For most of the history of philosophy, "knowledge" had a tidy three-part definition traced back to Plato: you know something if you believe it, it's true, and you're justified in believing it — a justified true belief. Guessing right doesn't count as knowing, because a guess isn't justified. Being confidently wrong doesn't count either, because it isn't true. All three together looked like the whole story, tight enough that generations of philosophers used it as a working definition without much argument. Then in 1963, Edmund Gettier published a paper barely three pages long that broke it, and philosophy still hasn't agreed on how to fully put it back together.
A belief that's true for the wrong reason
Gettier's cases share a shape: someone forms a belief with excellent justification, and the belief turns out to be true, but its truth has nothing to do with the justification that produced it — the justification and the truth are connected only by coincidence. In one of his original examples, Smith has strong evidence that his colleague Jones will get a particular job, and has separately, directly observed that Jones has ten coins in his pocket. From this, Smith justifiably believes: "the person who will get the job has ten coins in their pocket." Jones doesn't get the job. Smith does — and completely unknown to Smith, he himself happens to have ten coins in his own pocket at that moment. Smith's belief was justified. It turned out true. And almost everyone's intuition says Smith didn't know it — he got the right answer by accident, riding on a coincidence his justification never touched.
Why a three-page paper mattered so much
What makes Gettier's cases hard to wave away isn't that they're exotic — it's that they're built entirely from the three ingredients the classical definition already required. Nothing about the case needs a stretch or a science-fiction premise; it needs only a justified belief and an unlucky coincidence, which are both mundane. If the classical definition were correct, Smith's belief would count as knowledge by its own rules. Since it obviously isn't knowledge to nearly everyone who considers the case, the classical definition has to be missing something — a fourth condition, or a different account entirely, that would rule Smith's belief out without also ruling out ordinary, legitimate knowledge.
What we're still unsure about
Sixty years on, there's still no philosophical consensus on what that missing fourth piece is. Candidates include ruling out justifications that rely on any false step along the way ("no false lemmas"), requiring the belief to be caused by the fact that makes it true, and reliabilist accounts that ask whether the belief-forming process is generally trustworthy rather than examining this one justification in isolation. Each fix handles some Gettier-style cases and runs into new counterexamples of its own. This isn't a solved problem being explained after the fact — it's a genuinely open one, and treating epistemology as having "moved past" Gettier would be overstating how settled the field actually is.
This sits inside The Gettier Problem & Justified True Belief, one of seven topics in Epistemology, one of five domains in Philosophy, one of seventeen subjects the app can quiz you on.