Friday, August 28, 2009

How to refute mentalism

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.


geoff said...

A: The police believe that Mustard did it.

B: Mustard didn't do it, but this is perfectly consistent with what the police believe.

It seems that B's assertion flatly contradicts A's assertion. But all that B has done is assert that the falsity of a certain proposition (that Mustard did it) is consistent with another proposition about what the cops believe. Yet surely that doesn't give us any reason to think that you can't have false beliefs.

I'm not seeing how your case for (3) is different. No doubt I am just missing something, and maybe I'm just being mental...

Clayton said...

Hey Geoff,

I meant to get back to your earlier comments, but I can't keep up.

I think that B's assertion contradicts A's assertion, but that's because what the cops believe is X and B's assertion asserts that ~X is consistent with what the cops believe. So, I don't think that, "all that B has done is assert that the falsity of a certain proposition (that Mustard did it) is consistent with another proposition about what the cops believe." I think that one of the things that B has done is assert that the cops beliefs won't turn out to be mistaken if Mustard didn't do it. But, they would be mistaken.

Michael said...

I'm a little confused by the role (5) is playing here. If 'autonomous' means capable of yielding evidence without necessarily relying on collateral resources, and if evidence is factive, then saying that perception is an autonomous source of evidence seems to amount to saying that it's capable of yielding non-inferential knowledge. In that case it would indeed be odd to deny 4 and accept 5; but I would think that would just go to show that the mentalists in question would not accept neither 4 nor 5.

I'm not sure what they could/should accept instead of 5, but an example might be something like (what I take to be) Wright's view that perceptual knowledge is inferential (hence, not autonomous) because it depends on hinge propositions for which we have de facto warrant.

Well, who knows -- I tend to agree with you that this internalist mumbo-jumbo leads quickly to intractable skepticism. But given that (4) is just what's at issue in this dispute, I don't think I'm fully clear on what the argument in your post contributes to the case that this is so.

Mike Almeida said...

It seems right that (E) = the fingerprints are on the weapon is evidence for the conclusion that Mustard did it only if it is true that (E). But why wouldn't the mentalist deny that the prosecutor has (E)? What the prosecutor has is (E') = it genuinely seems that Mustard's fingrprints are on the weapon, and (E') is also evidence that Mustard did it. (E') does not entail the truth of (E), but there is no violation of (3). And the BIV also has true genuine seemings as evidence.
[Reading through, I now see that this might be something like what you attribute to Feldman. But I'm not sure I understand your reply.]