Any view claiming that to analyze a certain phenomenon reference must be made to something outside a certain sphere within which the phenomenon might have been thought to be confined.
In particular, externalism appears in certain analyses of mental notions such as belief and knowledge. An externalist view of belief holds, for example, that one can only believe that King Arthur ruled Britain if King Arthur at least existed, and perhaps also was causally connected to the believer (see causal theories of reference).
Otherwise, one's state of mind cannot be that belief but something else; for example, that the sentence 'King Arthur ruled Britain' (which does exist) says something true.
An externalist view of knowledge holds that to count as knowing something one must be suitably related (for example causally: see also causal theory of knowledge) to the thing or fact in question. Externalist theories are often subdivided into strong and weak versions, and so on, in various ways.
Also see: internalism
C McGinn, Mental Content (1989), especially part 1; with review by D Owens in Mind (1990), externalism about belief and thinking