Full Idea
If functionalism implies that there is nothing physically in common among the realisations of a given mental state, then there is no possibility of any uniform explanation of why they all give rise to a common physical result.
Gist of Idea
If a mental state is multiply realisable, why does it lead to similar behaviour?
Source
David Papineau (Philosophical Naturalism [1993], 2.2)
Book Reference
Papineau,David: 'Philosophical Naturalism' [Blackwell 1993], p.35
A Reaction
This is the well known interaction problem for dualism. The standard reply is to accept interaction as a given (with no apparent explanation). A miracle, if you like.