Externalism and Internalism are two lenses through which a philosopher may view the world, particularly applicable to areas such as motivation, truth, knowledge and justified belief. In the case of the former, the theory is that motivation etc. ultimately comes from sources outside the subject while the latter might argue that these factors arise from within the mind itself. To contextualize this specifically for the field of epistemology, Internalists hold that belief is produced via successful “cognitive operations”1 whereas Externalists prefer the explanation that a belief comes about in relation to environmental factors or other people.
There are subdivisions of each school of thoughts as well - in Externalism, Putnam focused on semantic externalism, Dretske and Lycan advocated a kind of phenomenal externalism and Rockwell’s more recent ideas have been centered around the idea of “embodied cognition” which would allow for experiences of the body (but not the mind) to fall into the category of external factors. Internalism and Externalism also differ significantly in their views on science, where Internalists would posit that the necessary ingredients are within us so science will evolve naturally regardless of the social context and Externalists see it quite the opposite.
1. Mattey, G. J. (2003).Internalism vs. Externalism. [ONLINE] Available at: http://hume.ucdavis.edu/mattey/phi102f03/fumerton.html. [Last Accessed 5/2/14].