Observer‐relative chances and the doomsday argument

Abstract

Suppose various observers are divided randomly into two groups, a large and a small. Not knowing into which group anyone has been sent, each can have strong grounds for believing in being in the large group, although recognizing that every observer in the other group has equally powerful reasons for thinking of this other group as the large one. Justified belief can therefore be observer‐relative in a rather paradoxical way. Appreciating this allows one to reject an intriguing new objection against Brandon Carter's ‘doomsday argument’. Carter encourages us to doubt that we are among only the first hundredth, say, or first millionth, of all humans who will ever have existed. He thereby reinforces whatever reasons we may have for suspecting that, unless we take great care, the human race will not survive long. Admittedly his argument is weakened if our world is indeterministic, so that there is no suitably guaranteed ‘fact of the matter’ of how many humans will ever have existed. But even then, it can caution us against believing that a lengthy future for humankind ‘is as good as determined’. Of all the objections the argument has yet faced, the new one is the most interesting.