Theism, in the field of comparative religion, is the belief in the existence of deities. In popular parlance, the term theism often describes the classical conception of god(s) that is found in the monotheistic and polytheistic religions. The term theism derives from the Greek theos meaning "god". The term theism was first used by Ralph Cudworth (1617–1688). In Cudworth's definition, they are "strictly and properly called Theists, who affirm, that a perfectly conscious understanding being, or mind, existing of itself from eternity, was the cause of all other things".