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Wellsprings
metaphysics

Universals

τὰ καθόλου

When we call many different things 'red' or 'triangles,' is there a single real thing — redness, triangularity — that they all share, or just a name we attach to the group?

The problem of universals asks whether general categories like "redness" or "dog-ness" exist as real entities, or whether only individual red things and individual dogs truly exist. Plato argued that universals are real, eternal forms existing in a realm beyond the physical world. Later philosophers, especially Aristotle, relocated universals into particular things themselves, making them real but dependent on individual instances. Medieval thinkers inherited this ancient debate and spent centuries refining positions on whether universals are substances, mental concepts, or merely linguistic conveniences.

The stakes of this question directly affect how we understand knowledge and science. Ancient philosophers recognized that we cannot acquire knowledge by studying only particular things—our understanding depends on grasping universal principles and shared characteristics. If universals are merely names we assign to similar individuals, then science becomes impossible; but if universals genuinely exist, either in a transcendent realm or within particulars, then systematic knowledge of the world becomes achievable. This tension shapes every subsequent attempt to explain how general laws of nature could be real and knowable.

Where this idea shows up

29 Greek sources

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