## We still don’t know if there have been alien civilizations

Pedants (a group in which I have occasionally been included) often complain that nobody uses the phrase “beg the question” correctly anymore. It’s supposed to refer to the logical fallacy of circular reasoning — that is, of assuming the very conclusion for which you are arguing. Because the phrase is so often used to mean other things, you can’t use it in this traditional sense anymore, at least not if you want to be understood.

I’ve never found this to be a big problem, because the traditional meaning isn’t something I want to talk about very often. Until today.

The article headlined Yes, There Have Been Aliens in today’s New York Times is the purest example of question-begging I’ve seen in a long time. The central claim is that “we now have enough information to conclude that they [alien civilizations] almost certainly existed at some point in cosmic history.”

The authors use a stripped-down version of the Drake equation, which is the classic way to talk about the number of alien civilizations out there. The Drake equation gives the expected number of alien civilizations in our Galaxy in terms of a bunch of probabilities and related numbers, such as the fraction of all stars that have planets and the fraction of planets on which life evolves. Of course, we don’t know some of these numbers, particularly that last one, so we can’t draw robust conclusions.

The authors estimate that “unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first” such civilization. Based on this number, they conclude that ” the degree of pessimism required to doubt the existence, at some point in time, of an advanced extraterrestrial civilization borders on the irrational.”

Nonsense. It’s not the least bit irrational to believe that this probability is so low. We have precisely no evidence as to the value of the probability in question. Any conclusion you draw from this value is based solely on your prior (evidence-free) estimate of the probability.

I mean the phrase “evidence-free” in a precise Bayesian sense: All nonzero values of that probability are equally consistent with the world we observe around us, so no observation causes us to prefer any value over another.

They’d revoke my Bayesian card if I didn’t point out that there’s no problem with the fact that your conclusions depend on your prior probabilities. All probabilities do (with the possible exception of statements about pure mathematics and logic). But it’s absurd to say that it’s “irrational” to believe that the probability is below a certain value, when your assessment of that probability is determined entirely by your prior beliefs, with no contribution from actual evidence.

This sort of argument is occasionally known as “proof by Goldberger’s method“:

The proof is by the method of reductio ad asburdum. Suppose the result is false. Why, that’s absurd! QED.