Abstract

A Boltzmann brain is a randomly-formed configuration of matter that is conscious. According to some theories that cosmologists take seriously, the universe is so spatiotemporally large that it contains a great many Boltzmann brains that are duplicates of you. In the light of this it seems to follow that you should have significant confidence that you are a Boltzmann brain. What's worse, your situation seems to be "cognitively unstable": It seems unstable to end up confident that you are a Boltzmann brain because you should then think that your apparent cosmological evidence was randomly generated and hence that your confidence was unwarranted. But it also seems unstable to end up confident that you are not a Boltzmann brain because then you should follow your cosmological evidence to the conclusion that many Boltzmann brain duplicates of you exist, and hence that you are probably a Boltzmann brain. A case involving unreliable vision exhibits a similar threat of instability. A simple Bayesian model of that case, however, shows that the threat is an illusion. And a corresponding model suggests that the same goes for the threat of instability associated with Boltzmann brains.