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The new tree prover has now replaced the old one at /logik/trees. I'm still grateful for all kinds of feedback, even if it's just letting me know that it works on your machine. Unlike the old version, which has retired to /logik/oldtrees/, the new one certainly doesn't work any more with Internet Explorer on the Mac and Netscape 4. If you're still using these browsers, you should really consider switching to Mozilla (or, on MacOS Classic, WaMCom).
If people disagree about whether a sentence S is true in a thought experiment, what could explain the disagreement?
1) They disagree about the meaning of S. Perhaps one party uses 'zombie' for revived corpses whereas the other uses it for people without phenomenal consciousness. The disagreement is 'merely verbal'.
That's not to say it isn't a serious disagreement, in particular if both parties think their usage corresponds to the folk conception, that is, if what they disagree about is whether S is true in the thought experiment according to the common, conventional usage of S in their community. In this case the disagreement can't be resolved by mere stipulation.
If you've followed this blog for a while, you'll have noticed that I'm occasionally worried about the status of shared truth-conditions in a linguistic community. Here's my current opinion.
First the problem. We can use language to communicate how things are. By saying "I have a headache" I can let you know that I have a headache roughly because it is common knowledge between us that people typically utter the words "I have a headache" only when they have a headache. In general, a sentence S can be used to convey the information that certain conditions obtain only if both speaker and hearer know that the hearer will take an utterance of S as evidence that the conditions obtain. Let's call those conditions the 'truth conditions of S'. (The name is a bit misleading because it is often used for the counterfactual conditions under which S would be true. In this sense, the truth conditions of "water isn't H2O" are nowhere satisfied. But clearly that sentence could and can be used to convey information, so these counterfactual conditions aren't the truth-conditions I'm talking of. The truth-conditions I'm talking of are the sentence's A-intensions.)
So Lewis says that a language L is used by a population P iff there prevails in P a convention of truthfulness and trust in L.
This requirement for language use seems far too strong, given Lewis's account of conventions.
The most obvious problem is the condition that for a regularity to be a convention, it must be common knowledge in the population that it is a convention. Lewis offers some weak readings of this condition, but even his weakest versions rule out that sufficiently many members of the population may doubt or deny that the regularity is a convention. So if there were sufficiently many French speakers who believe that their language is completely innate, they would not partake in the convention of truthfulness and trust in French, and thus not use French, on Lewis's account. It even suffices if sufficiently many French speakers merely believe that there are enough who believe that, or believe that there are enough who believe that there are enough who believe it.
The main difference between Lewis's account of language use in Convention and his account in "Languages and Language" (and later works) is that in the latter the convention required for a language L to be used is a convention of truthfulness and trust in L, whereas in the former it was only a convention of truthfulness. I wonder if there are any good reasons for this change.
Suppose in a certain community there exists a convention of truthfulness in L. On Lewis's analysis of conventions this means that within the community,
I'm always worried when a philosopher claims that it's a virtue of his theory that it rules out certain kinds of scepticism, or when a philosopher criticizes another philosopher (say, a contextualist) for not doing so.
I suppose it would be a good thing if newspapers always told the truth. But what would you say if I offered you a theory on which it is ruled out a priori that something false could be written in a newspaper? That wouldn't be a point in favour of my theory. For it seems intuitively obvious that something false could be written in a newspaper. A theory isn't good just because it entails something which, if true, would be good.
A zombie world is a world physically just like our world but in which there is no consciousness. Must a type-A materialist deny the conceivability of zombie worlds? No, not quite.
Compare the rather uncontroversial hypothesis that "the HI virus" denotes the (type of) virus responsible for most AIDS infections. Is it conceivable that a world could be biologically just like ours but not contain the HI virus? Yes, for it might turn out that scientists have been wrong all the time and no virus is involved in most AIDS infections. If it turned out this way, our own world would be a world biologically just like ours but not containing the HI virus.
Let P be a proposition of which you neither believe that it's true nor that it's false, say Goldbach's Conjecture. Since you know that you don't believe P (otherwise you couldn't have chosen it), your conditional subjective probability for [P and I don't believe P] given P should be close to 1. However, if you were to learn that P, your subjective probability for [P and I don't believe P] shouldn't be close to 1, but close to 0. So is this a case were you shouldn't conditionalize?
Merlin is bound to disappear at noon, taking with him all physical traces of his existence. Shortly before his magic disappearance, he casts a spell. As a result, at noon on the following day, the prince turns into a frog.1
In virtue of what does the spell cause the metamorphosis? For instance, it is not at all clear that by Lewis's standards of similarity, some world containing neither spell nor metamorphosis is more similar to actuality than any world not containing the spell but containing the metamorphosis. The problem is that the only trace left by the spell, after Merlin's magic disappearance, is the metamorphosis itself:
Suppose you and I both face a choice between several different options. Say, we both have to pick a ball out of a bag of 100 balls. We win a prize if we make the same choice. But we have no means to communicate. Moreover, our only relevant interest is to win the prize, otherwise we are completely indifferent about the options.
If one of the options is somehow salient, say one ball is red and all the others white, most people will choose that one. And wisely so, as many people following this strategy win the prize, whereas hardly anyone picking a white ball does. However, is this a rational decision among perfectly rational agents who know of each other's rationality and preferences? (I also assume that the agents know that they make exactly the same judgements about salience.)
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