Mentalism
Mentalism is the position that whether someone's belief is justified supervenes only on their mental states. It is a form of internalism about justification.
Arguments for mentalism
Earl Conee and Richard Feldman defend mentalism in their 2001 paper 'Internalism Defended'.[1] Their central argument is that it is a natural explanation of our intuitions in various thought experiments. One example is the famous new evil demon problem. Many people have the intuition that people in The Matrix can be justified in believing in the world they false appear to inhabit. Mentalism would explain this, since (unless externalism about mental content is correct) these people can have identical mental states to us, and our beliefs about the world are presumably justified. By contrast, paradigmatic externalist positions such as reliabilism struggle to explain this intuition. However, this does not provide a conclusive argument for mentalism: the intuition on which it rests can be rejected, or explained on externalist theories other than simple reliabilism.
Arguments against mentalism
In his paper 'Internalism Exposed', Alvin Goldman argues that whether a belief is justified will depend upon the logical and probabilistic relations of the proposition it expresses, and that these are not mental states.[2]