Full Idea
A class is, for Frege, the extension of a concept.
Gist of Idea
A class is, for Frege, the extension of a concept
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.8
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.91
A Reaction
This simple idea was the source of all his troubles, because there are concepts which can't have an extension, because of contradiction. ...And yet all intuition says Frege is right..