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Nisan comments on The Useful Idea of Truth - Less Wrong
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Koan answers here for:
Only propositions that constrain our sensory experience are meaningful.
If it turns out that the cosmologists are wrong and the universe begins to contract, we will have the opportunity to make contact with the civilization that the colonization starship spawns. The proposition "The starship exists" entails that the probability of the universe contracting and us making contact with the descendants of the passengers of the starship is substantial compared to the probability of the universe contracting.
Counter-example. "There exists at least one entity capable of sensory experience." What constraints on sensory experience does this statement impose? If not, do you reject it as meaningless?
Heh. Okay, this and dankane's similar proposition are good counterexamples.
What about the proposition "the universe will cease to exist when I die" (using some definition of "die" that precludes any future experiences, for example, "die for the last time")? Then the truth of this proposition does not constrain sensory input (because it only makes claims about times after which you have no sensory input), but does have behavioral ramifications if you are, for example, deciding whether or not to write a will.
Least convenient possible world - we discover the universe will definitely expand forever. Now what?
Or what about the past? If I tell you an alien living three million years ago threw either a red or a blue ball into the black hole at the center of the galaxy but destroyed all evidence as to which, is there a fact of the matter as to which color ball it was?
I suspect that the answer to the alien-ball case may be empirical rather than philosophical.
Suppose that there existed quantum configurations in which the alien threw in a red ball, and there existed quantum configurations in which the alien threw in a blue ball, and both of those have approximately equal causal influence on the configuration-cluster in which we are having (approximately) this conversation. In this case, we would happen to be living in a particular type of world such that there was no fact of the matter as to which color ball it was (except that e.g. it mostly wasn't green).
"Possible" is an important qualifier there. Since 0 and 1 are not probabilities, you are not describing a possible world.
The comment doesn't lose too much if we take 'definite' to mean 0.99999 instead of 1. (I would tend to write 'almost certainly' in such contexts to avoid this kind of problem.)
Yvain's objection fails if "definitely" means "with probability 0.99999". In that case the conditional probability P( encounter civilization | universe contracts) is well-defined.
Oh, I thought I retracted the grandparent. Nevermind---it does need more caveats in the expression for it to return to being meaningful.
I think it loses its force entirely in that case. Nisan's proposal was a counterfactual, and Yvain's counter was a possible world where that counterfactual cannot obtain. Since there is no such possible world, the objection falls flat.
If this claim is meaningful, isn't Nisan's proposal false?
No. Why would that be?
You're right, my principle doesn't work if there's something we believe with absolute certainty.
If we later find out that the alien did in fact leave some evidence, and recover that evidence, we'll have an opinion about the color of the ball.