Full Idea
If we accept Putnam's externalist conclusion about the meaning of a word, it is a short step to a similar conclusion about the contents of the twins' beliefs, desires and so on.
Gist of Idea
If content is external, so are beliefs and desires
Source
Gabriel M.A. Segal (A Slim Book about Narrow Content [2000], 2.1)
Book Reference
Segal,Gabriel M.A.: 'A Slim Book about Narrow Content' [MIT 2000], p.24
A Reaction
This is the key step which has launched a whole new externalist view of the nature of the mind. It is one thing to say that I don't quite know what my words mean, another that I don't know my own beliefs.