A reader is convinced by my arguments against presentism and eternalism but is not convinced that there is a genuine issue in dispute. He further suspects that the parties to the dispute are using 'exist(s)' in different ways. The reader issues a serious challenge. Can I meet it?

Presentists and eternalists give competing answers to Quine's question, "What is there?" Roughly, presentists maintain that only  present items exist, whereas eternalists maintain that past, present, and future items exist.  The dispute concerns the ontological inventory. It is essential to observe that the disagreement presupposes a prior agreement as to how 'exist(s)' is to be used.  It obviously cannot be used in the present tense. If it is, then both presentism and eternalism turn out to be trivial theses, presentism trivially true, and eternalism trivially false. (We have gone over this many times.)

So let me introduce the sign 'exist(s)*' to denote existence simpliciter. The dispute is then whether what exists* is restricted to what is present or is not so restricted.  This strikes me as a substantive difference.  The views are in genuine conflict. It is as genuine a conflict as that between those who say that only particulars exist* and those who say that both particulars and universals exist*.  The dispute is about what exists simpliciter, i.e., what exists*. 

Another example. Suppose on Monday morning you take delivery of 300 paving stones. By  Friday evening, you have made a walkway out of them. Do you now have 300 + 1 new things on your property or only 300?  Does the walkway count as something in addition to the paving stones? Some say yes. Other say no: you have 300 stones arranged walkway-wise. This is an ontological inventory dispute, a dispute about what exists.  It is arguably genuine — but only if  there is agreement as to the sense of 'exist(s).'

Quine famously told us that "Existence is what existential quantification expresses." ("Existence and Quantification" in Ontol. Rel., 97) To put that with all due scrupulosity, we must rewrite it as "Existence* is what existential quantification expresses."   Equivalently, existence simpliciter is what existential quantification expresses. Uncle Willard takes his quantifiers 'wide open,' or unrestricted. They range over whatever there is, whether abstract or concrete , universal or particular, past, present, or future. On he same page, Quine offers his definition of singular existence: a exists =df (∃x)(x = a). 

Suppose my reader agrees with the above.  He might still feel that there is no real difference between presentism and eternalism, that the metaphysical difference is not a difference that makes a (practical) difference.  The reader may be reasoning as follows: since the presentist and the eternalist accept all the same Moorean facts, there is no substantive difference between the positions.

Consider first the past. Among the gross facts not in dispute is the truth of

1) Scollay Square no longer exists.

What this says using tensed language is that

1T) Scollay Square existed but Scollay Square does not exist.

In tenseless language it goes like this:

1U) Every time at which Scollay Square exists* is a time earlier than the present time.

The reader may claim victory at this point. "You see? Two different ways of saying the same thing, a presentist way and an eternalist way. Hence there is no substantive difference between the two views."

But now consider the future. Here a substantive difference emerges. Suppose Dave is a father whose kids are slackers who may or may not procreate, but haven't done so yet.  If they do, then Dave will have one or more grandchildren.  If they do not, then Dave will have no grandchildren.   On presentism, future temporal items do not exist* which implies that neither of the following is now true:

2) Dave will have a grandchild

and

~2) Dave will not have a grandchild.

On eternalism, however, future temporal items exist* so that one of the above propositions is now true.  On eternalism, the future is as fixed as the past, whereas on presentism, the past alone is fixed.  This is a substantive difference and not a difference in two ways of saying the same thing.

RELATED:  Peter Unger on the Emptiness of the Presentist-Eternalist Debate