Definitions of knowledge aim to identify the essential features of knowledge. Closely related terms are conception of knowledge, theory of knowledge, and analysis of knowledge. Some general features of knowledge are widely accepted among philosophers, for example, that it involves cognitive success and epistemic contact with reality. Despite extensive study, disagreements about the nature of knowledge persist, in part because researchers use diverging methodologies, seek definitions for distinct purposes, and have differing intuitions about the standards of knowledge.
An often-discussed definition asserts that knowledge is justified true belief. Justification means that the belief fulfills certain norms like being based on good reasons or being the product of a reliable cognitive process. This approach seeks to distinguish knowledge from mere true beliefs that arise from superstition, lucky guesses, or flawed reasoning. Critics of the justified-true-belief view, like Edmund Gettier, have proposed counterexamples to show that some justified true beliefs do not amount to knowledge if the justification is not genuinely connected to the truth, a condition termed epistemic luck.
In response, some philosophers have expanded the justified-true-belief definition with additional criteria intended to avoid these counterexamples. Suggested criteria include that the known fact caused the belief, that the belief manifests a cognitive virtue, that the belief is not inferred from a falsehood, and that the justification cannot be undermined. However, not all philosophers agree that such modifications are successful. Some propose a radical reconceptualization or hold that knowledge is a unique state not definable as a combination of other states.
Most definitions seek to understand the features of propositional knowledge, which is theoretical knowledge of a fact that can be expressed through a declarative that-clause, such as "knowing that Dave is at home". Other definitions focus on practical knowledge and knowledge by acquaintance. Practical knowledge concerns the ability to do something, like knowing how to swim. Knowledge by acquaintance is a familiarity with something based on experiential contact, like knowing the taste of chocolate.