A dispute · spans 3 eras
The Problem of Universals
Do universal categories – redness, humanity – exist independently of the things that have them?
Three red apples share something we call "redness." Is redness a real thing – existing somehow alongside the apples, perhaps in a Platonic heaven? Or is it merely the name we give to a useful resemblance? The dispute organized medieval philosophy. Realists (Plato, Aquinas) said universals are real. Nominalists (Ockham) said only particulars are real and universals are names. Conceptualists said universals exist as ideas in minds. The dispute is alive: contemporary debates in metaphysics, philosophy of mathematics, and natural-kind theory all replay it.
// The sides
Realism
What red things or all human beings share is a real entity, existing independently of our minds and of the particular things that instantiate it; otherwise the truths of geometry would be truths about nothing.