Full Idea
In causal theories of knowledge and reference, the causal chain between object and thought must be of the "right" sort - the nature of rightness to be specified later, typically.
Gist of Idea
Causal theories require the "right" sort of link (usually unspecified)
Source
Daniel C. Dennett (Elbow Room: varieties of free will [1984], §3.3 n14)
Book Reference
Dennett,Daniel C.: 'Elbow Room - Free will worth wanting' [MIT 1999], p.58
A Reaction
This is now the standard objection to a purely causal account of reference. Which of the many causal chains causes the meaning? Knowledge of maths is a further problem for it.