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The classical analysis, traced to Plato's Meno and Theaetetus and codified in 20th-century analytic philosophy, holds that knowledge is justified true belief (JTB): an agent S knows that p if and only if (1) p is true, (2) S believes that p, and (3) S is justified in believing that p. The truth condition ensures knowledge tracks reality; the belief condition ensures the knower actually assents; and the justification condition distinguishes knowledge from true belief that is merely lucky. Each condition appeared independently necessary and jointly sufficient until Edmund Gettier's 1963 challenge.
Work through the three conditions one at a time, constructing counterexamples that violate each in isolation (believing something false, a true proposition you don't believe, a true belief held without justification). This builds intuition for why all three conditions are needed before Gettier destabilizes the picture.
Philosophy has long distinguished between merely believing something and actually *knowing* it. The justified true belief (JTB) account, traceable to Plato's *Meno* and *Theaetetus* and formalized in 20th-century analytic epistemology, proposes that three conditions are necessary and jointly sufficient for knowledge: truth, belief, and justification.
Each condition rules out a class of cases that clearly fall short of knowledge. The truth condition eliminates false beliefs: you cannot know that the earth is flat, because it isn't true — no matter how sincerely you believe it. The belief condition eliminates propositions you accept abstractly but don't genuinely assent to: a student who has memorized a theorem but doesn't really believe it applies to the current problem doesn't "know" it in the relevant sense. The justification condition — the most philosophically interesting — eliminates true beliefs held by luck or for bad reasons. A stopped clock is right twice a day; if you glance at it at exactly the right moment, your belief about the time is true, but you don't *know* the time, because your evidence is defective.
It is crucial not to confuse justification with certainty. Justification is not all-or-nothing and does not require infallibility. A detective who carefully assembles evidence and forms a reasonable conclusion is justified even if that conclusion turns out to be wrong. Justification is about the quality of the reasoning process, not a guarantee of truth. This is why the JTB account requires truth as a separate condition — because even good reasoning can occasionally produce false beliefs.
The elegance of the JTB analysis is that each condition appears necessary: remove any one of them and you get cases that fall short of knowledge. And the three conditions together appeared sufficient — philosophers thought that true belief held for good reasons just *is* knowledge. This confidence held for roughly two millennia until Edmund Gettier published a three-page paper in 1963 constructing cases where all three conditions are satisfied, yet intuitions strongly resist calling them knowledge. The JTB framework is thus both the starting point and the first target of contemporary epistemology.
Understanding JTB matters not just as historical background but because the three conditions continue to structure all subsequent theories of knowledge. Reliabilism, virtue epistemology, and contextualism each preserve the core insight — that knowledge requires more than a lucky true belief — while revising or supplementing the justification condition. Learning what JTB gets right prepares you to understand exactly where and why it falls short.
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