Full Idea
There is a scale of abstractness that leads downwards from sets through attributes to formulas as abstract types and on to formulas as abstract tokens.
Gist of Idea
Abstraction is on a scale, of sets, to attributes, to type-formulas, to token-formulas
Source
JP Burgess / G Rosen (A Subject with No Object [1997], III.B.2.c)
Book Reference
Burgess,J/Rosen,G: 'A Subject with No Object' [OUP 1997], p.199
A Reaction
Presumably the 'abstract tokens' at the bottom must have some interpretation, to support the system. Presumably one can keep going upwards, through sets of sets of sets.