Speaker: Roger White

Abstract: Some apparently well-motivated cosmological models have it that the world is at some point teeming with Boltzmann Brains—randomly assembled clumps of matter floating in a void, some of which may just happen to be duplicates of my brain or yours. Indeed on these models there are so many that the vast majority brains that exist at some point in my exact current brain state are Boltzmann Brains. This appears to raise a skeptical worry with more bite than familiar Brain in Vat stories as it is no mere science fiction fantasy. If I believe that in the actual world most subjects in my phenomenal state are mere Boltzmann Brains mustn’t I conclude that I am most likely one of them? But once I draw this conclusion the whole basis for it falls apart. I can hardly accept a cosmological theory if I think that most likely my observational data are all a hallucination! Some argue that this “cognitive instability” is a reason to reject any such theory. I will argue that this is a mistake, and then explore a non-skeptical resolution of the puzzle