According to the doctrine of Tensed Propositions (TP), which is accepted almost all presentists, when I say "It is night" at noon and when I say it at midnight, I express the same proposition, which is true at midnight and false at noon. The opponents of TP are apt to say that "It is night" is indexical, and as said at noon and at midnight expresses different propositions. TP is compatible with there being untensed propositions, like <It is night at t7>, but typical tensed sentences in English express tensed propositions.
Here is an argument against tensed propositions:
- (Premise) To be in agreement with what someone has said is to accept the proposition that she expressed.
- (Premise) If p is a tensed proposition, then x's accepting p at t1 and y's accepting p at t2, where t1 and t2 are different, does not constitute agreement between x and y.
- There are no tensed propositions.
Argument for (1): To agree is to accept what was said. But what was said was the proposition that was expressed.
Argument for (2): Suppose I say: "It is now exactly 7 p.m." and twenty seconds later you sincerely say: "It is now exactly 7 p.m." If there are tensed propositions, then both of our utterances express the same tensed proposition. But surely your assertion expresses a disagreement with me. The point generalizes to all tensed propositions. Granted, sometimes, especially if t1 and t2 are within a relevant time period (say, on the same day, and p is about "today") x will be right in believing p if and only if y is right in believing p, but nonetheless x and y's agreement does not consist in mutual acceptance of p.
Even if one does not think that (1) is always true, it is very plausible that typically in communicating we are trying to communicate a proposition. But also typically, our sentences are tensed. So if TP is true, then typically we are communicating tensed propositions. But it seems essential to communicating a proposition that agreement should be constituted by mutual acceptance of that proposition. But in the case of tensed propositions that isn't what agreement is constituted by (maybe it's constituted by accepting the second-order claim that the proposition was true when it was expressed). So it can't be that our typical communication involves tensed propositions. Hence, TP is false.
Josh Rasmussen said that arguments along these lines came up independently in conversation with Peter van Inwagen.