Full Idea
Behaviourism can be considered as an attempt to reduce the mental to the physical via definitional bridge principles (every mental expression being given a behavioural definition).
Gist of Idea
Behaviourism reduces mind to behaviour via bridging principles
Source
Jaegwon Kim (Philosophy of Mind [1996], p.217)
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.217
A Reaction
Effectively these would (if they had been discoverable) have been the elusive psycho-physical laws (which Davidson says do not exist). The objection to behaviourism is precisely that there is no fixed behaviour attached to a given mental state.