Full Idea
For us it is sets which constitute the most natural example of a hierarchical structure within the abstract realm; but for Aristotle it would have been definitions, via their natural division into genus and differentia.
Gist of Idea
Definitions formed an abstract hierarchy for Aristotle, as sets do for us
Source
Kit Fine (Aristotle on Matter [1992], §1 n4)
Book Reference
-: 'Mind' [-], p.37
A Reaction
I suppose everyone who thinks about reality in abstraction ends up with a hierarchy. Compare the hierarchy of angelic hosts, or Greek gods. Could we get back to the Aristotelian view, instead of sets, which are out of control at the top end?