Full Idea
The same principle that implies multiple realisability would seem to imply universal realisability. …Any object whatever could have syntactical ascriptions made to it.
Gist of Idea
If mind is multiply realisable, it is possible that anything could realise it
Source
John Searle (The Rediscovery of the Mind [1992], Ch. 9.V)
Book Reference
Searle,John R.: 'The Rediscovery of the Mind' [MIT 1999], p.207
A Reaction
This leads to rather weak reductio objections to functionalism. Logically there may be no restriction on how to implement a mind, but naturally there are very tight restrictions. Stick to neurons seems the best strategy.