Re: Once more with the Doomsday Argument

Lee Daniel Crocker (
Tue, 25 Aug 1998 18:39:55 -0700

LC> 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.

NB> 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.

Precisely; which is why you are not an acceptable random sample. If we are betting a short-lived civilization against a longlived one, then we must offer the bet to a random sample of ALL the postulated possible civilizations, not just the one at hand. You can't have it both ways: either other civilizations are postulated, in which case we are not a good sample, or else we are a good sample, in which case we may not postulate others.

Bayes' requires an initial P(H), and P(E|~H). The former doesn't matter much; it can be vanishingly small and still generate a final probability as high as 1/3 in my examples. But the latter--the prior probability of finding the evidence conditioned on the negation of the hypothesis--i.e., how likely are we to find our birth order small if we are a sample from the postulated civilizations other than ours-- is critical. A large final result requires this prior to be large; and the larger this is, the poorer a sample we are.

Lee Daniel Crocker <>
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