Full Idea
A theory of existence should 1) be consistent with what actually exists, 2) be consistent with what could exist, 3) not make existence impossible (e.g. in space-time), 4) not violate logic, 5) make knowing the theory possible.
Gist of Idea
Existence theories must match experience, possibility, logic and knowledge, and not be self-defeating
Source
J.P. Moreland (Universals [2001], Ch.6)
Book Reference
Moreland,J.P.: 'Universals' [Acumen 2001], p.135
A Reaction
A nice bit of metaphilosophical analysis. I still doubt whether a theory of existence is possible (something has to be 'given' a priori), but this is a good place to start the attempt.