Full Idea
In the Representation Theory of Mind, programs (the 'laws of thought') may be explicitly represented, but data structures (the 'contents of thought') have to be.
Gist of Idea
In CRTT thought may be represented, content must be
Source
Jerry A. Fodor (Psychosemantics [1987], p. 25)
Book Reference
Fodor,Jerry A.: 'Psychosemantics' [MIT 1993], p.25
A Reaction
Presumably this is because content is where mental events actually meet up with the reality being considered. It may be an abstract procedure, but if it doesn't plug into reality then it isn't thought, but merely activity, like that of the liver.