Final answer:
The original meaning of externalism, focused on the justification of beliefs by external factors, was extended to include mental states to address the mind-body problem and theories of mental causation.
Step-by-step explanation:
The question pertains to the extension of original meaning externalism to include externalism about mental states. Originally, externalism in epistemology referred to the idea that factors outside a person's internal mental state could contribute to the justification of beliefs. This was in contrast to internalism, which held that the justification of beliefs relied solely on factors accessible to the conscious mind.
The extension of externalism to mental states reflects the evolution of thinking within philosophy, especially in the context of the mind-body problem and theories of mental causation. For instance, in substance dualism as expounded by René Descartes, the mind is a non-material entity distinct from the physical body. This distinction highlighted questions about how mental states could affect the physical body if they were indeed external to the material realm.
Externalism about mental states was developed, suggesting that the mind, though not spatially extended, could still interact with the physical in causally significant ways despite being an external, non-accessible factor. In this view, the credibility or reliability of certain processes, such as reliable belief-forming processes mentioned by philosopher Alvin Goldman, could justify beliefs independently of the person's ability to internally access or recount the reasons for those beliefs.
While the two notions of externalism initially seem separate, upon closer inspection, they are linked by the overarching theme of how external factors contribute to internal states or processes, such as belief justification or mental causation.