Gale wrote:
> Bostrom wrote ("Investigations
> into the DA"):
>
> >The motivation for defining this axiom is that if we accept it then that
> >buys us a very neat "solution" to the DA problem, i.e. a way of
> >giving all the arguments advanced by Carter and Leslie their due
> >without changing our minds one bit about the future of our species
> >and its robotic descendants. The idea is that we could grant that a
> >shift must be made ... while insisting that this
> >shift is counterbalanced by the greater a priori likelihood of there
> >being many observers.
>
> The error here is the same error made by many who try to argue
> against the DA without realizing that the DA argues for a *shift*
> in priors which are otherwise given and not for an a priori likelihood
> of any particular form. Likewise, as I read KKP, they argue that
> in total parallelism with the DA, *another* shift should be made to any
Yes, except that they do not really *argue* for that.
> prior probabilities that the DA is applied to, and they do not argue
> that there is a "greater a priori likelihood of there being many
> observers."
Yet that is precisely what it amounts to. If you think that the
Self-Indication Assumption is justified then, if you didn't know your
birth rank, you would have to conclude, a priori, that the world
contains infinitely many observers. That seems wrong. When I'm saying
"a priori" I am referring to this prior probability before you know
or take account of your birth rank.
> In particular, the SI *Argument* is no more an Assumption (or Axiom
> as the paper has it) than is the Doomsday *Argument*. They are both
> just applications of Bayes Theorem for which one must specify a prior
> likelihood.
I disagree. The Doomsday argument rests on the Self-Sampling
assumption, and the Dieks-KKP cancellation of the Doomsday argument
rests on the Self-Indication assumption. The difference is that there
are independent arguments for the Self-Sampling assumption (namely
the Amnesia Chamber thought experiment, and Leslie's Emeralds
example) whereas the only reason I know of for accepting the
Self-Indication assumption is that it would cancel the Doomsday
argument, which is a dubious justification. In addition, there are
other problems with the Self-Indication assumption, such as the one
referred to above (that you would think with probability one that the
universe contains infinitely many observers if you don't know your
birth rank). That seems very wrong.
I had a long debate with Robin Hanson about this some 8 months ago on
the extropians list. You might be able to find it in the extropian
achives (linked from
http://www.transhumanism.com/lists.htm
> In other words, KKP are arguing that IF the DA shift is made THEN
> another shift (the SIA shift??) should also be made. I do not see that
> the arguments presented by Bostrom about the SIA address this cleanly.
Whether the DA shift should eb made depends on whether you happen to
know your birth rank, but the correctness of the SIA as a principle
doesn't hinge on that.
> Bostrom also wrote:
> >Moreover, if we
> >reject an inference of the DA type, as many of us are inclined to do,
> >then we must also abstain from applying the SIA: we should either
> >apply both considerations or none.
> >...
> >There is nothing deep or very subtle
> >about this. Either you cross the street or you stay on the pavement,
> >and in both cases you will be safe; but if you stop midway you'll get
> >run over by a bus.
>
> This appears to be quite consistent with the argument presented in KKP.
>
> Thus if Leslie's *conclusion* is to be accepted, then the DArg shift
> must be made
> while the SIArg shift is not made. And this is just what Bostrom has
> called stopping midway, and which he would, from the previous quote,
> seem to reject.
Actually, the quote is from a passage where my objective is to spell
out this objection to the DA in its strongest form.
> But he has also written in bold letters "The Doomsday
> Argument is Alive and Kicking", and he stimulated this post by
> commenting
> that the SIAssumption was problematic, so I would not hazard to guess
> from
> his writings how he might stand on Leslie's conclusion.
I find it useful to separate Leslie's conclusion from his argument.
Even if the DA is correct, it need not mean that intelligent life
will probably go extinct soon. Some of the applicability conditions
of the DA might not be fully satisfied, e.g. if there are lots of
extraterrestrials, or if humans evolve into posthumans which might
arguably belong to a different reference class (I'm not sure about
that), or if there are infinitely many observers, in which case it is
not clear what conclusion to draw.
Nowadays I do tend to think that at least in its pure form, the DA is
very hard to reject. It is more difficult to say how it applies to
the real world, but I think it does place interesting constraints on
what may count as plausible future scenarios.
Nick Bostrom
http://www.hedweb.com/nickb n.bostrom.domain.name.hidden
Department of Philosophy, Logic and Scientific Method
London School of Economics
Received on Sun May 30 1999 - 09:43:35 PDT