Nominalism and realism are theories related to epistemology (the study of knowledge). Both positions are anchored in their approaches to the concept of universals.
Realism is the view that there are universals that are related to but exist apart from thoughts and individual objects in our world. Consider two white horses for example. Realism asserts that there is a universal concept of “whiteness” which the two white horses share. Plato’s ‘theory of forms’ is the classic representation of realism. Plato argued that for every object in the physical world, there is a more perfect ‘form’ or ‘idea’ that exists in another realm. Plato never did identify where these forms or ideas existed. The fifth century theologian, Augustine, however, modified Plato’s realism by stating that universals exist in the mind of God.
Nominalism, on the other hand, asserts that reality only exists in particular objects. Universals, therefore, have no reality apart from objects. Thus, there are no concepts like “whiteness” that exist in another dimension. Two objects like horses or rocks may share “whiteness” but this whiteness is located in particular objects and not in some independent concept of “whiteness” that exists somewhere else. Forms of nominalism can be found in the views of William of Ockham, George Berkeley, and David Hume.
The concepts of nominalism and realism were an important part of medieval philosophy and theology. In general, the time period of 1200–1350 was a period in which realism was held. The time period of 1350–1500 was dominated by nominalism.