The Gettier problem is the difficulty, raised by Edmund Gettier in 1963, that a belief can be true and justified yet still fail to count as knowledge, because its truth depends on luck rather than on the justification.
For much of the twentieth century, philosophers treated knowledge as justified true belief. On that view, a person knows a proposition when they believe it, the proposition is true, and they have adequate justification for believing it. In a three-page paper, Edmund Gettier described cases in which all three conditions are met and yet we are reluctant to say the person knows. The cases were brief, but they reopened a question that had seemed settled, and the resulting literature has shaped epistemology ever since.[1]
The force of the examples lies in a gap between justification and truth. A belief can be well supported by evidence and also happen to be true, while the support and the truth come apart, so that the belief is true for reasons the believer knows nothing about. NoteSimilar worries had been noticed earlier by Bertrand Russell, whose stopped-clock example anticipates the structure.
The justified-true-belief account
The account holds that the three conditions are individually necessary and jointly sufficient for knowledge. Each looks indispensable: a false belief is not knowledge, a lucky guess is not knowledge, and a believed truth held for no reason is at best a hunch. The Gettier cases grant all three conditions and then show that something more is needed, because the conditions can be satisfied while the connection between the believer and the truth runs through an accident.[2]
It is possible for a person to be justified in believing a proposition that is in fact false. after Gettier, “Is Justified True Belief Knowledge?”
Gettier’s counterexamples
Gettier offered two cases, both built so that a justified false belief leads, by valid inference, to a justified true one.
The job and the coins
Smith has strong evidence that Jones will get a job and that Jones has ten coins in his pocket, so Smith concludes that the man who will get the job has ten coins in his pocket. As it turns out, Smith himself gets the job, and Smith, unaware of it, also has ten coins in his pocket. Smith’s belief is true and was justified, but it is true by coincidence, and we hesitate to call it knowledge.[3]
The disjunction case
In the second case, Smith justifiably believes that Jones owns a Ford and infers a disjunction that includes a place he has merely guessed. The disjunction turns out true because of the unguessed part, again making the true belief a matter of luck rather than of the evidence Smith possessed. NoteBoth cases share a pattern: a false but justified belief is a premise for a true conclusion.
Responses
Replies have taken several forms. One adds a fourth condition forbidding reliance on any false premise, the so-called no-false-lemmas approach. Another requires that the belief be produced by a reliable process that tracks the truth, shifting attention from justification to reliability. A third, defeasibility theory, demands that no further true information would undermine the justification. Each handles the original cases, but new variants have been constructed against each, and no single repair has won general agreement.[4]
Significance
The lasting effect of Gettier’s paper was less any particular fix than a change of agenda. It made the analysis of knowledge a central project and pushed epistemologists toward accounts that locate knowledge in a reliable connection between belief and fact rather than in justification alone. For the method of testing principles against cases that this debate exemplifies, see the entry on reflective equilibrium; for a related dispute over agency and luck, see free will and determinism.