Full Idea
If you crosswire your 'pain box' and your 'itch box', the functionalist says you are in pain if the inputs and outputs are for pain, even though the feeling is of an itch.
Gist of Idea
Crosswiring would show that pain and its function are separate
Source
report of Jaegwon Kim (Philosophy of Mind [1996], p.115) by PG - Db (ideas)
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.115
A Reaction
If functionalists would indeed say this, then the objection seems to me almost conclusive. But they might well say that such simple crosswiring won't work. Itching won't produce pain behaviour - it lacks the correct function.