Re: Once more with the Doomsday Argument

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

Lee Daniel Crocker wrote:

> A realistic
> set of propositions would be something more like {there will be fewer
> than 100 billion persons, there will be 100-200 billion persons, there
> will be 200-300 billion persons, ...} up to a suitable upper bound
> chosen empirically based on the size of the planet or universe.
> Without changing the results, this is equivalent to a set of urns with
> {1 ball, 2 balls, 3 balls, ... N balls}, where N is the multiple of
> 100 billion persons we choose as our upper limit. If we then assign
> equal initial probabilities to each of those urns

For the purposes of the DA, we should assign "priors" given by empirical considerations. Korb and Oliver had a similar point like the one you are making here. (I have a more detailed reply in "Alive and Kicking" (

> Let's say we had such evidence--that advanced
> civilizations really seemed to grow to a random size from 100 billion
> to 100 billion billion, and then collapse.

Then the no outsider requirement is not satisfied and the DA is weakened in proportion to the number of existing alien civilizaions.

> Would the fact that we
> find ourselves numbering less than 100 billion truly give us reason
> to suspect we are among the unfortunate slated to perish soon? That
> depends upon whether or not the question is being asked by someone
> who _fairly chose our civilization to pick on_ by offering this bet.

In a sense you are betting against yourself when you decide how to plan your life in view of your estimates of how long our species will survive.

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