Non-reductive (a posteriori, type-B) materialists say that even though phenomenal terms denote physical states or properties, the phenomenal way things are is not a priori entailed by the physical way things are. This means that no amount of physical information can tell us what our phenomenal terms denote. That is, non-reductive materialism implies that the projects of naturalising linguistic and intentional content are doomed. I would say that contraposititvely, since there are good reasons to believe in the project of naturalising linguistic and intentional content, non-reductive materialism is doomed.
If Tina is a time traveler who is free to change the past, it must be true that
1) if Tina had chosen 1928, a time traveler would have appeared in 1928.
Moreover, this must be true on a "non-back-tracking" interpretation. A back-tracking interpretation is one on which we consider how past events would have had to be in order to cause some later event. Let's see how (1) fares on Lewis' conditions for non-back-tracking counterfactuals (in "Counterfactual Dependence and Time's Arrow").
Brian Weatherson:
We know that positive conceivability is a good inductive guide to possibility. And we know negative conceivability is a good inductive guide to possibility.
What kind of induction is this? What we do know is that sometimes what seems conceivable on first sight later turns out to be incoherent (and thus inconceivable in the technical sense introduced by Dave Chalmers and deployed by Brian). We also know that this doesn't happen very often, and that it happens mainly when we consider rather complicated stories or hypotheses. So we have good inductive reason to assume that there is no hidden contradiction in, say, the hypothesis that there could be an apple in a basket. But this only supports the claim that prima facie conceivability is a good inductive guide to ideal conceivability.
The standard solution to worries about time travelers' freedom to 'change the past' rests on a distinction between legitimate and illegimitae facts in such considerations. (See e.g. this great paper by Ted Sider.) Assume for simplicity that x is free to do y iff he really would do y should he decide to do y. Now consider Tina the time traveler. Is she free to kill her earlier self? I.e. is it true that
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.