Doomsday Argument (was: a baysian solution)

From: Wei Dai <weidai.domain.name.hidden>
Date: Sat, 18 Apr 1998 14:41:34 -0700

On Sat, Apr 18, 1998 at 02:56:18PM +0000, Nick Bostrom wrote:
> I think that is wrong. It presupposes that I regard the name "Nick"
> as being randomly choosen from the set of all possible names. When I
> discover that I am called Nick, I would then get reason to believe
> that many people are called Nick. --But "Nick" is not randomly
> choosen from among all possible names. The only reason why "Nick" was
> choosen as a sample was that there existed somebody with that name
> who took his own name as a sample. And if you choose the name from
> the set of all actual names (names of living persons), then you don't
> get any information from finding that that name is instanciated.

If you don't regard "Nick" as being randomly chosen, then why do you
regard your birth rank as being randomly chosen? It seems the same logic
should apply to both, and in this case the two arguments cancel each other
out.

> Say that the percetage of B universes containing somebody called
> "Nick" is twice that of A universes. But the same would hold for any
> other name. Since I know that I have one name or another, then, if
> the above reasoning were correct, I could infer that the B universe
> was twice as likely as the A universe, even before I knew anything
> about my name. This is equivalent to accepting the Self-Indication
> Axiom. I show in my Doomsday-paper that there are serious problems
> with accepting the SIA.

Since "There is at least one person called Nick" and "There is at least
one person called Sam" are not mutually exclusive, you can't sum over all
possible names and conclude that the real universe is twice as likely to
be type B before you learn your name.

> (The difference between finding your name and finding your position
> in time is that you could have found that you position in time had
> been incompatible with there being 100 people, but you couldn't find
> a name that would have been incomplatible with either hypotesis.)

"There is at least one person called Nick" is not incompatible with the
"Real universe is type A" hypothesis, but it's less likely given that
hypothesis.
Received on Sat Apr 18 1998 - 14:45:14 PDT