Full Idea
Mental states have vastly diverse physical/biological realizations in different species and structures (e.g. pain in humans and in molluscs), so no mental state can be identified with any single physical/biological state.
Clarification
Because molluscs and humans have different nervous systems
Gist of Idea
If humans and molluscs both feel pain, it can't be a single biological state
Source
report of Hilary Putnam (The Nature of Mental States [1968]) by Jaegwon Kim - Mind in a Physical World n p.120
Book Reference
Kim,Jaegwon: 'Mind in the Physical World' [MIT 2000], p.120
A Reaction
But maybe mollusc and human nervous systems ARE the same in the respects that matter. We don't know enough about pain to deny that possibility.