I can concede that being cold is not luminous. That is not the kind of state I mean when I say that one’s occurrent states are luminous. Rather, I mean something like feeling like this is luminous. While both my feeling like this and my feeling like this* support my belief that I am feeling cold, only one of them can provide me with the knowledge that I am feeling like this. The idea is that, by merely being in the state of feeling like this, I am thereby in a position to know that I am feeling like this. No matter how similar feeling like this is to feeling like this*, feeling like this cannot play the sort of role for the belief that I am feeling like this* that it does for the belief that I am feeling like this.
Note that the claim that I am feeling like this is not trivial, for feeling like this is feeling a certain way, and it could be misrepresented. As an analogy, think about colors. If I claim that that box is that color, then I could either be making the trivial claim: it has whatever color it has, or I could be making the claim: it has the particular color that I perceive it as having (McDowell, Mind and World). Likewise, the claim is not supposed to be that I am feeling however I am feeling, but that I am feeling a particular way. I could misrepresent how I am feeling—e.g., I am feeling jealous of S but I form the belief that I am feeling angry at some wrong x has committed. The idea is that there is a particular color (feeling) picked out by the demonstrative, and it is that color (feeling) that is evaluated against the actual color (feeling).
(Thanks to T. Donaldson for pushing me to clarify this point.)
However, one might think that in the cases where I do misrepresent how I am feeling, introspection is thwarted by other factors. When introspection works properly, my belief that I am feeling like this cannot be wrong. Note that the analogy between representing one’s qualitative states and representing the colors of objects fails in an important respect. When I form the belief that that box is that color, I need to represent my qualitative state, my experience, and then on top of that my qualitative state is evaluated against a particular color. (I have to get things right twice over. However, with knowing what I am feeling, I am only trying to represent the qualitative state. Both my ability to know what colors objects are and my ability to know what I am feeling rely on my ability to reflect upon my qualitative states. However—contrasted with the case of colors—my ability to know what I am feeling is merely this ability. It is merely the ability to reflect on my qualitative states that is employed here, not a further ability to accurately perceive further states.
Now Williamson’s argument against the reliability of introspection because of the similarity of the bases does not go through, for being in a certain qualitative state cannot be one’s basis (in the sense described) for forming the belief that one is in a qualitatively different state. On this view, the belief-forming process is still reliable, but in a different way than Williamson proposes. What is reliable is the following capacity: when I am feeling like thisn, and I reflect on how I am feeling, I can thereby come to believe truly that I am feeling like thisn. This concedes the requirement that knowledge be formed in a reliable way but avoids the problems Williamson poses for claiming that beliefs formed by introspection on one’s occurrent states are formed in a reliable way. (If there is any vagueness to this cold, it will not challenge the idea that our introspective capacities are reliable.)
One caveat: such a defense of luminosity is not committed to the idea that there could be no general concepts modifying the demonstrative element in the experience and in the piece of knowledge. For example, if I am feeling this hot, the knowledge that I can thereby non-observationally acquire is that I am feeling this hot. That is, the state need not be merely demonstratively picked out. My being this hot does not support my belief that I am this hot*, no matter how close feeling this hot is to feeling this hot*.