Your analogy would more accurately be, "it's been dark all my life, so the sun's never going to rise again." The paper goes into this starting on page 12. Afte

Your analogy would more accurately be, "it's been dark all my life, so the sun's never going to rise again." The paper goes into this starting on page 12. After establishing estimates based on a variety of factors, they then adjust the results by pointing out that "there were no Type III Kardashev civilizations using more than 85% of the starlight in 10^5 surveyed galaxies" and that "the lack of observable settlement provides a strong bound on extraterrestrial intelligence". They proceed to offer approximations for Bayes conditional probabilities with different formulae for no stars having been observed to have civilizations, no civilizations within a detectable distance, and no settlements within nearby spacetime. After putting all of this together, they conclude in section 4.0 that, "The Fermi observation thus raises our 52% credence for being alone in the galaxy to somewhere between 53% and 99.6%, depending on the type of evidence considered. It likewise raises our 38% credence for being alone in the observable universe to somewhere between 39% and 85%." Essentially, they're saying that if the probability of detection is some value if there are intelligent civilizations in the observable universe, then how does it change the model that they haven't been observed? I am not a knowledgeable enough mathematician to judge whether this is an appropriate use of Bayes or not, but I would expect that someone would have already pointed it out if it weren't. They then conclude section 4.1 with, "...unless one assigns a very strong prior to [a systematic failure to detect observable civilizations], our qualitative conclusions still hold."