# Re: doomsday argument

```
Bruno wrote:
> Charles wrote:
>
> >(BTW, would I be right in thinking that, applying the SSA to a person who
> >"finds himself" to be 1 year old, the chances that he'll
> >live to be 80 is 1/80?)
>
> This argument (against Leslie Bayesian Doomsday argument) has been
> developped by Jean Paul Delahaye in the journal "Pour la Science"
> (french version of the "Scientific American").
> I have not the precise reference under the hand. I think it is
> a good point against too quick use of Bayes in infinite or continuous
> context.```
```
An interesting article by Ken Olum can be obtained from:

http://xxx.lanl.gov/abs/gr-qc/0009081

In short you don't get any information by observing your age, because you

1) I exist

2) I am one year old

When you compute your updated probability distribution according to Bayes'
theorem by taking into account 1) and 2), you find that the updated
probability distribution is the same as the original one.

The mistake is to use 2) and omit 1).

Nick Bostrom has argued against including 1) (Self Indicating Axiom) in
Bayesan reasoning. This leads to all sorts of nonsensical results. E.g.:

``Bostrom  points out that if one accepts the doomsday argument one must
also accept a number of very strange similar arguments. For example, suppose
that there
is some kind of a natural happening that we have no control over but
nevertheless wish to
avert. For example, suppose that we learn that a nearby star has a 90%
chance of becoming
a supernova, causing significant destruction on earth but not killing
everyone. Now we make
the plan (and make sure that it will be carried out) that if the supernova
occurs we will
start an aggressive program of space colonization, leading to a huge
increase in the number
of people that will eventually exist, while otherwise we will not. Now the
same doomsday
argument that says that the human race is likely to end soon tells us that
the supernova is
not likely to occur. If it did occur, we would then be in the first tiny
fraction of humanity.
Not only does this seem to allow us to affect things over which we should
not be able
to have any control, but it even works backward in time. By exactly the same
argument,
even if the supernova has or has not already occurred in the past, and its
effects have not
reached us, we can change the chance of its having occurred by the above
procedure.
Obviously this kind of paranormal and backward causation is ridiculous.
Bostrom says
that it is not as bad as it seems, but to do so he has to resort to some
rather strange
argumentation including the claim that given some action A and some
consequence C, one
can consistently believe \If we do A then we will have brought about C" and
\If we don't
do A then the counterfactual 'Had we done A then C would have occurred' is
false". It
would seem easier just to say that the type of argumentation that gives us
these paranormal
powers, and thus the Doomsday Argument as well, is simply incorrect. ´´

Saibal

```