It is often said that we have at least some reason for accepting that, “It is possible that a Maximally Great Being exists”. One of the reasons is that it seems to be epistemically possible that such a being exists. For all we know, it seems that such a being exists. This gives us some reason to think that a MGB or “God” exists.
The objection of course is that for all we know (epistemic possibility) such a being does NOT exist. The proponent of the ontological argument then can respond that our epistemic notions of it being possible for all we know, is stronger than our seemings that it is not possible for all we know. But even if that’s true, which is a point of contention, that doesn’t show that epistemic possibility is anywhere close enough to being sufficient to show that it is indeed metaphysically possible for a MGB to exist. In fact, there is serious problems in trying to show something is metaphysically possible just by it being epistemically possible; however, I’m sure there might be other reasons to accept premise 1.