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 made two observations: 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