Full Idea
It is generally conceded by reductive physicalists that a state of the brain cannot be intrinsically about anything, for intentionality is not an intrinsic property of anything, so there can be no internal objects for a physicalist.
Gist of Idea
Physicalism cannot allow internal intentional objects, as brain states can't be 'about' anything
Source
Howard Robinson (Perception [1994], V.4)
Book Reference
Robinson,Howard: 'Perception' [Routledge 2001], p.136
A Reaction
Perhaps it is best to say that 'aboutness' is not a property of physics. We may say that a brain state 'represents' something, because the something caused the brain state, but representations have to be recognised