Full Idea
Davidson's thesis is that if mental events of a particular kind cause physical events of a particular kind, and the two kinds are connected by a law, then they must both be physical kinds.
Gist of Idea
Davidson claims that mental must be physical, to make mental causation possible
Source
report of Donald Davidson (Mental Events [1970]) by Jaegwon Kim - Philosophy of Mind p.137
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.137
A Reaction
Davidson would pretty obviously be right. The whole problem here is the idea of a 'law'. You can only have strict law for simple entities, like particles and natural kinds. The brain is a mess, like weather or explosions.