Full Idea
If the mental is just that which plays a causal role then there is the possibility, which may even be an empirical possibility, that the causal role of tokens of the same mental type should be filled by tokens of significantly different physical types.
Clarification
'Tokens' are individual items
Gist of Idea
One mental role might be filled by a variety of physical types
Source
David M. Armstrong (Pref to new 'Materialist Theory' [1992], p.xiv)
Book Reference
Armstrong,D.M.: 'A Materialist Theory of Mind' [Routledge 1993], p.-10
A Reaction
This allows for multiple realisability in a physicalist framework. Fear has the same role in all animals, but may be realised in physically different ways. I go further, and say that two mental states could differ, while playing the same role.