Philosophy: No, we are not Boltzmann Brains. But it would be nice to have a good reason to justify our conclusion that we are not Boltzmann Brains.

This “we cannot think coherently at all if we are Boltzmann’s Brains, and the Crawling Chaos is coming toward us from all directions at lightspeed” is indeed a thing, but not quite what we are really looking for.

Here Sean Carroll shifts Decartes’s “cogito” into a different key—it is not the fact that we can imagine perfection that can only have come from the existence of a perfect being, but rather that we must believe that we can think coherently about an external world, and Boltzmann Brains cannot do so:

Sean Carroll (2017): Why Boltzmann Brains Are Bad: ‘What we can do, however, is recognize that it’s no way to go through life…. In a randomly-fluctuating scenario, there’s no reason for this “knowledge” to have any correlation whatsoever with the world outside our immediate sensory reach…. Use our reasoning skills… to deduce that… we are probably randomly-fluctuated observers, even after conditioning on our local data… [and] also… that we then have no reason to trust those reasoning skills… [and] no justification for accepting your own reasoning…. In a universe dominated by Boltzmann fluctuations… we can’t trust anything we think we know… <arxiv.org/pdf/1702.00850>