Full Idea
The multiple realisability emphasised by functionalists rules out the hidden essences (and the 'deferential' move in semantics) that one finds in the cases, for example, of "water" and "gold" emphasised by Kripke and Putnam.
Gist of Idea
Multiple realisability rules out hidden essences and experts as the source of water- and gold-concepts
Source
Colin McGinn (The Problem of Consciousness [1991], p.132)
Book Reference
Rey,Georges: 'Contemporary Philosophy of Mind' [Blackwell 1997], p.188
A Reaction
Presumably if they are 'hidden', then the people to whom we 'defer' for our concepts can't actually know about the essences we are supposed to be discussing. You can mean essences without knowing them. Cf. Loch Ness Monster.