Full Idea
The Armstrong/Lewis version of functionalism takes mental properties to be functional properties, but identifies these with what other functionalists would regard as their realisers.
Gist of Idea
Armstrong and Lewis see functionalism as an identity of the function and its realiser
Source
report of David M. Armstrong (A Materialist Theory of Mind (Rev) [1968]) by John Heil - Philosophy of Mind Ch.4
Book Reference
Heil,John: 'Philosophy of Mind' [Routledge 1998], p.98
A Reaction
Heil rejects this, but I am beginning to think that this is the answer. If functions do not have an ontological life of their own (the 'ringing' of the bell), then functionalist mental states can't either. Function is not an ontological category.