Justified True Belief

Justified True Belief (JTB) is the classical analysis of knowledge proposed by Plato in the Meno and Theaetetus and formalized in twentieth-century analytic philosophy: an agent knows that P if and only if (1) P is true, (2) the agent believes P, and (3) the agent is justified in believing P. The analysis dominated epistemology until Edmund Gettier's 1963 paper demonstrated that all three conditions can be satisfied without constituting genuine knowledge — a result so decisive it redirected the field. The problem of finding a fourth condition that excludes Gettier cases without generating new counterexamples has not been solved, suggesting that the JTB analysis mistakes a cluster of related phenomena for a single natural kind. See also Epistemology, Cognitive Reliability, Epistemic Luck.