# Re: Doomsday Example

Robin Hanson (hanson@econ.berkeley.edu)
Mon, 24 Aug 1998 21:48:47 -0700 (PDT)

Nick Bostrom writes:
>> >a small C universe ...spawns one thousand baby-univeses through a random
>> >process (like rolling fair dice) that has a 10% chance of yielding an
>> >A and 90% of a B.
>> >It seems clear that there will probably be about 100 A univeses and
>> >900 B universes.
>> Let N = "trillion trillion", and assume there are exactly 100 A
>> "universes" and 900 B "universes". ...
>> As described the only remaining uncertainty is where in this world I am.
>> If I treat stone slots and human slots equally, there are
>> 100*10 + 900(10+N) slots. If my prior is uniform across these slots,
>
>But the point of my example having a C universe etc. was that this
>prior is not a plausible one. The fair dice in the C universe, or
>rather let's say it's a fair coin, do you really want to say that the
>prior probability that this coin should land heads all thousand
>throws is almost one? That seems wrong.

What are you talking about? I stipulated the fraction of A vs. B "universes" to exactly match the the expected coin toss fraction. How can you then complain I'm saying the coins will all go one way?

>Suppose the only alternatives were (1): one in ten Earth-like planet
>evolve intelligent life; or (2) one in a thousand does. Suppose the
>prior probability is fifty-fifty.
>
>Now you observe that the Earth has evolved life. Based on your
>clarification above, I now take it that you think this observation
>should increase your confidence in (1). Is this right?

Yes.

>But then what happened to the selection-effect you spoke of in the
>"Early life"-paper? "Since no one on Earth would be wondering about
>the origin of life if Earth did not contain creatures nearly as
>intelligent as ourselves, the fact that four billion years elapsed
>before high intelligence appeared on Earth seems compatible with any
>expected time longer than a few billion years"

I don't see this as inconsistent with the other position.