I suggest that asking what exists as a general question is meaningless. It's only meaningful within a context where the ontology is well defined and an unambiguous answer is, in principle, possible. I think this was the philosophy of the young Wittgenstein. Recently I posted in a thread (since locked) where the OP asked if anything could exist without space and time. If the context was physical space and time, then the question is a meaningless tautology. So I assumed a more general context and answered that things like the natural numbers exist outside the context of space and time. A member responded critically and suggested that the natural numbers were imaginary and were akin to "vampires and werewolves." There is a clear context where one can ask sensible questions such as: 1) How many primes exist between two (arbitrarily large) primes x and y.? 2) Does a derivative exist everywhere for G(x)? One can ask if a language contains a particular word, or if a particular short excerpt came from a particular Beethoven symphony. I don't know what the ontological context of a werewolve is. For vampires, it's "bats".