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For the next few days I'll be in Konstanz at the 'Concepts and the A Priori' conference. I'll probably stay a bit longer in southern Germany and Switzerland to visit some friends and relatives and mountains before I return to Berlin in about a week ot two.
If the individuation of mental states depends at least partly on their causal roles, then it depends on the laws of nature (including possibly psychophysical laws). For if the laws differ between world 1 and world 2, a state with a given intrinsic nature can have causal role R in world 1 but lack R in world 2.
Assume world 1 is our world and world 2 is a world that contains a perfect spatiotemporal duplicate of our galaxy but lots of weird things elsewhere that contradict our laws. So the laws of world 2 are not the laws of our world. Then our duplicates in world 2 could have quite different mental states than we do.
But that sounds strange. I would have thought that my mental states do not depend upon what goes on outside the milkyway. We might also get the externalist problem about self-knowledge: If whether I believe P or Q depends on far away events, how can I know I believe P rather than Q if I don't know about these far away events?
Jonathan Schaffer argues (in Analysis 2001) that Relevant Alternatives Theories of knowledge (RATs) such as Lewis's fail because of Missed Clues cases:
Professor A is testing a student, S, on ornithology. Professor A shows S a goldfinch and asks, 'Goldfinch or canary?' Professor A thought this would be an easy first question: goldfinches have black wings while canaries have yellow wings. S sees that the wings are black (this is the clue) but S does not appreciate that black wings indicate a goldfinch (S misses the clue). So S answers, 'I don't know'.
We want to say that S doesn't know that the bird is a goldfinch. Yet it seems that S's evidence rules out all relevant alternatives. For situations with goldfinch-perceptions but no goldfinches are skeptical scenarios and usually regarded as irrelevant.
The computer is working again. Now I have to catch up with 200 non-junk mails.
My computer is currently broken. I hope to get it running again sometime next week. Until then I won't be reading emails very regularly.
It is widely assumed that Lewis takes the objective naturalness of semantic values to be an important constraint on semantics, needed to prevent radical indeterminacy of meaning. On rereading some of his remarks today, I found them a little confusing, and now I think the situation is far more complicated.
Lewis discusses Putnam's model theoretic argument for radical indeterminacy extensively in "New work for a theory of universals" (NW) and "Putnam's paradox" (PP). In both papers, he says there is something wrong with posing the problem as a problem about language, because in fact the interpretation of language is settled by the assignment of content to propositional attitudes (NW 49, PP 58f.). But, he says, focussing on attitudes only relocates the problem without solving it, so that he might as well talk about language in the rest of PP, which he does. He points at NW for a discussion of the properly relocated problem.
I've thought a bit more about the comments Michael Fara left last week, and I don't find my points very convincing any more. The following is partly a correction, but mostly just thinking out loud about a more general semantic question.
The general question is how to interpret sentences of the form
1) At i, A is F
2) At i, A is not F
where 'i' denotes something like a time or a place or a world. There are a dozen proposals for interpretations of (1) in the temporal case, invoking temporal parts or relations to times or whatever. Most of these proposals can be applied to other indices as well. But let's put that aside. Suppose we understand how to interpret instances of (1) in easy cases. The hard cases I have in mind are cases where A doesn't exist exactly once at i. The precise definition of these cases depends on the question I've put aside, but I hope it is reasonably clear what I mean anyway. Not existing exactly once at i means either not existing at i at all, or multiply existing at i. Plausible examples of the first kind: I do not exist in 1758; I do not exist on Alpha Centauri; I do not exist at any world containing only empty space-time. Controversial examples of the second kind: if I get split into two persons tonight, I will doubly exist tomorrow; if river R has two branches where is crosses the border to country C, R doubly exists at the border to C; if at some world, two people resemble me to exactly the same degree in all extrinsic and intrinsic respects, I doubly exist at that world.
I have a problem with the new, free-variables powered version of my tree prover (only existing here on my hard disk at the moment): It doesn't terminate on some valid formulas, at least not within reasonable time.
A very nice feature of ordinary tableaux is that there is a mechanical procedure for building a ("canonical") tableau that will always close as long as there is any closed tableau for the input formula. To my knowledge, no such procedure has been found for free-variable tableaux. The problem, I think, is to decide at each stage whether to apply an ordinary expansion rule or the Closure rule. For many trees, it is best to apply Closure after every expansion. But for some formulas, this procedure will leave the tree forever open. The common response to this problem is apparently to try out all possible decisions at every point, using backtracking and iterated deepening of the search space.
This is still a bit vague, but anyway.
As I remarked in the first part of this little series, from an implementation perspective, it is not surprising that applying one's beliefs and desires to a given task requires processing. Consider a 'sentences in boxes' implementation of belief-desire psychology: I have certain sentence-like items stored in my belief module, and other such items in my desire module. When I face a decision, I run a query on these modules. Suppose the question is whether I should take an umbrella with me. The decision procedure may then somehow find the sentences "It is raining" and "If I take an umbrella, I don't get wet" (or rather, their Mentalese translations) in the belief box and "I don't get wet" in the desire box. From these it somehow infers the answer, that I should take the umbrella.
Why not simply use a notion of content on which belief isn't closed under strict implication? Then it will be much easier to say that reasoning always delivers new content.
There is no shortage of fine-grained notions of content. We could use English sentences, or classes of intensionally isomorphic sentences, or bundles of tuples of objects and properties ('singular propositions') together with modes of presentation, or whatever. The tricky part is to say what determines whether a subject has a belief or desire with such a content.
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