Full Idea
Presumably it is a proper function of the belief-manufacturing mechanisms in John to produce beliefs-that-p only if and when p.
Gist of Idea
The function of beliefs is to produce beliefs-that-p when p
Source
Ruth Garrett Millikan (Thoughts without Laws [1986], p.69), quoted by Christopher Peacocke - A Study of Concepts 5.2
Book Reference
Peacocke,Christopher: 'A Study of Concepts' [MIT 1999], p.130
A Reaction
This is the 'teleological' account of belief, which is trying to fit belief into an evolutionary view of humans. It is doubtful whether you can say mental states are just their 'proper' function, because then piano-playing becomes a puzzle.