A three-page paper upended a definition of knowledge that had stood for 2,000 years

Philosophers had long defined knowledge as justified true belief, but Edmund Gettier published cases where someone has a belief that is both justified and true, yet seems to be true only by luck rather than because their reasoning actually tracked reality, meaning it shouldn't count as knowledge. The paper was barely three pages long but launched decades of attempts by epistemologists to patch or replace the traditional definition.

— Edmund L. Gettier, Is Justified True Belief Knowledge? — Analysis, 1963