 Is (K) true? On the one hand, if (K) is true, then what it says is true, so no one knows it. On the other hand, that very reasoning seems to be a proof of (K). Believing a proposition by seeing it to be proved is a sufficient for knowledge of it, so someone must know (K). But then (K) is false! Since no one can know a proposition that is false, (K) is not known.