|problem of universals|
The philosophical debate over whether universals — i.e. qualities or relations that two or more entities have in common (e.g. redness or circularity) — really exist outside of our minds, and if so, what are they and where do they dwell.
1. Plato believed they did exist outside of our minds (see Theory of Forms).
Aristotle disagreed. To him, universals only exist when they are instantiated, i.e. they exist only in things, never apart from things. There is no Platonic form called redness, which exists apart from the set of all red things.