Full Idea
Folk psychology is a powerful instrument of prediction, …which associates with each mental state a typical causal role.
Clarification
'Folk psychology' is the way we describe thoughts and motives in ordinary language
Gist of Idea
Folk psychology makes good predictions, by associating mental states with causal roles
Source
David Lewis (Lewis: reduction of mind (on himself) [1994], p.416)
Book Reference
'A Companion to the Philosophy of Mind', ed/tr. Guttenplan,Samuel [Blackwell 1995], p.416
A Reaction
This seems a good account of why we should take folk psychology very seriously, even if it is sometimes wrong (e.g. about people who are mentally ill).