Nominalism was a popular term in medieval Scholastic philosophy. The doctrine stated that abstractions, known as universals, are without essential or substantive reality, and that only individual objects have real existence.

These universals, such as animal, nation, beauty, circle, were held to be mere names, hence the term nominalism. For example, the name circle is applied to things that are round and is thus a general designation; but no concrete identity with a separate essence of roundness exists corresponding to the name. The nominalistic doctrine is opposed to the philosophical theory called extreme realism (see Realism), according to which universals have a real and independent existence prior to and apart from particular objects.

Nominalism evolved from the thesis of Aristotle that all reality consists of individual things; the extreme theory of realism was first enunciated by Plato in his doctrine of universal archetypal ideas. The nominalist-realist controversy became prominent in the late 11th and 12th centuries, the nominalist position being expounded by the Scholastic Roscelin, and the realist by the Scholastics Bernard of Chartres and William of Champeaux.

The issue between nominalism and realism was not only philosophical but also theological, for Roscelin maintained that the Trinity (Father, Son, and Holy Spirit), conceived in the traditional theology of the church as constituting a unity of one divine nature, cannot be understood, according to the individualizing method of nominalism, except as three distinct and separate gods, a doctrine known as tritheism. The church was therefore irreconcilably opposed to nominalism. The implications for ethics were also far-reaching. If there is no common nature for all individuals, then there is no “natural law” that governs all people; actions are morally right or wrong only because they are commanded or forbidden by God.

A theory intermediate between nominalism and realism is that of conceptualism, in which universals, although they have no real or substantive existence in the external world, do exist as ideas or concepts in the mind and are thus something more than mere names. Another alternative theory is moderate realism, which locates universals in the mind but also admits a real basis in particular objects. The defense of nominalism undertaken by the 14th-century English Scholastic philosopher William of Ockham prepared the way for various modern nominalistic theories such as those of instrumentalism, pragmatism, semantics, and logical positivism.