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Full Idea
The definitive characteristic of any experience is its causal role, its most typical causes and effects; but we materialists believe that these causal roles which belong by analytic necessity to experiences belong in fact to certain physical states.
Gist of Idea
Experiences are defined by their causal role, and causal roles belong to physical states
Source
David Lewis (An Argument for the Identity Theory [1966], §I)
Book Ref
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.100
A Reaction
This is the Causal version of functionalism, which Armstrong also developed. The word 'typical' leads later to a teleological element in the theory (e.g. in Lycan). There are other things to say about mental states than just their causal role.