A lucky guess may be both believed and true without being knowledge. The traditional justified-true-belief analysis adds justification, but Gettier constructed cases in which a belief is justified and true only through epistemic luck. Competing analyses therefore try to identify what rules out that luck while preserving ordinary cases of knowing.
This problem asks what knowledge is, not whether it comes from experience or reason; that question appears under Experience or Reason?. It also differs from the broader skeptical question of whether knowledge is possible, treated under The Possibility of Knowledge.