Epistemology
Gettier Problem
Justified true belief is not knowledge — three pages that shook epistemology
The Gettier problem is a challenge to the traditional definition of knowledge as "justified true belief" (JTB), introduced by Edmund Gettier in a 3-page 1963 paper. Examples: someone has a justified belief that turns out true, but only by luck. Suggests JTB is insufficient for knowledge — need more. Famous Gettier cases: Job and ten coins; sheep in field; clocks. Generated huge literature: hundreds of papers proposing 4th condition (no false lemmas, sensitivity, safety, virtue). Challenges centuries-old definition. Most influential 3-page paper in philosophy.
- AuthorEdmund Gettier (1963)
- Length3 pages
- TargetJTB definition of knowledge
- ConclusionJTB insufficient for knowledge
- Common featureJustified belief is true, but by luck
- IndustryHundreds of proposed solutions
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why Gettier problem matters
- Epistemology. Definition of knowledge.
- Philosophical methodology. Power of thought experiments.
- AI/automated reasoning. What is knowing?
- Evidence and justification. Connection to truth.
- Critical thinking. Quality of reasoning.
- Science philosophy. Justification standards.
- Pedagogy. Famous example of philosophical progress.
Common misconceptions
- Gettier solved problem. Generated debate; not settled.
- Just artificial cases. Reveal real conceptual issue.
- Solution is obvious. Many sophisticated approaches.
- Knowledge undermined entirely. Just JTB definition.
- Just for philosophy. Implications for AI, science.
- Outdated. Active research area still.
Frequently asked questions
What's the Gettier problem?
Challenge to JTB. Knowledge defined as Justified True Belief — believe X, X is true, you have justification. Gettier: this isn't sufficient. Cases where someone has JTB but doesn't seem to have knowledge. Belief is justified; happens to be true; but justification connection to truth is lucky. Real knowledge requires more.
What's a classic Gettier case?
Smith's case (Gettier's original). Smith and Jones apply for job. Smith has good evidence Jones will get it (boss says so). Smith counts coins in Jones's pocket: 10. Smith concludes: "The man who gets the job has 10 coins." But: Smith gets the job (not Jones). Smith also has 10 coins (didn't know). Smith's belief "the man who gets job has 10 coins" was justified, true, but Smith doesn't really know it.
Why is luck the issue?
Belief justified by reason X (Jones with 10 coins). Belief true for reason Y (Smith with 10 coins). Justification disconnected from truth. Belief true by luck — coincidence. Knowledge: justification should track truth. When luck intervenes: not knowledge. Gettier cases all have similar structure — disconnect between justifying reason and reason for truth.
What solutions have been proposed?
Many. (1) No-false-lemmas (Clark, 1963): justification can't depend on false beliefs. Smith's reasoning relied on "Jones will get job" (false). Add: no false premises. (2) Causal theory (Goldman): belief must be caused by fact. (3) Sensitivity: would believe ~P if ~P. (4) Safety: in similar situations, belief tracks truth. (5) Virtue epistemology: knowledge is virtuous true belief. None settles it.
Why did 3 pages matter?
Until Gettier, JTB definition reigned (~2000 years from Plato). Few questioned. Gettier showed simple cases violate JTB. Set agenda for epistemology since. "Post-Gettier" era. Philosophical impact: showed centuries-old idea inadequate; analytic precision can reveal fundamental mistakes; thought experiments reveal much. Highly influential.
Are some philosophers skeptical of Gettier project?
Yes. (1) Knowledge first epistemology (Williamson): can't define knowledge as belief + conditions; knowledge primary. Reverse the analysis. (2) Maybe JTB suffices in real cases; Gettier cases artificial. (3) Maybe knowledge has no precise definition; a "family resemblance" concept. Various ways out of project. But: most epistemologists take Gettier seriously.
What about "knowledge" in everyday use?
Practical sense often less strict. We say we "know" things based on JTB. Gettier cases mostly artificial. Doesn't undermine ordinary practice. But: theoretical question of what knowledge precisely is — unsettled. Philosophy of language, virtue epistemology, contextualism: various sophisticated responses to issue.