Re: predictions

From: Hal Finney <>
Date: Mon, 2 Mar 1998 09:36:40 -0800

Nick Bostrom, <>, writes:
> > > This is what want to dispute. If I observe heads at time 1 there is a
> > > 2/3 chance that I observe heads at time 2. This might sound
> > > paradoxical, but the strangeness, I suspect, comes from the fact that
> > > the normal conditions for thinking about personal identity are not
> > > satisfied when there exist several copies of one mind.
> [...]
> I agree that there are two continuations of the experimenter who
> observe heads at time 2. But, I think, the total number of
> continuations of the experimenter at time 2 is three: the two you
> mention plus the one that exists in the other universe where the coin
> landed tail. That observer-instance is no less a continuation of my
> present observer-instance than are the two observer-instances that
> observe heads. So the correct probability, on definition A, is 2/3.

But wouldn't this 2/3 be "the probability that I observe heads at time 2"
irrespective of whether you observed heads at time 1? One of the
three continuations did not see heads at time 1, but you are counting him
in your 2/3. The question was, "what is the probability of observing
heads at time 2, given that you observed heads at time 1?" I think
Wei's answer of 1 in the conventional interpretation, or 2 using his
modified definition of probability, seems correct.

Received on Mon Mar 02 1998 - 09:56:43 PST

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