A three-page paper from 1963 broke philosophy's definition of knowledge.
You glance at a clock reading two. It is two o'clock, and you have every reason to trust the clock. But it stopped exactly twelve hours ago. Your belief is true and justified, yet surely you didn't know the time. Edmund Gettier used cases like this to upend a definition dating back to Plato.
For a long time many philosophers held, at least roughly, that knowledge is justified true belief. To know something, you must believe it, it must be true, and you must have good reason for it. The idea is traced to Plato, although he raised doubts about it himself in the Theaetetus. In 1963 the American philosopher Edmund Gettier published a three-page paper, 'Is Justified True Belief Knowledge?', offering two counterexamples.
Both involve a man named Smith who reasons correctly from evidence that happens to be false, such as the belief that a colleague called Jones will get a job, and lands on a conclusion that turns out true by luck. Smith has a justified, true belief, but it seems wrong to say he knows. Later philosophers multiplied such cases. In Roderick Chisholm's version, you see a dog disguised as a sheep and conclude there is a sheep in the field, which there is, hidden behind a hill. In Alvin Goldman's 'fake barns', a driver happens to look at the one real barn in a district full of barn facades.
Gettier was not actually the first. Bertrand Russell described the stopped clock, and around 770 AD the Indian philosopher Dharmottara gave examples of a man who mistakes a swarm of insects for smoke above a real fire, and a traveller whose mirage of water happens to sit above real water. Alvin Plantinga has even joked that Gettier created a tradition in the very act of destroying it, since explicit statements of the definition before 1963 are hard to find.
The fallout was enormous. Philosophers tried adding a fourth condition, for instance that the belief must not rest on false premises, or that it must be caused in the right way. Each fix seemed to invite new counterexamples. The Gettier problem is now a central topic in epistemology, and it has shifted many thinkers toward theories that focus on how reliably a belief was formed.
Source: Wikipedia — Gettier problem · Text summarised from Wikipedia (CC BY-SA 4.0)
