Familiarly, the presentist faces an objection from special relativity. If presentism is true then only presently existing things exist. What is present is simultaneous with your reading this. So what exists are all and only the things that are simultaneous with your reading this just now. But what is simultaneous with your reading this just now is relative to a frame of reference. Therefore, what exists is relative to a frame of reference. But that’s nuts, so presentism is false.
What does the presentist need to do? Well, they win the game, presumably, if they can give some reason for privileging one reference frame over any other, for in that case they can claim simply that what exists is what is simultaneous with your reading of this according to the privileged reference frame. But how can they have a reason for thinking that one reference frame is privileged (without resorting to claiming that God smiles upon one and not the others)?
What we need is a truthmaker for the claim that some reference frame is privileged. Easy! Remember that to exist is to be present. So consider all the things that exist – they are all and only the present things. So surely the privileged reference frame is just that one according to which exactly these things are simultaneous. In that case the truthmaker for the fact that this is the privileged reference frame is just what makes it true that those things exist – namely, those things.
So the thought is this. Everyone in this debate agrees that there is a unique set, S, which is the set of the existing entities. (If you give up on that you don’t get to say ‘but that’s nuts’ in the original argument against presentism.) This (correct me if I’m wrong – I may be (not for the 1st time!) misunderstanding the physics) lets us single out a unique reference frame: the unique reference frame according to which exactly the members of S are simultaneous. And so, if we’ve got good reason to think that everything that exists is present then we’ve got good reason to think that this frame is the privileged reference frame. Since everything in S exists then everything in S is present; so they had better be simultaneous; so the reference frame that says they are simultaneous is obviously the privileged one.
Convinced? I imagine not. Nor am I. But what’s wrong? I’m finding it hard to put my finger on it. What more is needed from the presentist? Or what are they doing that’s illegitimate?