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In a three-page 1963 paper, Edmund Gettier refuted the JTB analysis by constructing cases where an agent has justified true belief but intuitively lacks knowledge. In one case, Smith justifiably believes Jones will get the job and has ten coins in his pocket; Smith also (unknowingly) has ten coins in his own pocket. Smith infers 'the man who will get the job has ten coins in his pocket' — a belief that turns out true, but for the wrong reason, since Smith himself gets the job. Such Gettier cases share a structure: justification is present, truth is present, belief is present, yet knowledge seems absent because the truth and justification are connected only by accident.
Construct several Gettier cases yourself, trying to vary the structure. Identify what each case has in common: the belief is justified by evidence that fails to track the actual truth-maker. Then ask what additional condition, if added to JTB, would block all such cases.
You already know that the justified true belief (JTB) analysis proposes three necessary and jointly sufficient conditions for knowledge: you know that P if and only if you believe P, P is true, and your belief is justified. This seemed to capture what knowledge is — not lucky guessing (you need justification), not false belief (you need truth), not mere inclination (you need actual belief). Gettier's 1963 paper showed in three pages that the analysis is wrong, and it changed epistemology permanently.
The key to understanding Gettier cases is the accidental connection between justification and truth. In standard cases of knowledge, your justification is a reliable indicator of the truth — you see a red barn, and that perception justifies "there's a red barn," and the truth and the justification are connected because the barn itself caused your perception. Gettier cases sever this connection. Your justification is legitimate, the belief turns out true, but the truth is reached by a route different from the one your justification tracks. The coin-in-the-pocket case is the original: Smith justifiably believes "Jones will get the job" (on good evidence) and "Jones has ten coins in his pocket" (having personally counted them). Smith infers "the man who will get the job has ten coins in his pocket." The inference is valid; the conclusion is true. But it's true because Smith himself gets the job, and Smith happens to have ten coins in his pocket — facts Smith didn't know. Smith's justification was for a true belief that reached the truth by accident.
The counterexample method you know is precisely what Gettier deployed: find a case that satisfies the definition yet clearly lacks the property being defined. The philosophical power of his cases comes from their simplicity and reproducibility — you can construct Gettier cases yourself by following a simple recipe. Start with a justified belief in a false proposition P. From P, infer a true proposition Q. Your belief in Q is justified (because it follows from a justified belief), it is true (by construction), but you don't know Q because the false proposition P was your route to it. The truth of Q and your justification are accidentally related.
Post-Gettier epistemology spent decades trying to add a fourth condition to JTB that would close the gap. The no false lemmas response says: your belief must not depend on any false intermediate belief. This blocks Gettier's original cases (both depend on the false belief "Jones will get the job"). But Gettier-style cases can be constructed without false lemmas. In a classic fake barns case: you're driving through a region filled with barn facades, indistinguishable from real barns. You look at the one real barn in the area and form the justified true belief "that's a barn." No false intermediate belief — yet most philosophers say you don't know. The environment was rigged against reliable barn-perception. This reveals that the real problem isn't false lemmas; it's epistemic luck: your belief is true, but it could easily have been false given how things were arranged. Eliminating luck from the conditions for knowledge became the central project of post-Gettier epistemology.
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