Was the interstellar object 'Oumuamua a light sail constructed by aliens (hypothesis A) or can it be explained by more natural processes (hypothesis N)? To compare these two hypotheses, a Bayesian analysis of the Sagan standard is introduced. I show that apparently contradictory answers are not contradictory when one is careful about the specific question one is trying to answer. Different estimates of prior odds play the dominant role in the controversy. The existence of technological alien civilizations is largely an issue of evolutionary biology, not astronomy. I argue that, based on tens of millions of independent evolutionary experiments here on Earth, the probability of technological alien civilizations is somewhere between zero and tiny. This extremely low prior decreases the probability of A being favored in the posterior odds, but counterintuitively increases the power of the new evidence to favor A.
Keywords: Bayesian analysis; Technological alien civilizations; ‘Oumuamua.