My own conclusion is that seeking to put forth ultimate answers which others should--must?--accept as valid is a waste of time. Do human beings have a rational capacity? Yes. Have we (at this point) definitively answered, rationally, every question our brains are capable of asking? No--but that cannot invalidate the possibility of sufficiently valid knowledge within material existence.

Whether or not material existence is all that exists is an unanswered question--rationally. Any answer to that question can only be a belief, i.e., an "internal conviction," i.e., extra-rational knowledge. Believe whatever you want--accepting that no belief of anyone can be of any consequence whatsoever to anyone who eschews that belief. For rational beings, though, whatever is sufficiently validated within material existence must be accepted as knowledge--accepting that more information could possibly invalidate it at any time.