The Problem of Universals

Is 'redness' a real thing, or just a word we use for things that happen to be red? Abelard, Aquinas, and Ockham each gave different answers.

When you see two red things and call them both red, what exactly are you pointing at? The problem of universals asks whether abstract categories (redness, humanity, justice) exist on their own or only in our minds. Plato thought universals were the most real things there are. Realists in the medieval tradition followed him: humanity exists, and individual humans merely participate in it. Nominalists like Ockham disagreed sharply. Only particular things exist. Universals are names, useful tools for sorting the world, but not inhabitants of it. Abelard staked out the middle: universals are neither free-standing realities nor mere words, but mental concepts grounded in real resemblances between things. Aquinas sided closer to realism, holding that universal forms exist in the mind of God and in things themselves. The argument shaped logic, theology, and eventually the foundations of science.