Methodological solipsism
In epistemology and the philosophy of mind, methodological solipsism has at least two distinct definitions:
The second definition was promoted by Jerry Fodor (1980). 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. Very briefly, the question is whether it is possible for two people, one living in the actual world where water is H2O and the other living in some possible world (Twin Earth) where water has all the same qualities of our water but is actually composed of XYZ, to have the same beliefs (or other propositional attitudes) about water. The externalist says that this is not possible, while the internalist insists that it is.