 If one claims that something is possible, it is sometimes natural to take this to mean that one does not know it to be false. For example, suppose that you ask someone whether Socrates ever went to Sparta and she answers that it is possible. It is natural to understand her as saying that she does not know that Socrates did not go to Sparta. Thus, the possibility that is expressed here is a kind of epistemic possibility (in particular, one according to which \(p\) is epistemically possible for an agent \(X\) just in case not-\(p\) is not known by \(X)\). This notion of epistemic possibility is clearly distinct from the notion of metaphysical possibility, since there are cases of epistemic possibilities that are not metaphysical possibilities. Of Goldbach’s Conjecture (that every even number greater than two is the sum of two primes) and its denial, each is epistemically possible but one (we know not which) is not metaphysically possible. And there are cases of metaphysical possibilities that are not epistemic possibilities. That there are only two planets in our solar system is metaphysically possible but not epistemically possible for most of us, given that most of us know that there are not only two planets in our solar system. (This is not to deny that there may be some notions of epistemic possibility—for example, maximally complete ways the universe can coherently be conceived to be—for which it is at least plausible to suppose that every metaphysical possibility is also an epistemic possibility. Even if this is so, the notions of metaphysical possibility and epistemic possibility are distinct.)