Full Idea
Mentalism isn't gratuitous; you need it to explain rationality. Mental causation buys you behaviours that are unlike reflexes in at least three ways: they're autonomous, they're productive, and they're experimental.
Gist of Idea
Rationality has mental properties - autonomy, productivity, experiment
Source
Jerry A. Fodor (The Elm and the Expert [1993], §4)
Book Reference
Fodor,Jerry A.: 'The Elm and the Expert' [MIT 1995], p.89
A Reaction
He makes his three ways sound all-or-nothing, which is (I believe) the single biggest danger when thinking about the mind. "Either you are conscious, or you are not..."