The problem of universals is one of the oldest and most central issues in metaphysics, concerned with the status of properties, kinds, and relations. In its most basic formulation, the problem asks: What is the ontological status of general features that can be shared by multiple particular things? For example, when we say that two apples are red, or that two circles are round, we seem to be attributing to them the same property or character. The question is whether there really is some single entity—a universal—that is present in all the individual things that instantiate it.

This question touches on many core areas of philosophy, including ontology, epistemology, logic, and the philosophy of language. It affects how we understand similarity, classification, meaning, and even causation.

Definition of Universals

A universal is generally understood as something that can be instantiated or exemplified by many different particular things. For example, the property of redness is a universal because it can be instantiated in every red object. The relation of being taller than is also a universal because it can hold between many pairs of objects.

This contrasts with particulars, which are individual things or instances. An individual red apple is a particular; its redness is a universal.