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Full Idea
If the famous brain in a bottle is your exact duplicate in brain states, but only experiences the computer's virtual reality, so that you share no objects of acquaintance, then according to externalists you share no beliefs whatsoever.
Clarification
The 'brain in a bottle' has not body, only inputs from a computer
Gist of Idea
If you don't share an external world with a brain-in-a-vat, then externalism says you don't share any beliefs
Source
David Lewis (Lewis: reduction of mind (on himself) [1994], p.424)
Book Ref
'A Companion to the Philosophy of Mind', ed/tr. Guttenplan,Samuel [Blackwell 1995], p.424
A Reaction
A very nice reductio ad absurdum of the idea that all concepts and beliefs have external meaning.