Full Idea
Functionalism holds that it is in the nature of certain mental states to have certain effects; therefore there can be no mental epiphenomena.
Clarification
Epiphenomena are side effects which play no causal role in events
Gist of Idea
Functionalism defines mental states by their causal properties, which rules out epiphenomenalism
Source
Tim Crane (Elements of Mind [2001], 2.14)
Book Reference
Crane,Tim: 'Elements of Mind' [OUP 2001], p.51
A Reaction
I strongly resist the idea that a thing's identity is its function. Functionalism may not say that. Mind is an abstraction referring to a causal nexus of unknowable components.