Full Idea
If someone knew that a thinker had, without realising it, been transported to Twin Earth, they would almost certainly be a higher authority on the content of the thinker's thoughts than would the thinker.
Clarification
On Twin Earth, water looks like H2O, but isn't H2O
Gist of Idea
If someone is secretly transported to Twin Earth, others know their thoughts better than they do
Source
Mark Rowlands (Externalism [2003], Ch.8)
Book Reference
Rowlands,Mark: 'Externalism' [Acumen 2003], p.142
A Reaction
They would certainly be a higher authority on the truth of the thinker's thoughts, but only in the way that you might think I hold a diamond when I know it is a club. If the thinker believes it is H2O, the fact that it isn't is irrelevant to content.