An Argument for Evidence Internalism
Evidence internalism is a view about what evidence is (i.e., the ontology of evidence). Some evidence internalists claim that a person's evidence consists entirely of non-factive mental states, like beliefs or perceptual experiences. Other evidence internalists think evidence is entirely propositional. Propositions (whether believed, known, etc.) are one's evidence. So, according to evidence internalism, evidence could be one of those things (as is argued by those who favor the phenomenal conception) or a combination of those things. What does it mean to say that evidence is "non-factive"? If Jones believes that Joel is cool, it need not be true that Joel is cool in order for that belief to count as part of Jones's evidence (and of course, Joel is cool). Maybe Joel is not cool (let's pretend). Beliefs can be false and still be evidence. Evidence can be non-factive in this sense. Now, consider non-factive mental states. If Jones has a handish experience (appears to see hands), there need not be actual hands causing the experience in order for the experience to count as evidence. Experiences can be non-veridical and still be evidence. That's what it means for evidence to be non-factive. Evidence externalism denies that all evidence is non-factive and internal in the sense outlined above. Evidence externalists might say that one's evidence is both a matter of non-factive mental states (like perceptual seemings) and the way things stand in the world. Concrete objects like footprints and fingerprints can be evidence. The internalist has to deny this. Rather than the actual fingerprints, a proposition or belief like, "those are Jones's fingerprints," is what counts as evidence, though the actual fingerprints might be the occasion for the belief that those are Jones's fingerprints.
Consider two fellows: Brian and Brain. Brian is having a handish experience. So is Brain. They both take their handish experiences as evidence for believing they have hands. Intuitively, that's good evidence for believing in hands. Here's the catch: Brian lives in the real world, where real hands cause handish experiences. His hands are causing him to have a handish experience. Brain is a brain in a vat of chemicals and a scientist is causing him to have handish experiences. He doesn't have hands. Intuitively, both of them are justified in believing they have hands (even though one of them -- Brain -- ends up having a false belief in the end). If one experience is veridical and the other is non-veridical, then what explains the intuition that they are both justified? Here's a simple explanation: they are both justified because they have the same evidence. If so, then the evidence is not stuff that is external to them. It must be non-factive stuff internal to them, like mental-states (beliefs, perceptions, etc.) or propositions that they can have doxastic attitudes toward.
More precisely, facts about their environment (their hands, in particular) are different. Brian has hands. Brain does not. Those are different "external facts." So if their evidence is identical, the facts external to them cannot be part of their evidence. If A is identical to B, then A and B have all the same properties. Hence, if a good explanation for why they are both justified is that they have the same evidence, then there can't be salient differences between their evidence. And the only thing that seems to count as evidence and that is the same between them is their experience and/or beliefs --- the internal, non-factive stuff). They both have handish experiences (and/or beliefs about seeming to see hands). Therefore, their evidence has to be something internal to them (either their phenomenal/sensory states, their beliefs, or something else). It isn't something factive and external to them (i.e., actual hands). Therefore, evidence is non-factive and internal. The crucial premise here is that the best explanation for why they are both justified even though their external circumstances are different is that they have the same evidence. The externalist could say this: "they are both justified, but not because they have the same evidence. Brian has external evidence, while Brain has internal evidence." Recall, the externalist allows that evidence can be both factive and non-factive, mental and concrete. But doesn't that proposal complicate our theory of evidence unnecessarily? Why not just go with the simple internalist explanation? ¯\_(ツ)_/¯
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