Axiom, a truth requiring no proof. The term is sometimes applied to any important and generally accepted truth, as, in political economy, "cheap money drives good money out of circulation"; in logic, "he who admits a principle admits its consequence"; in natural history, "mountain ranges restrict and direct migrations"; in geography, "an increased altitude lowers the temperature"; in sociology, "character is affected by associations," etc. The term is restricted more properly, however, to self-evident mathematical truths, as, for instance: "The whole is greater than any of its parts"; "Things which are equal to the same thing are equal to each other"; "Two straight lines cannot enclose a space," etc. Euclid recognized fifteen geometrical axioms.