Full Idea
Psychologists mean a by-product by an 'epiphenomenon', ...but the philosophical meaning is too strong: it yields a concept of no utility whatsoever. Since x has no physical effects (according to the definition), no instrument can detect it.
Gist of Idea
If an epiphenomenon has no physical effects, it has to be undetectable
Source
Daniel C. Dennett (Consciousness Explained [1991], 12.5)
Book Reference
Dennett,Daniel C.: 'Consciousness Explained' [Penguin 1993], p.402
A Reaction
Well said! This has always been my half-formulated intuition about the claim that the mind (or anything) might be totally epiphenomenal. All a thing such as the reflection on a lake can be is irrelevant to the functioning of that specified system.