Drake’s equation (i.e. a decomposition of the probability that alien life must exist as a chain of conditional probabilities which sound more reasonable. After a beer or two. Or three) makes an interesting case for thinking about the nature of probabilities. What do we mean really when saying ‘those aliens must in all likelihood exist‘? No way it can point to a relative frequency. Maybe a propensity interpretation? But statements as ‘they tend to exist‘ make not much sense either. The only viable option – as far as I can see – is that evidence suggested by Drake should be in the eye of the beholder. Subjective. Belief-based. Bayes. In the good ol’ De Finetti sense. Earthlings and any alien civilisation far far away must be exchangeable. We are as curious for them as they for us. This seems a solid foundation for educated guessing about this.