Universals

A ripe tomato and a fire engine are both red. Do they share a thing — redness — or merely resemble each other, with "red" a word we find useful?^[1] This is the problem of universals, and it is one of the oldest live disputes in philosophy because both answers cost something.

Realism about universals says the property is real. Plato put universals in a separate realm the particulars participate in, which raises the question of how a thing here relates to a Form there. Aristotle kept universals but put them in their instances — redness exists, but only as the redness of red things. Nominalism says only particulars exist and universals are names or concepts, which is economical but owes an account of why the naming isn't arbitrary: something must make "red" apply to the tomato and not the sky.^[2] Trope theory splits the difference — each thing has its own particular redness, and resemblance among those does the work.

The dispute is not idle vocabulary. It decides what a scientific law is about, whether mathematics describes anything, and whether two things can be exactly alike and still be two — which is why it keeps reappearing in arguments that seem to start somewhere else.