Upvoted for mentioning that ethics and epistemology are subject to similar questions. That's a huge insight, familiar in academic philosophy, but AFAICT rare among self-identified rationalists and little discussed on lesswrong.

Of course, the academic philosophy way to handle the insight has usually been worse than useless: take the Mysterious Phenomenon of "epistemic normativity" as reason to believe in metaphysically basic moral normativity, then use that to ground epistemology, and thus go from one field that can be naturalized and one that is claimed to remain a mystery, to -1 fields naturalized and two fields made Mysteriously Metaphysical.

Whatever you call it, they've got to identify some alternative, even if only tacitly by following some approximation of it in their daily life.