It is plausibly maintained that all relations are existence-entailing. To illustrate from the dyadic case: if R relates a and b, then both a and b exist. A relation cannot hold unless the things between which or among which it holds all exist. A weaker, and hence even more plausible, claim is that all relations are existence-symmetric: if R relates a and b, then either both relata exist or both do not exist. Both the stronger and the weaker claims rule out the possibility of a relation that relates an existent and a nonexistent. (So if Cerberus is eating my cat, then Cerberus exists. And if I am thinking about Cerberus, then, given that Cerberus does not exist, my thinking does not relate me to Cerberus. This implies that intentionality is not a relation, though it is, as Brentano says, relation-like (ein Relativliches).)
But if presentism is true, and only temporally present items exist, then no relation connects a present with a nonpresent item. This seems hard to accept for the following reason.
I ate lunch an hour ago. So the event of my eating (E) is earlier than the event of my typing (T). How can it be true that E bears the earlier than relation to T, and T bears the later than relation to E, unless both E and T exist? But E is nonpresent. If presentism is true, then E does not exist. And if E does not exist, then E does not stand in the earlier than relation to T. If, on the other hand, there are events that exist but are nonpresent, then presentism is false.
How will the presentist respond? Since E does not exist on his view, while T does, and E is earlier than T, he must either (A) deny that all relations are existence-symmetric, or deny (B) that earlier than is a relation. He must either allow the possibility of genuine relations that connect nonexistents and existents, or deny that T stands in a temporal relation to E.
To fully savor the problem we cast it in the mold of an aporetic tetrad:
1. All relations are either existence-entailing or existence-symmetric.
2. Earlier than is a relation.
3. Presentism: only temporally present items exist.
4. Some events are earlier than others.
Each limb of the tetrad is exceedingly plausible. But they cannot all be true: any three, taken together, entail the negation of the remaining limb. For example, the first three entail the negation of the fourth. To solve the problem, we must reject one of the limbs. Now (4) cannot be rejected because it is a datum.
Will you deny (1) and say that there are relations that are neither existence-entailing nor existence-symmetric? I find this hard to swallow because of the following argument. (a) Nothing can have properties unless it exists. Therefore (b) nothing can have relational properties unless it exists. (c) Every relation gives rise to relational properties: if Rab, then a has the property of standing in R to b, and b has the property of standing in R to a. Therefore, (d) if R relates a and b, then both a and b exist.
Will you deny (2) and say that earlier than is not a relation? What else could it be?
Will you deny presentism and say that that both present and nonpresent items exist? Since it is obvious that present and nonpresent items cannot exist in the present-tense sense of 'exists,' the suggestion has to be that present and nonpresent (past or future) items exist in a tenseless sense of 'exist.' But what exactly does this mean?
The problem is genuine, but there appears to be no good solution, no solution that does not involve its own difficulties.