Familiarly, presentism faces an objection from special relativity. According to special relativity, so the objection goes, there is no such thing as absolute simultaneity: events that are simultaneous according to some reference frame are not simultaneous according to another. As a result, what counts as the present is different from one reference frame to another. If presentism is true, then, what exists must vary from one reference frame to another. But this is uncomfortable. Let us spell this argument out in a bit more detail.
1) There is no such thing as absolute simultaneity. (Premise from special relativity.)
2) Only the present time exists. (Premise from presentism.)
3) Only presently existing things exist, and only presently occurring events are occurring. (From 2.)
4) The present time is the time that includes all and only what is simultaneous with your reading of this . (Analytic of ‘the present time’.)
5) What is simultaneous with this utterance varies from one reference frame to another. (From 1.)
6) What things exist, and what events are occurring, varies from one reference frame to another. (From 3, 4 and 5.)
Presentism looks unacceptable then insofar as special relativity and what we might call ontological absolutism – the claim that what things exist, and what events are occurring, is absolute and not relative to anything; a fortiori it is not relative to a reference frame – are both acceptable.
What does the presentist need to do to meet this objection? A sufficient condition for meeting it, presumably, is that they 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. What is simultaneous with your reading of this according to non-privileged reference frames is neither here nor there: there is no pressure to think these things exist unless they are simultaneous with your reading of this according to the unique privileged reference frame.
But how can the presentist 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 could ground the privileged status of one reference frame over another, given that the physical facts don’t appear to distinguish between them?
I hear that question as a question about truthmakers. What we need is a truthmaker for the claim that some reference frame is privileged. If there is some ontological ground for distinguishing one reference frame from the others – some thing or things that mark to pick out one reference frame – then we can, without being arbitrary, take that reference frame to be the privileged one: the one that reveals what there is. So what might the truthmaker be for the fact that some reference frame is the privileged one?
Remember that, for the presentist, to exist is to be present. So consider all the events that exist – they are all and only the presently occurring events. So surely the privileged reference frame is just that one according to which exactly these events 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 events exist – namely, those events.
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. (That just follows from the assumption of ontological absolutism, and if you reject that then there is no problem in the first place.) This 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 (and that nothing outwith S is simultaneous with any of them) is obviously the privileged one.
I imagine that seems way too quick. But ask yourself what the demand against the presentist is. It’s not that they must be able to discover what is present. The preceding remarks do not help the presentist do that. It may be in principle impossible for the presentist to know what exists because it is in principle impossible for them to know if their reference frame is the privileged one. But the objection against the presentist wasn’t epistemic, it was ontological. The objection wasn’t that discovering what is present, and hence what exists according to the presentist, is made difficult because there is no absolute simultaneity; the objection was that there is no absolute fact of the matter as to what is present, and hence no absolute fact of the matter as to what exists according to the presentist, because there is no absolute simultaneity. That is what I think is answered by the preceding remarks: insofar as we have reason to think that there is an absolute fact of the matter as to what exists, the presentist has reason to think that there is an absolute fact of the matter as to what is present, since granting the presentist that there is a unique set of existing entities allows them to uniquely specify a reference frame – the reference frame according to which they are all only the present things. What other reference frame could be a candidate for being privileged, given that the presentist thinks that to exist is to be present?
(And the epistemic objection doesn’t seem too worrying to me in any case. It seems to reduce to: you can’t know what exists, because you can’t know if you’re in the privileged reference frame. But why is that any better than any of the myriad sceptical challenges that aim to undermine your knowledge by showing that there are empirically equivalent possibilities where you get it wrong?)
I think the objection against the presentist has seemed more forceful than it is due to the assumption that the presentist should be able to pick out a unique reference frame independently of picking out the class of existing entities. But why hold that assumption? It’s not as if those entities exist in virtue of that reference frame being privileged. If that were the case then it may well be objectionably circular to presuppose their existence in an explanation of why that reference frame is privileged. But existence facts are, plausibly, brute: when it is true that a exists it is true solely in virtue of a. So the existence of the members of S can be taken for granted in any metaphysical explanation we choose to give - indeed, if the truthmaker theorist is right, all explanation comes to a halt when, and only when, we reach propositions concerning what the members of S are.
The members of S don’t exist because they are all present according to the privileged reference frame; rather, it is that reference frame that is privileged because it is the one according to which all and only the members of S are present. All of us, presentist and non-presentist alike, have to simply accept as brute the existence of some entities. For the non-presentist, accepting a set of things whose existence is brute does not serve to privileged one reference frame, but that is precisely because they don’t accept that to exist is to be present. For the presentist, however, these entities must be present, because everything is present, and so it follows immediately that only a reference frame according to which all and only these things are simultaneous is one that gets things right. Since we are sure of at least one thing that it is present – namely the reading of this sentence – and since there is a unique reference frame according to which that event is simultaneous with exactly the members of S, we know that that reference frame is the privileged one.
The argument from relativity might have force against other versions of the A-theory – such as the moving spotlight or growing block views – that postulate a privileged present without saying that what it is to exist is to be present. Such views require a privileged frame of reference but can’t say that it is the frame of reference according to which all the existing things co-exist: that frame of reference would get things wrong since both views admit the existence of past entities as well as present ones. And so they need another answer as to what grounds the privileged status of the reference frame that gets things right, and it’s not clear what they can say. But presentism, I think, faces no problem: and so this is one reason to think that presentism is the best version of the A-theory available.