You might have thought that all truths are, in principle, knowable, even if it turns out that nobody actually ever does manage to know it. But, surprisingly enough, this is provably false. For consider some such truth P that will never actually be known. Then the statement Q: "P is true but will never be known" is true. But Q is clearly unknowable, for if you knew Q then you would know the first conjunct (P), but that would contradict the second conjunct (P is never known), thus making Q false, which is a contradiction. Since knowing Q would yield a contradiction, and is thus impossible, then Q is an unknowable truth.
So we've managed to prove a priori that either there are no never-known truths P, or else there are unknowable truths Q. It's a neat argument. I just came across it here. "Fitch's knowability paradox", I think they call it. Maybe epistemology isn't so bad after all.