externalist

Externalist is a term used in philosophy to describe a proponent of externalism, the view that the content of mental states, the meanings of words, or the justification for beliefs depend on factors outside the thinker. Externalism contrasts with internalism, which holds that content and justification are determined by internal states accessible to the subject.

In philosophy of language and mind, semantic externalism claims that the mental content of a term or belief is partly determined by the actual external world. The most famous argument is Putnam's Twin Earth thought experiment: the term "water" refers to H2O in Earthly contexts but would refer to something different on Twin Earth if the local liquid had the same apparent properties yet a different chemical makeup. This extends to mental content in general, making the environment part of the content-determining conditions.

In epistemology, externalist theories of justification and knowledge, such as reliabilism and proper functionalism, hold that whether a belief counts as knowledge can depend on external factors like the reliability of the belief-forming process, rather than on the agent's internal perspective alone.

Notable figures associated with externalism include Hilary Putnam, Tyler Burge, and Fred Dretske. The externalist program has generated extensive debate and has influenced fields beyond philosophy, including cognitive science and linguistics, raising questions about the nature of reference, consciousness, and the accessibility of justification.