- One sees that the wall is red.
- One has reflective access to the fact that one sees that the wall is red.
- One knows that the wall is red on the basis of the fact that one sees that the wall is red.
Since Duncan also accepts a closure principle on knowledge, he accepts:
- One knows that the wall isn't a white wall illuminated by red light.
Like all forms of Mooreanism, Duncan's view is in tension with certain skeptical intuitions. For example, it is in tension with this intuition:
(S) One can't tell by introspection that one is faced with a red wall rather than a white wall with red light.
As Duncan puts it,
If, in the non-deceived case, one has reflective access to the relevant factive reason as epistemological disjunctivism maintains, then why doesn't it follow that one can introspectively distinguish between the non-deceived and deceived cases after all, contrary to intuition? ... In short, the problem is that it is difficult to see how epistemological disjunctivism can square its claim that the reflectively accessible reasons in support of one's perceptual knowledge can nonetheless be factive with the undeniable truth that there can be pairs of cases like that just described [ordinary perceptual cases and corresponding deceptions] which are introspectively indistinguishable. (21)(Duncan defines 'introspective indistinguishability' as the inability to know by introspection alone that the cases are distinct. (p. 53))
If I wanted to be a neo-Moorean of broadly Duncan's style (something I might well want to do), I'd just deny S, along with the many other skeptical intuitions that come out false on this view. But Duncan doesn't want to go that way; as this passage indicates, he considers S and claims like it to be 'undeniable truths'. (On p. 92 he even says that disjunctivists in particular are "unavoidably committed to denying that agents can introspectively distinguish" between the relevant cases.) I confess I don't see why it's so important to hold on to this particular skeptical intuition while happily rejecting others, such as the intuition that an ordinary person at the zoo doesn't know that she isn't looking at a cleverly disguised mule.
How does Duncan go about resolving the tension between his disjunctivism and S? By leaning on the 'by introspection' qualifier. He does think that, if one in the good case, one can reason thus, resulting in knowledge of the conclusion: "I have factive reason R. Only in the good case would I have factive reason R. Therefore, I'm in the good case." But, he says, this is consistent with intuitions like S, which are about introspective abilities. And while one may be able to tell by introspection what reasons one has, one cannot tell by introspection that factive reasons obtain only in the good cases. This is something one can come to know by a priori reflection, but not by introspection. (And maybe the same goes for the epistemic standing of the inference from the two premises to the conclusion.)
This is ultimately a much milder concession to skeptical intuitions than at first it appeared. Although he preserves the letter of his interpretation of the claim that we can't introspectively distinguish the good cases from the bad cases, he does so by pointing out that "introspectively" is a stronger qualifier than one might have realised. He does think (p. 95) that one can reflectively distinguish between good and bad cases, where reflective distinguishability is the ability to know distinct base on a combination of introspection and a priori reasoning.
So two thoughts. First the smaller one: is it really right to exclude a priori reasoning from the considerations that establish 'introspective distinguishability'? It's very hard for me to even make sense of just what that constraint is. (In The Rules of Thought, Ben and I argue that we can't divorce any kind of thought from a priori reasoning.) Consider these two cases: (1) I am presented in ordinary circumstances with a blue ball. (2) I am presented in ordinary circumstances with a black ball. Given the way my perceptual faculties work, we should consider these cases to be distinguishable in the relevant sense if any are. But is it clear that I can know them to be distinct without using a priori reasoning? It's not like the proposition that they're distinct is made available to me directly via introspection. Instead, I have introspective access to how one case looks, and to how another case looks, and I observe that they're different. From this I infer, using something like Leibniz's law, that they're distinct.
Second, supposing Duncan is right about introspective distinguishability: maybe this just shows that the worry wasn't properly articulated in the first place. I submit that someone motivated by the kinds of skeptical pressures that would drive someone to say that you can't tell good cases and bad cases apart by introspection, isn't going to feel better if you allow a priori reasoning along with introspection. The key skeptical intuition in the first place was just that it shouldn't be that easy to tell the good cases and the bad cases apart. And there's no getting around it: that's just an intuition that disjunctivists need to deny. Once we come to appreciate this fact, I'm not sure how important it is to conform to the letter of certain idiosyncratic statements of the intuition.