- If presentism is true, then necessarily: if t is present, then x exists at t if and only if x exists simpliciter, x exists definitely at t if and only if x exists simpliciter definitely, x exists vaguely at t if and only x exists simpliciter vaguely.
- There can be vagueness about existence at a time.
- There cannot be vagueness about existence simpliciter.
- Any time can be present.
I think 3 is very plausible. The idea that, say, I might vaguely exist seems absurd. However, 2 is also very plausible. Think of the vagueness in the moment of a brute animal's death. (Human death is not the cessation of existence, so human death is beside the point.)
The weakness in the argument is that one might take 2 to be strong evidence for the denial of 3. Suppose there is vagueness in the moment of conception, so that at at the actual world w0, it is vague whether Bucephalus exists yet at t0. Then take a world w1 which is a duplicate of everything in w0 up to and including t0, but with everything getting annihilated after t0. Plausibly, at w1 it will be vague whether Bucephalus exists. Here, I would bite the bullet if I accepted 2. I would say that it's vague whether a world w1 that contains Bucephalus is a duplicate of w0 up to and including t0, or whether the duplicate is a world w2 that doesn't contain Bucephalus. This is a hard bullet to bite, but it is better than allowing for vague existence.