Full Idea
In extensionalist semantics only existent objects can be referred to, ...but in everyday thought and discourse we regularly and apparently without undue confusion speak about nonexistent objects.
Gist of Idea
Extensionalist semantics forbids reference to nonexistent objects
Source
Dale Jacquette (Intro to 'Philosophy of Logic' [2002], §4)
Book Reference
'Philosophy of Logic: an anthology', ed/tr. Jacquette,Dale [Blackwell 2002], p.5
A Reaction
This is the reason why Meinong, whose views are presented by Russell as absurd, are undergoing a revival. The full-blown view will even treat 'round squares' as objects about which we can reason - and why not? Don't open a shop which sells them.