Published online by Cambridge University Press: 01 January 2020
Externalism with respect to content is the thesis that the contents of an individual's mental states are fixed, not just by the intrinsic physical characteristics of the individual, but also in part by the external circumstances of the individual. This idea can be illustrated by means of thought experiments involving pairs of ‘twins’ who are identical with respect to their intrinsic features yet who differ mentally because of differences in their environments. The thoughts of a thirsty earthling may turn to water. Suppose that an earthling has a molecule-for-molecule twin on a very distant planet, a planet identical to Earth except that wherever Earth has H2O, Twin Earth has XYZ, which is superficially indistinguishable from water. When the twin is thirsty, her thoughts will turn instead to twater, as we may call it. Externalism about content is a metaphysical thesis, but it has epistemological implications. Many have argued that externalism is incompatible with a suitably rigorous thesis about the authoritative, introspective self-knowledge we seem to have with respect to our own thought contents.