Full Idea
The Twin Earth argument does not refute internalism, since it depends on the 'Content-Determines-Reference' principle, which internalists can reject.
Clarification
Reference is fixed by meaning, rather than by external connection
Gist of Idea
The Twin Earth argument depends on reference being determined by content, which may be false.
Source
Tim Crane (Elements of Mind [2001], 4.37)
Book Reference
Crane,Tim: 'Elements of Mind' [OUP 2001], p.126
A Reaction
The idea is that content should be understood in a context (e.g. on a particular planet). Indexicals count against a totally narrow view of content (Twins thinking 'I am here').