Universals & the Predicables

A universal (to katholou, 'the on-the-whole') is what is common to many particulars — humanity shared by all human beings, redness by all red things — as opposed to the individual. The status of universals is one of the oldest and deepest questions in philosophy: are they real beings, or only names?

Plato and Aristotle

For Plato the universal is supremely real: it is the Form, existing in itself, eternal and separate, in which particulars participate. Aristotle brought the universal down into things, holding that the form exists only in the individuals that share it, drawn out by the mind but not subsisting apart.

Porphyry's question

The debate was set for the whole later tradition by Porphyry, whose short Isagoge ('Introduction') to Aristotle's logic listed the five 'predicables' — genus, species, differentia, property, and accident — the ways a universal can be said of things. Porphyry famously raised, then declined to settle, the question whether genera and species exist in reality or only in the mind. That deferred question became the great medieval 'problem of universals', dividing realists from nominalists for centuries.