Logos (UK /ˈloʊɡɒs/, /ˈlɒɡɒs/, or US /ˈloʊɡoʊs/; Greek: λόγος, from λέγω lego "I say") is an important term in western philosophy, psychology, rhetoric, and religion. It is a Greek word meaning "a ground", "a plea", "an opinion", "an expectation", "word", "speech", "account", "to reason", but it became a technical term in philosophy beginning with Heraclitus (ca. 535–475 BC), who used the term for a principle of order and knowledge.