Full Idea
Functionalism has as much trouble with physical accounts of reference as of meaning. Reference is the main tool used in formal theories of truth. But 'truth' isn't folk psychology, it is central to logic, which everyone wants.
Gist of Idea
Functionalism can't explain reference and truth, which are needed for logic
Source
Hilary Putnam (Representation and Reality [1988], Int p.xiv)
Book Reference
Putnam,Hilary: 'Representation and Reality' [MIT 1992], p.-5
A Reaction
All logic is defined in terms of truth and falsehood resulting from reasoning, but it could be that 'true' and 'false' have no more content that 1 and 0 in binary electronics. They are distinct, but empty.