I'm sure this is coming up in relation to Plantinga's
modal ontological argument for the existence of god. Our tendency to use the term "possible" in two different ways (as described in my earlier post) plays into the plausibility of this argument, I believe.
Put god aside for a moment, and let's say you ask me "Is 7903 prime?" Now let's say I don't have internet access and I'm not very good at math. You are looking for a quick answer. So, I say "Possibly." Now, what I really mean by this is "I don't know whether 7903 is prime or not." But we talk this way all the time, we say things are "possible" when what we really mean is we just don't know one way or the other -- that's the epistemic sense of "possible". In actuality, 7903 is not prime. And since every number is either prime or not prime by necessity, then it's literally
impossible that 7903 be prime (i.e. there are no possible worlds where it is prime). Now notice if we apply Plantinga's modal principle, then if it's possible that P is necessarily true, it follows that P is necessarily true. So by that principle, I was wrong when I said that it was possible that 7903 is prime. In fact, I did not know whether it was possible or not -- it's the sort of thing where it's not that easy to know whether it is possible.
Enter god. Plantinga says that god by its nature would be a necessary being. That is, if god exists, then god exists in every possible world. In other words if P is the proposition that god exists, then P is a necessary proposition (it is either necessarily true or necessarily false). Now, if we are asked "Is it possible that God exists?" many of us would quickly reply "sure, it's possible". If we take that to be a claim about real possibility and not simply our state of knowledge, then by the modal principle we are committed to the existence of god, since if it's possible that a necessary being exists, then the necessary being exists. However, what I'm saying is that our answer "sure, it's possible" is misleading. In fact it is not that easy to know whether the existence of god is possible in the real sense, we only have immediate access to our epistemic state.
So, what I think is happening here is, the proposition that god (defined as a necessary being) exists seems "possible" in the sense that we just don't know whether it's true (the epistemic sense of possible). For Plantinga (or anyone) to assert that the proposition that god (defined as a necessary being) is possibly true in the literal sense of possible would be begging the question -- effectly assuming that thing that stands in need of proof.