Full Idea
We could know the necessary and sufficient stimulatory conditions of every possible act of utterance, in a foreign language, and still not know how to determine what objects the speakers of that language believe in.
Gist of Idea
You could know the complete behavioural conditions for a foreign language, and still not know their beliefs
Source
Willard Quine (Speaking of Objects [1960], pt.III,p.11)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.11
A Reaction
I just don't believe this, because the same scepticism then creeps into discussions of native speakers of a single language, and all communcation is blighted - which is nonsense.