Stop at the only real barn in fake-barn country: do you know it's a barn?
A driver passes a string of fake barn facades, pulls up by chance at the one genuine barn, and believes there is a barn in front of them. The belief is true and well grounded, yet most philosophers say it is not knowledge. Edmund Gettier's puzzles like this broke a definition that had seemed secure.
The classic formula treats knowledge of facts as justified true belief. Truth is required because you cannot know something false, and belief because you cannot know what you do not accept; the exclamation that you do not believe something, you know it, mostly signals confidence. Justification is the contested part. It is there to exclude lucky guesses: someone who calls heads correctly on a coin toss has not thereby known the outcome.
Gettier's twentieth-century counterexamples showed justification and truth can line up by accident. Responses scattered. Some philosophers dropped justification in favour of reliability or intellectual virtues. Others added conditions: no reliance on false beliefs, no defeaters, or a requirement that the person would not hold the belief if it were false. Some demanded infallibility, while pragmatists recast knowledge as whatever works in inquiry. Consensus remains thin, and some argue knowledge is a basic state that cannot be broken into parts at all.
Knowing that is only one variety. Knowing how, such as swimming or riding a bike, is a practical competence, and it is widespread in the animal world: an ant knows how to walk without, presumably, representing any proposition about walking. Knowledge by acquaintance is familiarity from direct experience, like knowing a person. The ancient Greeks split the territory with separate words, among them episteme for unchanging theory, techne for craft skill and metis for strategic cunning. English lumps them together under a word traced to the Old English cnawan.
Where does knowledge come from? Perception supplies most empirical knowledge, introspection reveals our own mental states, and memory, reasoning, intuition and testimony add more. Foundationalists hold that some sources justify beliefs on their own; coherentists say the whole web must hang together; infinitists think an endless chain of reasons is needed. Beyond individuals, knowledge can belong to groups, sit in libraries or fill an expert system. It differs from intelligence, which is the ability to acquire and use information rather than the information already held.
Source: Knowledge
