I started this as a comment on Brian Weatherson's latest posting. But it grew so long that I decided to post it here instead and test my trackback implementation on it.
Imagine a world in which there are nothing but two atoms.
This is ambiguous. Does it mean I should imagine a world in which there are two atoms and nothing else, not even the fusion of these atoms? Or is "nothing but" restricted to things distinct from the two atoms? I can follow the instruction on the latter interpretation but not on the former: a world with two atoms and nothing that is not identical to one of them is inconceivable to me.
Apologies if you've noticed strange errors here or in the RSS feeds. I made some changes to the blogger which broke things for half an hour or so.
In §7 of "Naming the Colours", David Lewis considers the view that colour terms can be analysed in terms of colour experiences which in turn are identified by "a simple, ineffable, unique essence that is instantly revealed to anyone who has that experience".
Then if it were also common knowledge that everyone in the community becomes acquainted with magenta early in life (and if the community were properly dismissive of sceptical doubts about inverted spectra, etc.), it would be common knowledge throughout the community that magenta is the colour that typically causes experiences with essence E.
Lewis goes on to reject this porposal because it contradicts (type-A) materialism. But he doesn't reject the general idea itself: "[The doctrine of revelation] is false for colour experiences. [Footnote:] Maybe revelation is true in some other cases -- as it might be for the part-whole relation."
If a statement p is impossible, then empirical information and a priori reasoning usually suffice to establish its impossibility. So if despite carrying out the relevant empirical investigations and a priori reasonings no impossibility shows up, this is a good reason to believe that p is possible. One might be tempted to say that our knowledge of possibility is always based on such a failure to detect the respective impossibility. This is what Bob Hale calls an asymmetric approach to modal epistemology. (See his "Knowledge of Possibility and of Necessity", Proceedings, 2003.)
In "The Varieties of Necessity", Kit Fine defends Modal Pluralism. Does he thereby threaten Modal Realism? He says he does (in footnote 5). But does he really?
Well, what is Fine's thesis of Modal Pluralism? Here is his summary:
I conclude that there are three distinct sources of necessity -- the identity of things, the natural order, and the normative order -- and that each gives rise to its own peculiar form of necessity. Neither form of necessity can be subsumed, defined, or otherwise understood by reference to any other form of necessity. (p.279 of Conceivability and Possibility)
It seems that he is mixing several different theses here. In particular,
Sometimes people say that for logical reasons there can be no examples of unknown or unknowable truths. The logical reason is this: to know that p is an unknown truth requires knowing that p is true, which contradicts the requirement of p being unknown.
Before I give examples of unknown and unknowable truths let me give examples of philosophers who died more than 100 years ago: Hume, Leibniz, Kant, and the philosopher first born in the 16th century. One might have thought that it is impossible for physical reasons to give such examples. After all, a philosopher who died more than 100 years ago just isn't there any more, so he can't be given as an example. But not so. In order to give an example of a dead philosopher it suffices to name or describe one; it is not necessary to dig him out.
With all these great comments coming in, I've decided to make them available in the RSS feed. (That also makes it easier to track reactions to a comment). While I was at it, I also built in an option to include only excerpts rather than the full postings. As a result, this blog now has the ridiculous number of six RSS feeds: One for my postings (that's the old feed), one for comments, one for postings and comments combined, and then all of them again but with excerpts only: postings, comments, combined. The feeds should now also support Conditional GET. Hope that works.
I guess I should to clarify my argument. The position I want to argue against consists of the following two claims:
1) "pain" denotes a physical entity, say CFF.
2) For no P that only contains physical terms is "P 
I am in pain" a priori.
"" is the material conditional. I've chosen the pain example only for brevity: if you think it matters, feel free to replace "pain" by something like "the phenomenal quality of my current red-experience".
Brandt Van der Gaast points out that Michael McDermott proposes something like the semantics I sketched on behalf of type-B materialism in his "The Narrow Semantics of Proper Names" (Mind 1988). That's true. But I think McDermott is almost silent on the matter crucial to type-B materialism, and there is no acceptable way to fill the silence without spoiling type-B materialism.
In The Conceivability of Naturalism, Crispin Wright notes:
When we disjoin or existentially generalise on names, the results -- for instance, "Tom or John was to blame", "Someone was to blame"-- had better not be conceived as forms of expression involving reference to disjunctive, or existentially general objects. There are no such objects.
What does he mean? Is his point merely the semantical hypothesis that "Tom or John" and "someone" should not be treated as refering expressions? It is probably easy to create a semantics where they are assigned a reference. I'm not even sure (though I believe it) that such a semantics would be perverse, given that lots of people have argued that expressions like "Tom and John" should be assigned some kind of reference to account for sentences like "Tom and John ate the cake". But at any rate, this presumably isn't Wright's point. For even if there was a disjunctive object consisting of Tom and John, it doesn't follow that "Tom or John" must be interpreted as refering to it. So the converse is also invalid: it doesn't follow from the fact that "Tom or John" doesn't refer that there are no disjunctive objects. The passage rather sounds like Wright has independent reason to believe in the non-existence of disjunctive and existentially general objects; a reason that merely gives further support to the semantic claim that "Tom or John" and "someone" don't refer.