Full Idea
The domain of the abstract can be seen as exemplifying a hierarchical structure, with differences of level reflecting the number of steps of abstraction, via appropriate equivalence relations, required for recognition at different levels.
Gist of Idea
There is a hierarchy of abstraction, based on steps taken by equivalence relations
Source
Bob Hale (Abstract Objects [1987], Ch.3.III)
Book Reference
Hale,Bob: 'Abstract Objects' [Blackwell 1987], p.61
A Reaction
I think this is right, and so does almost everyone else, since people cheerfully talk of 'somewhat' abstract and 'highly' abstract. Don't dream of a neat picture though. You might reach a level by two steps from one direction, and four from another.