universals, in philosophy, term applied to general or abstract objects such as concepts, qualities, relations, and numbers, as opposed to particular objects. The exact nature of a universal deeply concerned thinkers in the Middle Ages. The extreme realists, following Plato, maintained that universals exist independently of both the human mind and particular things. In nominalism universals are considered arbitrary constructions of the human mind. In conceptualism universals exist only in the mind, as concepts, but they are not arbitrary, as they reflect similarities among particular things. Conceptualism led to the moderate realism of St. Thomas Aquinas and John of Salisbury.
See R. I. Aaron, Theory of Universals (2d ed. 1967); D. M. Armstrong, Universals and Scientific Realism (2 vol., 1980).