Presentism holds that only presently occurring events exist. You and I are watching an exciting game of tennis. Our particular interest is drawn by Federer's next serve which is at a match point. With eager anticipation I speculate about how the serve will go. Federer is serving. You briefly respond to my remark during the serve itself, saying that my speculation doesn't look right. Federer has served and wins the match. We continue disagreeing about the serve for the next fifteen minutes. Such a conversation is perfectly natural.
It seems essential to understanding how the conversation goes that we are all the time talking of the same serve. I claimed before the serve that it was going to be and then continue to maintain that it was a beautiful topspin. You claim that the serve was a poorly executed topspin-slice which only won the game by a fluke.[note 1]
It is difficult, however, for the presentist to maintain that we are talking about the same thing. After all, my initial remark, according to presentism, is about the future existence of a serve (note that on open-future presentism, it is not even yet the case that the serve will exist). Your next remark is about an existing serve. My next one is about a serve having existed. What unifies the topic of these remarks? After all, they are only a disagreement if they concern the same serve.
Moreover, consider the state of affairs that I am claiming to obtain, the state of affairs of the serve being an excellent topspin. Let us suppose I am right. Then it seems I am continually pointing to the same state of affairs throughout the conversation. I am certainly not changing my mind—I am too pigheaded for that. Consider now the truthmaker of my claim.
There are two kinds of presentist views about the truthmakers of claims apparently about past or present events. On a Bigelow kind of view, these truthmakers are presently existing but tensed concrete (i.e., non-abstract) states of affairs. On this view, I am initially claiming the obtaining of the concrete state of affairs of an excellent topspin being about to soon be served. You respond, during the serve, with a denial of the occurrence of the concrete state of affairs of an excellent topspin being presently served. I pigheadedly affirm the obtaining of an excellent topspin having been served. How is there any disagreement here? I first maintain the occurrence of one state of affairs, to which you respond with the denial of the occurrence of a second, and then I respond with an affirmation of the occurrence of a third? It seems plain that according to this kind of presentist we are talking about three different states of affairs, one of which (the one talked about during the serve itself) includes a serve, and the other two of which do not (since there are no past or future serves according to presentism).
Moreover, while the point is most vividly made concerning conversation that straddles the time of the event talked about, the point also can be made in regard to a distant past event. Suppose you and I are discussing the Battle of Waterloo. What makes it be the case that we're talking about the same battle? We can't just be talking of the present state of affairs of there having been such-and-such a battle. For there were many battles in the past. Rather, we're talking of the present state of affairs of there having been a battle x seconds ago. But the x keeps on changing, so the state of affairs we're talking about keeps on changing on us.
The second kind of presentism, ably defended by Merricks, holds that there are no truthmakers for typical past and future tensed propositions. Then my first claim, made before the serve, has no truthmaker. Your denial of the claim, during the serve, is the denial of a claim that in fact does have a truthmaker—the presently occurring serve. And then my subsequent re-affirmation is the re-affirmation of a claim that, again, does not have a truthmaker. Plainly you and I are not talking about the same thing, since there is no truthmaker homogeneity (two propositions are truthmaker-homogeneous iff they both lack truthmakers or they both have truthmakers of the same kind) between what I say and what you deny.