You can't know what you know without already knowing how to tell

To know whether a belief counts as knowledge, you need a criterion for telling knowledge apart from mere opinion. But to trust that criterion, you'd need to already know it's the right one — which requires yet another criterion, and so on forever. The ancient skeptic Sextus Empiricus used this regress to argue that certainty is unreachable; philosopher Roderick Chisholm revived the puzzle in the 20th century as a central problem of epistemology.

— Sextus Empiricus; revived by Roderick Chisholm, Outlines of Pyrrhonism — c. 2nd century AD; Chisholm's 'The Problem of the Criterion', 1973