Knowledge is an epistemic achievement commonly expressed as knowing a fact, knowing how to perform an activity, or knowing a person or thing through acquaintance. In philosophy, its nature, acquisition, justification, and limits are central concerns of epistemology. No single analysis commands universal agreement, but philosophical accounts generally distinguish knowledge from unsupported opinion, mistaken belief, and accidentally correct guessing. (plato.stanford.edu)
Forms of knowledge
Propositional knowledge, or knowledge-that, concerns something that can be stated as true or false: for example, knowing that a particular train has departed. Philosophers often represent it as “S knows that p,” where S is a subject and p a proposition. Much analytical epistemology concentrates on the conditions under which such an attribution is correct. (plato.stanford.edu)
Knowledge-how concerns practical competence, such as knowing how to swim or play an instrument. Gilbert Ryle emphasized that possessing facts about an activity does not necessarily amount to knowing how to perform it. Intellectualist theories nevertheless explain knowledge-how through an appropriately practical form of propositional knowledge; opposing theories emphasize abilities or dispositions. Neither equating all know-how with explicit instructions nor sharply separating it from factual knowledge is uncontroversial. (plato.stanford.edu)
Knowledge by acquaintance refers to familiarity through contact or experience, rather than merely knowing descriptions. Knowing a city firsthand illustrates this distinction. These categories can overlap: an experienced musician may possess factual understanding, practical competence, and acquaintance with particular sounds. (plato.stanford.edu)
Truth, belief, and justification
A familiar analysis identifies propositional knowledge with justified true belief. Its three conditions require that a proposition be true, that the subject believe it, and that the belief possess adequate epistemic justification. Truth distinguishes knowledge from error; justification distinguishes an epistemically supported belief from a lucky guess. Calling this the “traditional” definition should not suggest that it was uniformly accepted throughout philosophical history. (plato.stanford.edu)
The Gettier problem, named after Edmund Gettier’s 1963 paper, challenges the sufficiency of those conditions. A person may possess good evidence for a belief that is true only because of an unforeseen coincidence. For example, someone reading a stopped clock precisely when its displayed time is correct appears to have a justified true belief without knowing the time. Such cases motivate additional conditions addressing epistemic luck or alternative analyses of knowledge. (plato.stanford.edu)
Theories differ over what supplies appropriate support. Foundationalism identifies basic beliefs that support others; coherentism emphasizes relations within a system of beliefs. Reliabilism gives a central role to dependable belief-forming processes. Virtue epistemology examines intellectual abilities and virtues through which cognitive success is achieved. These approaches are not interchangeable: some primarily explain justification, while others directly analyze knowledge. (plato.stanford.edu)
Sources and methods
Commonly discussed sources include perception, memory, introspection, reasoning, and testimony. They perform different functions: perception supplies information about surroundings, memory preserves previously acquired cognition, and introspection concerns aspects of one’s mental life. Each raises questions about reliability and possible error. (plato.stanford.edu)
Deductive reasoning derives conclusions that follow necessarily from premises, although knowledge of the conclusion also depends on suitable knowledge of those premises. Inductive reasoning extends beyond what the premises logically guarantee. David Hume helped formulate the problem of induction: how can past regularities justify expectations about unobserved cases without presupposing the reliability of induction itself? (iep.utm.edu)
The distinction between a priori and a posteriori knowledge concerns the role of experience in justification. A priori justification does not depend on experiential evidence in the relevant sense; a posteriori justification does. Mathematics and logic supply standard examples discussed in connection with the former, while observational claims illustrate the latter. Experience needed to learn concepts is distinct from experience needed to justify a claim using them. (iep.utm.edu)
Historical development
In Plato’s Theaetetus, successive proposals identify knowledge with perception, true judgment, and true judgment accompanied by an account. The dialogue subjects these proposals to criticism rather than establishing an uncontested definition. Its treatment of the difference between correct judgment and knowledge remains influential. (plato.stanford.edu)
Early modern debates contrasted rationalism, emphasizing reason as a source of knowledge, with empiricism, emphasizing experience. René Descartes used methodological doubt to seek secure foundations. Immanuel Kant subsequently argued that cognition involves both sensory input and organizing structures supplied by the mind, challenging a simple choice between the two traditions. (iep.utm.edu)
Limits and skepticism
Philosophical skepticism questions whether knowledge is available in particular domains or more generally. Arguments involving dreams or systematic deception ask how ordinary evidence could establish that appearances accurately represent reality. These challenges concern the adequacy of epistemic grounds, not simply the fact that people sometimes make mistakes. (plato.stanford.edu)
Fallibilist accounts allow knowledge without requiring the elimination of every possibility of error. On such views, uncertainty and knowledge need not be incompatible. Philosophers nevertheless disagree about which alternatives must be excluded and whether conversational context affects the standards governing knowledge attribution. (iep.utm.edu)
Social dimensions
Much knowledge depends on other people. Social epistemology studies testimony, expert disagreement, collective inquiry, and institutions that produce or distribute epistemic resources. Through education, individuals acquire claims they have not independently established. This dependence raises questions about trust, expertise, and the grounds for accepting another person’s statements. (plato.stanford.edu)
Scientific inquiry also distributes cognitive work across communities. Peer review, criticism, and the organization of research can influence how evidence is assessed. Social epistemologists investigate when these arrangements support knowledge and when conformity, distorted incentives, or unequal participation obstruct inquiry. Institutional acceptance and epistemic justification therefore remain distinct objects of investigation. (plato.stanford.edu)