Full Idea
That mental features supervene on neuronal features in no way diminishes their causal efficacy. The solidity of the piston is supervenient on its molecular structure, but this does not make solidity epiphenomenal.
Gist of Idea
Mental events can cause even though supervenient, like the solidity of a piston
Source
John Searle (The Rediscovery of the Mind [1992], Ch. 5.V)
Book Reference
Searle,John R.: 'The Rediscovery of the Mind' [MIT 1999], p.126
A Reaction
Searle's examples never seem to quite fit what he is saying. Molecules and solidity are supervenient because they are identical (solidity is the presence of certain molecules). Solidity doesn't have causal powers that molecules lack.