He later went on to distinguish this thesis from another that he called methodological individualism.
Fodor's motivation for introducing these concepts into the philosophical (and now psychological) lexicon was the need to defend some sort of internalist conception of the mental from the problems posed by the famous "Twin Earth" thought experiment of Hilary Putnam.
Fodor defines methodological solipsism as the extreme position that states that the content of someone's beliefs about, say, water has absolutely nothing to do with the substance water in the outside world, nor with the commonly accepted definition of the society in which that person lives.
Moreover, the only thing that other people have to go on in ascribing beliefs to someone else are the internal states of his or her physical brain.
In contrast, Fodor defines methodological individualism as the view that mental states have a semantically evaluable character—that is, they are relational states.