Logos (UK /ˈloʊɡɒs, ˈlɒɡɒs/, 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 "ground", "plea", "opinion", "expectation", "word", "speech", "account", "reason", "discourse", but it became a technical term in philosophy beginning with Heraclitus (c. 535–475 BCE), who used the term for a principle of order and knowledge.