# Justified True Belief (JTB) The classical definition of knowledge, traced back to [[Plato]]'s *Theaetetus*: a claim counts as knowledge when **three conditions hold simultaneously**. - **Belief** — the knower must actually hold the proposition to be true. Merely recording or quoting it is not enough. - **Truth** — the proposition must correspond to how things really are. A sincerely held false belief is not knowledge. - **Justification** — the knower must have good reasons for the belief. A lucky guess that turns out to be correct does not qualify. Remove any one condition and what remains collapses into something weaker: opinion, accidental fact, or hunch. For ~2500 years JTB was the working definition of knowledge in Western philosophy. In 1963, [[Edmund Gettier]] showed in three pages that the three conditions can all be satisfied while the result still fails to be knowledge — see [[Justified true belief is not always knowledge]]. Sixty years of epistemology has tried to patch the definition (no-false-premises clauses, reliabilism, tracking, safety conditions, virtue epistemology) without reaching consensus. For [[Personal Knowledge Management (PKM)]], the JTB triad is a useful diagnostic lens for what a vault actually holds. - A vault contains far more than its owner *believes* — steelmans, captures, AI summaries, "things I'm not sure about" notes. Marking which notes represent current belief vs. material-under-consideration keeps the epistemic state readable. - A vault cannot verify *truth* directly. It can track source quality and internal consistency, which are proxies, not truth itself. This pushes intellectually honest practice toward confidence markers rather than truth claims. - *Justification* is where PKM does the real work: tracking *why* a claim is held and how strongly, not just *that* a source exists. This is the layer where source chains, primary-source preference, and explicit reasoning traces pay off. ## References - Plato, *Theaetetus* — the original definition - Stanford Encyclopedia of Philosophy — "The Analysis of Knowledge" - Gettier, E. (1963). "Is Justified True Belief Knowledge?" *Analysis*