This article examines the essential conditions of propositional knowledge. It focuses on the conditions that must be satisfied for a person to have knowledge, specifically knowledge that something is so. Traditionally, knowledge has been analyzed in terms of justified true belief. This article first addresses philosophers' disagreements concerning the truth and belief conditions. After introducing the justification condition, it presents counterexamples (specifically Gettier-type counterexamples) challenging the standard analysis of knowledge. These challenges have provoked several attempts to replace or to supplement the justification condition for knowledge. This article presents and assesses several of these, including early causal theories, the nonaccidentality requirement, reliable process and conditional analyses, the reliable-indicator analysis, the conclusive reasons analysis, defeasibility analyses, analyses in terms of cognitive or intellectual virtues, and Plantinga's proper functionalism.
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