Re: another paradox and a solution

From: Nick Bostrom <>
Date: Mon, 2 Mar 1998 01:32:59 +0000

 Wei Dai <>

> On Sat, Feb 28, 1998 at 01:57:12AM +0000, Nick Bostrom wrote:
> > Only on the AUH, right?
> I'm not sure what you mean by "on the AUH."

"On the assumption that the All-Universes Hypothesis is true."

> > Isn't it the case that on the AUH, *everything* remains the same no
> > matter what I do?
> No, because your actions affect your future perceptions. I think Tegmark's
> distinction between the inside view and the outside view is very important
> here. From the outside, everything is fixed and timeless. But from the
> inside there is a flow of time and actions have consequences.
> > In one sense, yes; in another sense, no. I think it's the same as
> > with any other physical theory.
> >
> > The sense in which I *can* make a difference,on the AUH, is I think
> > the following: If I choose to do action X, then that indicates that X
> > is a rather probable thing to happen (it has a big measure, even
> > though in some universes I will not do X in a similar situation). It
> > might not matter whether we say that I "cause" X to have a big
> > measure by wanting it, or we say that the ground why I want X is
> > that the event of me bringing X about by wanting it has a relatively
> > big measure.
> Ok, but how do you apply decision theory when the probability of your goal
> being reached is either 1 or 0, independent of your decisions?

The probability might in some kind of objective sense be either 1 or
0, but that's the same in the standard view of the world. Either
something happens or it doesn't. The relevant perspective is the
subjective one, and even on the AUH it is possible (and in practise
will always be the case) that I don't know whether my goal will be
reached or not. In that case I have to look at subjective
probabilities (which are relative to what I know and can figure out
at a given time), and these can be anything between 0 and 1.

> > It looks to me now as if your definition of this probability agrees
> > with the (second) one I suggested (and which you say that Tegmark
> > also uses), except that I assumed a finite number of discrete
> > universes, whereas you stated the definition in terms of measures.
> First let's agree to use measures since that works even when the number of
> universes is finite. Your definition is

I suppose it has to work somehow, but for me it is still a magical
postulate. But for the sake of argument, we can assume that these
measures make sense.

> A. The measure of my future continuations perceiving X, divided by the
> measure of my future continuations.
> My definition is:
> B. The measure of my future continuations perceiving X, divided by the
> measure my of present self.
> Tegmark defines it a different way, but I think it is equivalent to
> definition A. The point of introducing definition B is that definition A
> is not self consistent.

I am not convinced about that.

Nick Bostrom
Department of Philosophy, Logic and Scientific Method
London School of Economics
Received on Sun Mar 01 1998 - 17:38:16 PST

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