Full Idea
Functionalists claim that pains and the like are higher-order, relational properties that things have in virtue of the pattern of causal interactions that they (can or do) enter into.
Gist of Idea
Functionalists see pains as properties involving relations and causation
Source
Jerry A. Fodor (In a Critical Condition [2000], Ch. 2)
Book Reference
Fodor,Jerry A.: 'In Critical Condition' [MIT 2000], p.13
A Reaction
The whole idea of a property being purely 'relational' strikes me as dubious (or even nonsense). "Is north of" is a relation, but it is totally derived from more basical physical geographical properties.