According to mentalism, if two subjects are in the same non-factive mental states, these subjects have the same evidence. Mentalism is attractive to those who like to say things like BIVs have the same evidence that we do, Neo has the same evidence we do, etc... That is to say, mentalism is attractive if you're mental and you think you have no more reason to believe that you have hands than a BIV does.
That's not the argument, that's the snarky comment. Here's the argument.
(1) Evidence is propositional, it either consists of propositions or facts.
(2) If you know p non-inferentially, p is included in your evidence (Immediate knowledge suffices for evidence a.k.a, 'IKSE').
(3) Evidence consists of truths (ET).
(4) We do know some propositions about the external world non-inferentially.
It follows from (1)-(4) that we have evidence that BIVs don't. Examples might include that this is a hand, that this looks like a hand, that this is not this (where 'this' and 'this' pick out something and something by perceptually), etc...
You can deny (2), but then you need to say something about what you take evidence to be. If evidence is what we have to go on in trying to settle some question or what we're entitled to reason from, I can't see how more than immediate knowledge could be necessary to get some proposition into your evidence.
As for (3), semantic intuitions seem to show that evidence ascriptions are factive. I'm bothered by the suggestion that there can be false propositions that constitute evidence. For me, the worry is brought out by this exchange:
Scarlet: Does the prosecution have solid evidence against Mustard?
Green: Yes, they have all sorts of evidence against him: namely, that he was the last one to see the victim alive, that his alibi did not check out, that his fingerprints were on the murder weapon, and that he had written a letter containing details the police think only the killer could have known.
Now, consider a second:
Plum: How good is the prosecution’s evidence against Mustard?
Peacock: It seems that the evidence is pretty strong. However, Mustard’s prints are not on the murder weapon, his alibi checks out, and he was not the last one seen with the victim. This is all perfectly consistent with the evidence that the prosecution does have.
It seems as if Peacock’s assertion flatly contradicts Green’s assertion. But all that Peacock has done is assert that the falsity of certain propositions is consistent with other propositions about what the prosecution’s evidence consists of. So, unless we say that claims about what someone’s evidence consists of entail that those claims are true, it’s hard to see how Peacock’s assertion could contradict Green’s assertion. Peacock’s assertion speaks to the veracity of the prosecution’s claims rather than speaking directly about the evidence that they have.
As for (4), you can deny that but it seems to come with costs. First, it seems unlikely that we could have any knowledge of the external world without some non-inferential knowledge of the external world. Second, it seems that (4) is an odd thing to deny if you accept (2), (3), and (5):
(5) Perception is an autonomous source of reasons or justification or evidence.
I don't know what devout mentalists would deny, but I know that I'd rather deny mentalism than any of these. I recall from earlier Andy saying that Feldman prefers a version of modest foundationalism that seems to have the implication that our evidence consists of things like 'It seems to me that blah blah blah', but if the view is that our evidence is propositions about us rather than propositions about objects and, say, their appearance properties I'd want to know not why such propositions are in our evidence but why we'd have to say that only such propositions are in our evidence. My guess is that the answer will be something like intuition, mentalism requires saying stuff like that, or a denial of IKSE.