# Re: Doomsday Example

Nick Bostrom (bostrom@ndirect.co.uk)
Fri, 28 Aug 1998 17:08:34 +0000

Robin Hanson wrote:

> >Let me put it like this. The amnesia heuristic sets a lower bound for
> >what should be included in the reference class.
> >
> >For example, it clearly makes sense to say that you might not have
> >known (indeed you may not know) the exact hour of your birth. If you
> >didn't know that, then you would use some probability distribution
> >over possible birth hours compatible with what else you know. If you
> >conditionalize this distribution on your exact birth hour, you should
> >get back the distribution you held before you forgot the birth hour.
> >If you don't get back the original distribution then that indicates
> >that you had forgot to take account of some effect. The doomsday
> >argument claims that there is such an effect that you have neglected
> >to take into account.
>
> Well if you're going to rest the whole DA on a mismatch between these
> two calculations, you have do a lot better convincing me there *is*
> a mismatch.
> I thought Dieks did a reasonable job of calculating what
> the person with amnesia should calculate according to the usual view.

Dieks calculation rests on the self-indication axiom: that finding that you are an observer indicates that there are many observers. Only yourself can answer whether that is what you believed before you heard about the DA. Speaking for myself, I didn't believe that, and most other people don't seem to have believed that either, since it was not generally accepted that by just sitting back in your armchair you could decide that the universe is infinite with probability one, unless it is impossible that it is infinite. Most people did, and presumably still do, believe that there is some finite probability that the universe is infinite and a finite probability that it is finite. If that is what you believe, then your prior was certainly not the one Dieks presupposes.

> Let me repeat as forcefully as I can: There are standard approaches
> to formally modeling our propects for doom, and which don't imply doom
> soon. To disagree with them, you must dispute some aspect of those
> models, either their state spaces, priors, or information partitions.

I don't know what "standard approaches" you are referring to. If you mean the one you outlined, I have already said that I think your specification of the state space is incoherent, since it presupposes that in a world with just one apple and one pear, there is a fact of the matter as to weather you are the apple and I'm the pear, or vice versa. I simply can't make any sense of that.

I should also stress that I *do not* see the conclusion of the doomsday argument to be that doom is likely to strike soon, or maybe even ever. The name is misleading. I *do* see it as placing interesting constraints on what are plausible future scenarios and theories of the world at large. The implications are complex, and there seem to be many possibilities that are ruled out but also many very different possibilities that are still permitted. At some point I will post a summary of what I think some of the lessons are.

Nick Bostrom
Department of Philosophy, Logic and Scientific Method London School of Economics
http://www.hedweb.com/nickb n.bostrom@lse.ac.uk