According to most proponents of presentism, propositions are tensed. Thus, when yesterday you said that it is raining and I said it today, we expressed the same proposition, which, perhaps, was false yesterday and is true today. Moreover, presentists believe that one cannot refer de re to non-present objects.
For presentism to have any hope of being able to express all of reality, the presentist needs an "Ago" operator, where Ago(t,p) is a proposition that backdates p by t (units of time). If p is expressed by a present tense sentence, Ago(t,p) can be expressed by a past tense sentence. Thus, if p is the proposition that it is raining, Ago(3 days,p) is the proposition that three days ago it was raining.
Here is a plausible fact about the logic of "Ago":
- Ago(t,p) is now true iff p was true t units of time ago
- No proposition that refers de re to a presently non-existent entity can be true. (If we like, we can probably qualify this as: "no positive proposition".)
- Ago(70 years,p) is true.
- p was true 70 years ago.
- p was not true 70 years ago.
Thus, the presentist cannot hold (1)-(3) together.
Thus, the straightforward presentist reading of the claim that 70 years ago there was someone who would vote for Obama as the claim that 70 years ago someone existed who would vote for Obama is one that doesn't fit with (1) and (2). But there is a non-straightforward way of giving the truth-conditions that does:
- There (is) a time t at which there (is) a person x who (votes) for Obama at t and who also existed on October 29, 1939,
An interesting question is whether such truth conditions are available for all possible examples of this sort.
There is, however, a different route for the presentist. She could deny (1). This would be analogous to Robert Adams' move of allowing that a proposition might be true at a world without being such that were that world actual, the proposition would be true. Such a view a view, when married to Crisp's concept of abstract times, would have the problematic consequence, that in general, at a time t, the time t was not present. (For t will contain propositions that make de re reference to now-actual objects that didn't exist at t, and so at t the maximal proposition would have been different from what it actually is.) To me, (1) seems very plausible.
Probably, most presentists will simply deny (2), allowing for de re reference to non-existents by means of haecceities. They will then open themselves to Lewis's objection that they are not really presentists, but there is probably a way out of that. It would, however, be an interesting thing if they had to deny (2)—this would mean that presentists cannot be serious actualists in the sense involved in (2). And if presentists could not be are not serious actualists, then their claim that only present objects are actual is not quite as revolutionary as it seems.