Epistemology is the study of knowledge, including its nature and its extent, and of justification (that is, whether a belief is formed in a valid way to justify its being deemed “knowledge”). Epistemology attempts to distinguish knowledge from merely true belief. The philosophically skeptical ask whether we possess knowledge in general or only with respect to a specific domain. These two issues, knowledge vs. belief and the of issue of skepticism, are closely related, as many proposed definitions of knowledge are simultaneously intended to answer various skeptical challenges.
The so-called standard analysis of knowledge maintains that to know that a “proposition” or “statement,” traditionally represented by the letter p (or P), one must have a justified true belief that p. There is considerable debate over how to articulate the requirement of justification. “Internalists” maintain that justification must be accessible to the subject (so that if one knows that p, one knows that one knows that p), while “externalists” argue that it is sufficient for the belief to be formed by reliable methods, regardless of whether or not this fact is accessible to the subject.
Other issues concern whether the concept of knowledge is closed under logical entailment, and if the standards of knowledge are fixed or if they vary with context. There is also the question of whether the justification for a belief must ultimately rest on unassailable foundations, or if a web of beliefs can be justified together on grounds of coherence or overall utility. A related issue is whether there are different types of knowledge, and whether one type is more important than the others. It is common to distinguish between knowledge a priori—that which can be known without appeal to experience—and knowledge a posteriori—knowledge acquired through experience. The relative importance of a priori and a posteriori knowledge is the primary issue that separates rationalists from empiricists.
Finally, while most examples of knowledge considered in the literature are propositional knowledge (knowledge that p), this is sometimes contrasted with practical knowledge (knowledge-how) and theories of direct acquaintance (knowledge-of). Recent work has increasingly attempted to approach these issues by studying the way in which the concept of knowledge is used in our everyday discourse.
