Re: predictions

From: Nick Bostrom <>
Date: Mon, 2 Mar 1998 15:40:33 +0000

Wei Dai <> wrote:
> > > A2. If I observe heads at time 1, at time 2 I will observe heads with
> > > probability 1.
> >
> > 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.
> How does 2/3 follow from definition A? At time 2, there are two
> continuations of the experimenter who observes heads at time 1: the
> original and the clone, both observing heads. So according to definition A
> the probability that he will observe heads at time 2 is 2/2 = 1.

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.

Nick Bostrom
Department of Philosophy, Logic and Scientific Method
London School of Economics
Received on Mon Mar 02 1998 - 07:46:50 PST

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