Full Idea
Frege's Constraint says if a subject believes an object is both F and not-F (as in 'Frege cases'), then the subject thinks of that object under distinct modes of presentation. Having distinct mental files of the object is sufficient to generate this.
Gist of Idea
The Frege case of believing a thing is both F and not-F is explained by separate mental files
Source
François Recanati (Mental Files in Flux [2016], Pref)
Book Reference
Recanati,François: 'Mental Files in Flux' [OUP 2016], p.-10
A Reaction
[compressed] When you look at how many semantic puzzles (notably from Frege and Kripke) are solved by the existence of labelled mental files, the case for them is overwhelming.