Re: Doomsday Example

Robin Hanson (
Wed, 26 Aug 1998 13:59:53 -0700

[I've summarized my thinking on this topic in an essay at: RH]

Nick Bostrom writes:
>> >> Doomsday argument folks also seem to want talk about the possibility
>> >> that I might have been some other human at some other place in
>> >> space-time.
>> >
>> >That is only shorthand. You can replace this phraseology if
>> >you find it problematic. Instead you can talk about the possibility
>> >that Robin Hanson, while remaining Robin Hanson, could forget certain
>> >facts, and about what probabilities RH would then assign to the
>> >hypothesis that RH is living in such-and-such a place and time.
>> I could sortof accept the idea that I
>> might have been someone else instead. But I find it very hard to believe
>> that you could make Robin hanson forget not so much that he was no longer
>> Robin Hanson, *and* enough so he couldn't tell he wasn't a Martian living on
>> Hermes in 2200. And even if you could I don't see the relevance of that poor
>> hypothetical creature and what I should infer from what I know. If you're
>> going to rest the DA on this construction, it seems implausible from the
>> get go.
>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.

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. Simply noting that someone could have amnesia is far from enough.

>> I'm not sure the SIA is sufficient to deal with all the cases of
>> interest. I'd rather instead accept an approach to defining states
>> and priors.
>It would be nice if it were that simple, but we're still waiting for
>a coherently specified state space. Do you have any specific cases in
>mind where the SIA would be inadequate?

I don't know how to do inference with "axioms" like "the fact that I am an observer gives me some reason to believe that the world contains many observers." As I've repeated, I just know how to do inference with state spaces, priors, and information partitions.

Robin Hanson RWJF Health Policy Scholar, Sch. of Public Health 510-643-1884 140 Warren Hall, UC Berkeley, CA 94720-7360 FAX: 510-643-8614