Full Idea
Evans's 'Generality Constraint' says that if a thinker is capable of attitudes to the content Fa and possesses the singular concept b, then he is capable of having attitudes to the content Fb.
Gist of Idea
Concepts have a 'Generality Constraint', that we must know how predicates apply to them
Source
report of Gareth Evans (The Varieties of Reference [1980], 4.3) by Christopher Peacocke - A Study of Concepts 1.1
Book Reference
Peacocke,Christopher: 'A Study of Concepts' [MIT 1999], p.1
A Reaction
So having an attitude becomes the test of whether one possesses a concept. I suppose if one says 'You know you've got a concept when you are capable of thinking about it', that is much the same thing. Sounds fine.