Full Idea
It's a mistake to try to construe the notion of an instance in terms of the notion of a good instance (e.g. Platonic Forms); the latter is patently a special case of the former, so the right order of exposition is the other way round.
Gist of Idea
Don't define something by a good instance of it; a good example is a special case of the ordinary example
Source
Jerry A. Fodor (In a Critical Condition [2000], Ch. 4)
Book Reference
Fodor,Jerry A.: 'In Critical Condition' [MIT 2000], p.43