may be particular variants on universalism that just are notational variants on nihilism. Say, ones where the quantifiers are stipulated in terms of plural quantification over simples.)