Full Idea
You need an externalist semantics to explain why the contents of beliefs should have anything to do with how the world is.
Gist of Idea
Externalist semantics are necessary to connect the contents of beliefs with how the world is
Source
Jerry A. Fodor (The Elm and the Expert [1993], §4)
Book Reference
Fodor,Jerry A.: 'The Elm and the Expert' [MIT 1995], p.98
A Reaction
Since externalist semantics only emerged in the 1970s, that implies that no previous theory had any notion that language had some connection to how the world is. Eh?