Full Idea
Since no laws exist connecting mental and physical properties, purely physical laws must do the causal work, which means mental events enter into causal relations only because they possess physical properties that figure in laws.
Gist of Idea
If mental causation is lawless, it is only possible if mental events have physical properties
Source
report of Donald Davidson (Mental Events [1970]) by Jaegwon Kim - Philosophy of Mind p.138
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.138
A Reaction
Surely no such laws exist 'yet'? I can see no plausible argument that psycho-physical laws are impossible. However, the conclusion of this remark seems right. Interaction requires some sort of equality.