Full Idea
My fundamental idea is that 'form-sets' are intersubstitutable constituents of states of affairs with the same form, and 'base-sets' are special form-sets which can be used to construct other form-sets. Ontological categories are the base-sets.
Gist of Idea
Categories are base-sets which are used to construct states of affairs
Source
Jan Westerhoff (Ontological Categories [2005], Intro)
Book Reference
Westerhoff,Jan: 'Ontological Categories' [OUP 2005], p.7
A Reaction
The spirit of this is, of course, to try to achieve the kind of rigour that is expected in contemporary professional philosophy, by aiming for some sort of axiom-system that is related to a well established precise discipline like set theory. Maybe.