In A Treatise of Human Nature, Hume says, "To form a clear idea of any thing, is an undeniable argument for its possibility, and is alone a refutation of any pretended demonstration against it." I think the modern way to put this is to say that conceivability is a guide to metaphysical possibility.
In Dialogues Concerning Natural Religion D9.6-7, Hume rejected that idea of necessary existence as meaningless. That seems to fit with his view that all knowledge comes from relations of ideas or matters of fact. But why shouldn't he also reject the idea of contingent existence as meaningless? It doesn't seem to be a relation of idea or matter of fact.
Am I missing something?