Full Idea
If one of your reasons for doubting that believing-that-P is a physical property is that believing is multiply realizable, then you have the same reason for doubting that being an airfoil (or a mountain) counts as a physical property.
Gist of Idea
Lots of physical properties are multiply realisable, so why shouldn't beliefs be?
Source
Jerry A. Fodor (Making Mind Matter More [1989], p.153)
Book Reference
'The Philosophy of Mind', ed/tr. Beakley,B /Ludlow P [MIT 1992], p.153
A Reaction
This merely points out that functionalism is not incompatible with physicalism, which must be right.