Re: Doomsday Example

Nick Bostrom (
Tue, 25 Aug 1998 18:50:10 +0000

> Date: Mon, 24 Aug 1998 21:48:47 -0700 (PDT)
> To:
> From: Robin Hanson <>
> Subject: Re: Doomsday Example
> Reply-to:

> 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?

Sorry, I misunderstood you. But let's drop that stipulation and consider the example that I formulated. It seems you have to say that when you find that you are a human, you have to conclude that all the 1000 universes almost certainly are of type A. That means, you have to infer that the coin landed heads a thousand times. But is that really what you would infer? It seems very wrong.

> >Suppose the only alternatives were (1): one in ten Earth-like planets
> >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.

What do you mean when you say that quick evolution is compatible with any expected time longer than a few billion years? Nobody thought quick evolution were *logically* incompatible with a very long expected time. So presumably you were pointing out that they were not even probabilistically incompatible. But now, it seems, you say that quick evolution gives probabilistic grounds for ruling out a very long expected time.???

Nick Bostrom
Department of Philosophy, Logic and Scientific Method London School of Economics