Re: Objection to the Doomsday Argument?

Hal Finney (
Fri, 14 Aug 1998 12:31:10 -0700

Doug Bailey, <>, writes:


> Thanks for the comments Nick. I think what I am attempting
> to articulate is that I don't see how any species can apply
> the DA and not conclude that its extinction is imminent. Can you
> provide details or characteristics of an entity and its species
> where the entity could apply the DA to its situation and not
> reach the "doom" conclusion?

Many times I've convinced myself that I have a refutation for the DA, only to decide on further consideration that it is more difficult than it seems.

In this case I think Doug's comment is correct, although it is not a matter of a "species" applying the DA but rather a member of the species.

However the point is that for most members of most species, the conclusion is correct. For most of them, it is true that they are about in the middle of the set of members of their species. That's just the law of averages. So of all those species members who apply the DA to conclude that "doom is imminent" (specifically, that they are about halfway along in the membership of their species), most of them are correct.

One possible escape, then, is to suggest that we are special because we are living in the time when the DA was just recently discovered. We are not a typical member of a long-lived species who learns about a DA which was discovered ages ago. If we were then it would be correct that the odds are that we are halfway along the species' existence. But we are not a random selection, we are special because this is the first era when the DA has been discovered. Every species must go through such an era, and for those observers who live in exactly that era, it tells them nothing about the expected time remaining for their species.

This argument would be a bit stronger if the DA had been discovered more recently. However it has only been popularized within the last couple of years and so we really are the first people to be faced with it.

I don't care for formulations of the DA which act as though I might have been born at different times. This raises thorny problems about the meaning of identity, whether I could be said to be the same person if I were born thousands of years in the future.

Rather, it is simpler to say that for most observers the DA will be valid. However, it will not be valid for those observers who are specially chosen as being part of a certain milestone in the history of their race - the discovery of fire, or of writing, or of the DA. You can easily imagine that the universe is such that most species do not discover fire halfway along their history, and the same reasoning applies to the discovery of the DA.