Full Idea
If a subject can be credited with the thought that a is F, then he must have the conceptual resources for entertaining the thought that a is G, for every property of being G of which he has conception. This condition I call the 'Generality Constraint'.
Gist of Idea
The Generality Constraint says if you can think a predicate you can apply it to anything
Source
Gareth Evans (The Varieties of Reference [1980], p.104), quoted by François Recanati - Mental Files 5.3
Book Reference
Recanati,François: 'Mental Files' [OUP 2012], p.65
A Reaction
Recanati endorses the Constraint in his account of mental files. Apparently if I can entertain the thought of a circle being round, I can also entertain the thought of it being square, so I am not too sure about this one.