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From: Wei Dai <weidai.domain.name.hidden>

Date: Fri, 17 Apr 1998 10:42:15 -0700

On Thu, Apr 16, 1998 at 08:23:33PM +0000, Nick Bostrom wrote:

*> But A and B can stand for *classes* of possible universes. In every
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*> univese of class A, the coin landed heads (say), and in every
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*> universe of type B, the coin landed tail. Each of these possible
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*> universes contains a person called Nick. Suppose I know that exactly
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*> one of these universes is the real one, and that I am called Nick.
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*> Then I can formulate the question: "What is the probability that the
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*> person called Nick should have a birth rank <=100, given that the
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*> real universe is one of type A, i.e. one where there are 200 people?"
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*> Now, this question cannot be ruled out on the grounds that
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*> probability assertions including personal pronouns are meaningless --
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*> for it contains no personal pronouns. Yet, this is enough to set the
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*> Doomsday argument rolling.
*

How do you know that every possible universes contains exactly one person

called Nick? Suppose you didn't, then the percentage of type B universes

that contains a person called Nick would be about twice the percentage of

type A universes that contains a person called Nick (since a type B

universe has twice as many people as a type A universe). Since these are

exactly the universes that would survive as possible universes after you

learn your name, this effect would cancel out the DA.

Let me give an example. Suppose in all possible universes everyone is born

conscious in the same mind state. One day after each person's birth he is

told his name. One day after that he is told whether his birth rank is <=

100. There are 200! type B universes, each corresponding to a different

assignment of 200 possible names to birth ranks. Similarly there are

200!/(100!100!) type A universes. Intuititvely in each universe every

person is randomly assigned a name that has not already been taken.

Consider someone just born who has a prior of 1/2 for the real universe

being type A. After learning his name, half of type A universes would be

assigned probability 0, so the probability of the real universe being type

A becomes 1/3. Then after learning his birth rank is <= 100, half of type

B universes drop out, so this probability goes back to 1/2.

-----End of forwarded message-----

Received on Fri Apr 17 1998 - 10:43:25 PDT

Date: Fri, 17 Apr 1998 10:42:15 -0700

On Thu, Apr 16, 1998 at 08:23:33PM +0000, Nick Bostrom wrote:

How do you know that every possible universes contains exactly one person

called Nick? Suppose you didn't, then the percentage of type B universes

that contains a person called Nick would be about twice the percentage of

type A universes that contains a person called Nick (since a type B

universe has twice as many people as a type A universe). Since these are

exactly the universes that would survive as possible universes after you

learn your name, this effect would cancel out the DA.

Let me give an example. Suppose in all possible universes everyone is born

conscious in the same mind state. One day after each person's birth he is

told his name. One day after that he is told whether his birth rank is <=

100. There are 200! type B universes, each corresponding to a different

assignment of 200 possible names to birth ranks. Similarly there are

200!/(100!100!) type A universes. Intuititvely in each universe every

person is randomly assigned a name that has not already been taken.

Consider someone just born who has a prior of 1/2 for the real universe

being type A. After learning his name, half of type A universes would be

assigned probability 0, so the probability of the real universe being type

A becomes 1/3. Then after learning his birth rank is <= 100, half of type

B universes drop out, so this probability goes back to 1/2.

-----End of forwarded message-----

Received on Fri Apr 17 1998 - 10:43:25 PDT

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