This chapter develops the Carter-Leslie ‘doomsday argument’ outlined in the Introduction. It can be read in isolation, though, since it includes all the main points made earlier, much expanded. It also makes many further points in the argument’s defence. Several of them work smoothly only if our world is a deterministic world, or at least a world whose indeterminisms are unlikely to have much influence on how long the human race will last.
Prima facie, we should prefer theories which make our observations fairly much to be expected, rather than highly extraordinary. Waking up in the night, you form two theories. Each has a half-chance of being right, you estimate. The first, that you left the back door open, gives the chances as 10 per cent that the neighbour’s cat is in your bedroom. The second, that you shut the door, puts those chances at 0.01 per cent. You switch on the light and see the cat. You should now much prefer the first theory.
Consider next your observed position in time. If the human race is going to last for at least a few thousand more centuries at its present size, let alone at the much larger size to which it would grow if it spread through its galaxy, then you are very exceptionally early among all the humans who will ever have been born, perhaps among the earliest 0.01 per cent. But if the race is instead due to end shortly—which, when one thinks of the ozone layer,