EPISTEMOLOGY. This branch of philosophy studies the nature, origin, and validity of knowledge; it is sometimes called "theory of knowledge." Epistemology has been central to modern philosophy since the sixteenth century, although it originally developed in Greek philosophy in close relation to ontology (theory of being) and metaphysics. (It is an open question whether epistemology can be completely disentangled from metaphysics.)

In Greek philosophy, and especially in Plato and Aristotle, the two words used to mean "knowledge" are epistēmē and gnōsis, the former having the narrower, more scientific connotation in opposition to doxa ("belief"), the latter the wider one, covering also perception, memory, and experience. Plato and Aristotle relate these two conceptions to the terms noēsis ("thinking, intuition") and sophia ("wisdom"). The Western Christian tradition, however, has paid more attention to epistemology than to gnoseology; the latter plays a greater role in Eastern Christian philosophy...