Full Idea
Extensional semantics is blatantly circular. For 'Fa' to be interpreted as true, we must know that object a belongs to the extension of the predicate F, so we must already know which objects belong to the extension.
Gist of Idea
Extensionalist semantics is circular, as we must know the extension before assessing 'Fa'
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
I'm delighted to read this, because it was the first thought that occurred to me when I encountered the theory. Presumably this leads Quine to take predication as basic, because you can't break into the circle. Or, vote for intensionalism?