The statement "p is an unknown truth" cannot be both known and true at the same time.
Therefore, if all truths are knowable, the set of all truths must not include any of the form
"p is an unknown truth"; Thus there must be no unknown truths, and thus all truths must be known.
At least according to Frederic Fitch.
Later, after a heated debate about the question if all true logical statements are indeed tautological, one
of the philosophers finally screamed "enough is enough" and stormed out of the room.