Universals: Nominalism and Realism

Core Idea

The problem of universals asks what accounts for objective similarities among particulars. Realism holds that universals (abstract properties shared by many particulars) are real entities that exist in each instance. Nominalism denies universals, explaining similarity through resemblance classes or class membership. This problem remains central because the answer shapes ontology and the theory of properties.

Explainer

From your study of universals and particulars, you understand the basic distinction: a particular is a concrete individual thing (this red apple, that red fire truck), while a universal would be the redness they share. The problem of universals asks: what exactly is being shared? Is there a real entity — redness itself — that is literally present in every red thing, or is the similarity among red things explained some other way?

Realism holds that universals are genuine entities. When two objects are both red, there is a single property, redness, that is wholly present in each. This explains similarity by positing a common constituent. The most radical version — Platonic realism — places universals in a realm of abstract objects that exist independently of any particular instance; redness exists whether or not anything is currently red. A more moderate Aristotelian realism holds that universals are real but only ever exist instantiated in particulars — redness exists only insofar as there are red things. Both versions face the challenge of explaining how an abstract or multiply-located entity can causally interact with the physical world, and how we come to have knowledge of it.

Nominalism denies that universals exist and tries to explain the appearance of shared properties without them. Class nominalism identifies properties with sets: being red just means being a member of the class of red things. Resemblance nominalism says that what makes two things red is that they sufficiently resemble each other and the paradigm cases of red things. The challenge for all nominalisms is to explain objective similarity without invoking the very universals they're trying to eliminate. What grounds the resemblance relation? If we say two red things resemble each other, are we smuggling in a shared property through the back door?

Your study of trope theory offers a third path: tropes are particular instances of properties — the redness of *this apple* is a distinct entity from the redness of *that truck*, even though they resemble each other. Trope theory avoids Platonic abstract universals while still giving properties ontological status. The debate between these positions isn't merely terminological. It shapes questions about natural laws (do laws hold because properties are universals that necessitate causal relations?), mathematics (are numbers universals?), and the metaphysics of science (what are natural kinds?). Choosing between nominalism and realism is one of the first and most consequential decisions in constructing an ontology.