Full Idea
I believe that the relation of mental states to behaviour is purely contingent.
Clarification
'Contingent' is the opposite of 'necessary'.
Gist of Idea
Mental states only relate to behaviour contingently, not necessarily
Source
John Searle (The Rediscovery of the Mind [1992], Ch. 1.V.5)
Book Reference
Searle,John R.: 'The Rediscovery of the Mind' [MIT 1999], p.23
A Reaction
I don't think I agree, though it will depend on where you draw the line between mental states and behaviour. Since there have never been two identical states since the beginning of time, it is a little hard to test this one.