Why Boltzmann Brains Are Bad
  Some modern cosmological models predict the appearance of Boltzmann Brains:
observers who randomly fluctuate out of a thermal bath rather than naturally
evolving from a low-entropy Big Bang. A theory in which most observers are of
the Boltzmann Brain type is generally thought to be unacceptable, although
opinions differ. I argue that such theories are indeed unacceptable: the real
problem is with fluctuations into observers who are locally identical to
ordinary observers, and their existence cannot be swept under the rug by a
choice of probability distributions over observers. The issue is not that the
existence of such observers is ruled out by data, but that the theories that
predict them are cognitively unstable: they cannot simultaneously be true and
justifiably believed.
