It's interesting that when I think of philosophy (which "plato") suggests, I think of a completely different kind of "Category" theory. It puts category in a different category.
If Plato were alive today, he would categorically deny that...
Eilenberg & Mac Lane needed a word like 'set', 'group', 'class', etc, except of course those were all taken. They claim to have gotten the word 'category' from Kant, although they didn't intend an actual semantic relation.
We should just pin some category theory posts to the top of HN.
<neo-voice> I know category theory </neo-voice>
Kidding. Where's the tl;dr?
tl; dr Collections of objects and maps between them are themselves good objects to consider. So meta, much abstraction, wow.