Full Idea
Davidson rejects ontological reduction of mental to physical because propositional attitudes are holistic; there must be extensive coherence among someone's attitudes to treat them as a rational person, and this has no counterpart in physical theory.
Gist of Idea
Reduction is impossible because mind is holistic and brain isn't
Source
report of Donald Davidson (Mental Events [1970]) by Keith T. Maslin - Introduction to the Philosophy of Mind 7.5
Book Reference
Maslin,Keith: 'An Introduction to the Philosophy of Mind' [Polity 2001], p.202
A Reaction
I don't find this view persuasive. We treat the weather in simple terms, even though it is almost infinitely complex. Davidson has a Kantian overconfidence in our rationality. A coherence among the parts is needed to be a tree.