Full Idea
The problem thoughts pose for causation is that causation depends directly only on intrinsic properties, whereas the causal powers of token thoughts depend on their contents, which are not intrinsic.
Clarification
'Token' thoughts are individual thoughts
Gist of Idea
Causation depends on intrinsic properties
Source
DH Mellor / T Crane (There is no question of physicalism [1990], p.194)
Book Reference
'Mind vol.XCIX 1990', ed/tr. Blackburn,Simon [OUP 1990], p.194
A Reaction
This, as we find later in the paper, depends on an externalist account of thoughts. Could a relational property not be causal? Edinburgh's being wetter than London is caused by its being further north?