know

Know is a verb that expresses awareness of a fact or possession of information, and it also appears as a noun in phrases such as know-how and knowledge. In ordinary language, to know something means to have mental access to information one regards as true, or to be certain of it in practice.

In philosophy, knowledge is a central topic within epistemology. The core idea is propositional knowledge, or knowing that something is the case. Other kinds of knowledge are also recognized: acquaintance knowledge (being familiar with a person, place, or thing) and procedural knowledge (know-how), which concerns abilities rather than propositions.

Classically, knowledge has been analyzed as justified true belief: a person knows p if p is true, the person believes it, and they have justification for believing it. This account faced challenges from Gettier problems, which showed that justified true belief does not always suffice for knowledge. In response, various theories have been proposed, including reliabilism, foundationalism, coherentism, and virtue epistemology, each offering a different account of what justifies knowledge.

Justification for knowledge can derive from perception, memory, reason, or testimony. Knowledge claims therefore depend on evidence, the reliability of sources, and the absence of defeaters. Skeptical challenges question whether knowledge is possible in some domains or under certain conditions, and debates continue about the limits and scope of human knowledge.

Etymology: know derives from Old English cnāwan and is related to cognates in other Germanic languages. In modern usage, the term interacts with a range of constructions that express belief, certainty, and epistemic attitude.