The nature of meaning, its definition, elements, and types, was discussed by philosophers Aristotle, Augustine, and Aquinas. According to them "meaning is a relationship between two sorts of things: signs and the kinds of things they mean (intend, express or signify)". One term in the relationship of meaning necessarily causes something else to come to the mind. In other words: "a sign is defined as an entity that indicates another entity to some agent for some purpose". As Augustine states, a sign is “something that shows itself to the senses and something other than itself to the mind” (Signum est quod se ipsum sensui et praeter se aliquid animo ostendit; De dial., 1975, 86).