Full Idea
For us, sets 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
Aristotle sees hierarchies in definitions using genus and differentia (as we see them in sets)
Source
Kit Fine (Aristotle on Matter [1992], 1 n4)
Book Reference
-: 'Mind' [-], p.37
A Reaction
Genus and differentia are only part of the story in Aristotle, and this remarks strikes me as perceptive. It is precisely the mapping of the explanatory hierarchy which Aristotle seeks in a good definition.