– Broadly, the Fregean/post-Fregean concept of existence is that existence is the instantiation (sp?) of a concept. No doubt there are some subtleties here and there but that’s the basic gist. For a horse to exist means that the concept of horse-ness is instantiated.
– Frege noted that there is an odd quality about terms like ‘existence’ – namely, that if existence is a universal predicate (i.e. a predicate which is true of everything) then non-existence is true of nothing.
– This seems to have the consequence that nothing can either exist or not-exist – which is, to say the least, counter-intuitive.
– The consequence of this is that the difference between existence/nonexistence is reduced form ontological to propositional. Surely, though, there are things that don’t exist and things that do, and the difference is more than how they both look when put into formal logic. Perhaps Frege’s logic isn’t equipped to deal with existence?
– It quickly becomes clear that this is a way of speaking about existence, rather than speaking about existence as such (David Bentley Hart points out as much in ‘The Experience of God). Existence is just assumed, rather than explained. And perhaps this is fine – but one certainly does feel that such a conception of existence is thin.