Philosophy: Now I am really confused:

Sean Carroll (2017): Why Boltzmann Brains Are Bad: ‘The idea that we should reason as if we are typical observers… has been criticized by Hartle and Srednicki… [in an] argument is closely related to the Presumptuous Philosopher[s]…. [Imagine] there are… two scenarios in front of us, with equal priors: one in which humans are the only intelligent observers in the Solar System, the other of which over 99% of the intelligent observers are Jovians. HS point out that adopting the likelihoods (18), with the role of OOs and BBs replaced by humans and Jovians, respectively, would lead us to conclude that there was less than a one percent chance that the Jovians existed, even though we haven’t actually gone to look for them. That’s because, in the scenario where there are any Jovians at all, a typical intelligent observer in the Solar System is a Jovian, so the fact that we are human counts as strong evidence against their existence. Once again, we seem to have helped ourselves to very strong conclusions about the universe without looking at it… <arxiv.org/pdf/1702.00850>

Suppose you were in a dining room along with the 300 undergraduates of a residential college at a university, and suppose none of them were Computer Science majors. Then you would have a reasonably justified true belief that that university did not have a CS major—that people were, rather, EE, or EE&CS, or some such.

Or suppose you intercepted a spaceship from the solar system, and it turned out its crew were humans. Then that would count as evidence against the existence of Jovians, or at least against the existence of Jovians who could build rockets.

And, indeed, the fact that I see no Jovians anywhere on earth counts as strong evidence against the existence of Jovians (who can build rockets). The idea that we would have had to go “looking for them” before we can start interpreting the universe by taking account of background probabilities…

So I do not understand Hartle and Srednicki…