Full Idea
All you need for inferring from John's utterance to the world is the sort of thing that a semantics (i.e. referential semantics) provides.
Gist of Idea
Semantics (esp. referential semantics) allows inferences from utterances to the world
Source
Jerry A. Fodor (LOT 2 [2008], Ch.3.5)
Book Reference
Fodor,Jerry A.: 'LOT 2: the Language of Thought Revisited' [OUP 2008], p.86
A Reaction
Fodor is very good at saying nice simple things like that. But it is not enough to infer what objects are being discussed. All the hard cases must be covered (denials of existence, reference to non-existence, intentional contexts, modal claims).