Re: Doomsday Example

Nick Bostrom (
Tue, 25 Aug 1998 02:34:13 +0000

Robin Hanson wrote:

> Leslie writes: "First: Dieks would be very obviously wrong if he
> had just said that more observers meant more opportunities of
> being an observer rather than, say, being an unconscious collection
> of atoms, ... things which do not actually observe anything and
> which therefore must be left out of any calculations about what
> observers could be expected to observe."
> I think I fundamentally disagree with Leslie here. The universe
> doesn't know whether we exist, and doesn't care. If we're going
> to create a state space and prior describing possible universes,
> we should do so in a way that is faithful to our best understanding
> of the physics of universes, and neutral relative to whether some
> set of atoms is organized so as to create an "observer."

I don't think Leslie disagrees with any of that. Note that he says "what observers should expect to observe" which is not the same as "what there should be".

> We ordinarily want to talk about what would happen if I died or
> if humans had never evolved. And it seems to me the
> natural neutral physics-oriented way to do is to talk about
> what would otherwise happen to the material that now makes me up.
> It seems natural to me to talk about this as saying "I could have
> been a rock," though Nick thinks this misuses the word "I". Maybe
> so, but since the basic notion I'm trying to speak of makes sense,
> there aught to be some similar words I could use to describe it.
> Let my "I" stand for that.

I don't understand this. If humans never evolved, you would never have existed. Nowhere around would there be an observer with the RobinHanson-properties. What you now refer to when you say "I" would not have existed.

> 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. That's actually quite simple.

... But to say that there is a fact of the matter as to whether you are the pear and I'm the apple, or the other way around, in a possible world which contains only these two objects - and you seem to accept that you are committed to this view - now *that* seems to me metaphysical in excelsior.

I don't see why you should would want to stick to that view. You can admit that only observers can enter the reference class while still holding on to your view that we should accept the SIA and thereby cancel the DA.

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