Aristotle's universals

Aristotle had a similar, but in several points distinct view on universals. He believed, that universals are simply types, properties or relations that are common to their various instances. Universals exist in their instances, they exist only when they are instantiated. They exist only in re, in things, never apart from them. Therefore, an universal is something identical in each instance. All yellow things have something in common, namely their ``yellowness''. There is no universal ``yellow'' apart from any yellow thing. On the contrary, in all yellow things there is the same universal, ``yellowness''. Thus universals do exist in space and time.

Aristotle's universals can be multiply instantiated. The same universal appears in each of its instances. This may seem confusing at first, saying that exactly the same thing exists in multiple objects all over the world. We have to keep in mind that universals are non-physical objects, and there is no reason to believe, that universals behave like physical objects.

How do we form concepts according to Aristotle? We abstract from a lot of instances. Thereby we pay attention to a quality all instances have in common. So by focusing on the common quality a bunch of bananas and some yellow cars and a lemon have in common, namely the same universal, ``yellowness'', we form the concept of ``yellowness''.

When we refer to ``yellowness'', we do not simply refer to ``all yellow things'', but rather to some quality, all yellow things have in common.