In response to your question I would have to say that we must answer the question of "how do we know?" first. Why? because if we have no concrete conception of how we get to know something (through reason + account + cognitive understanding (which could be a necessary precondition for knowledge of some subject) then how can we actually and absolutely know anything. We must be certain in the ways we attain knowledge before ever assessing that we can know anything at all. Once we develop the criterion that could allow us to know anything at all then we search for what we can know. But it must be differentiated between what we can know about the external world and what we can know about everything else (logic, math, subjectivity, metaphysics and morals. Try reading C. S. Pierce, Hilary Putnam, W. V. Quine and perhaps some positivists like Moritz Schlick, Carnap or Ayer for other answers to such questions.