Full Idea
If you know the content of a thought, you know quite a lot about what would cause you to have it.
Gist of Idea
Knowing the cause of a thought is almost knowing its content
Source
Jerry A. Fodor (The Elm and the Expert [1993], §4)
Book Reference
Fodor,Jerry A.: 'The Elm and the Expert' [MIT 1995], p.92
A Reaction
I'm not sure where this fits into the great jigsaw of the mind, but it strikes me as an acute and important observation. The truth of a thought is not essential to make you have it. Ask Othello.