Re: Objection to the Doomsday Argument?

Robin Hanson (
Sun, 16 Aug 1998 11:44:29 -0700 (PDT)

Nick B. writes:
>> 2) Our concept of "like us" can be biased by where we are in time.
>It comes down to this: From what class should you regard
>yourself as being a random sample? ...
>This, I think, means that you cannot regard yourself as a random
>sample from the set of all material objects. How far can we push this

The reference class of "animal", "conscious mind", "primate" seem available without this extreme problem, and seem much less biased to me.

>> For example, Oliver & Korb show that accepting one's birth rank
>> as a random draw from some total population, with a uniform prior
>> over that total population, an observation of a low rank changes
>> the expected value by less than a factor of ten.
>No they don't show that. ... So the quotient E(N) / E(N|R=r) ~
> ~ log U, which becomes arbitrarily large as U goes to infinity.

You're right, but log U goes to infinity very very slowly.

>> As another example, Leslie admits in his "shooting room" example
>> that if the probability of "doom" is constant with time independent
>> of population size, the doomsday argument fails.
>Leslie thinks the DA works in the shooting room in the deterministic
>version. (see p. 254). He doesn't think it works in the radically
>indeterministic case,

I couldn't make any sense of Leslie's determinism discussions.

>> Finally, Nick shows how the argument gets weakened as one allows
>> for alien creatures "like us" who won't get hurt by our local doom.
>Yes, but the fact that we are not posthumans could be taken to
>indicate that there will not have been a great many alien

Only if posthumans are taken to be more "like us" than monkeys.

>> If one instead assumes that the probability of finding oneself in
>> a universe is proportional to population of that
>> universe, the doomsday argument evaporates.
>Yes, this was the objection that I once thought refuted the DA. But
>on second thought there turned out to be big problems with it: the
>no-coincidence objection, and the infinity-objection. I outline these
>in my Investigations paper. How do you suggest we deal with these

I accept the conclusion that if infinite universes are possible, we are most probably in such a universe. I can't make sense of the no-coincidence argument. It seems to me that I can't just assume that I would exist, so my existence says something. In fact, powerful anthropic arguments come from realizing that our mere existence says something about the universe we live in. So how can we just assume that we'd be guaranteed to exist?

(As a formal theorist, I'll say that I really think these discussions become much clearer in the context of specific formal models of the inferences in question. Can this argument be formalized?)