I trust the words of my grandparents, who said that they and a number of other couoles saw a formation of what could only be UFOs, flying silently up the river Clyde in Scotland in 1938. They both descibed to me detail what sounded like impossible maneuvers for aircraft in those days, witnessed by everyone before even jets. My grandfather was a very enthusiastic collector of sci fi books after that, which I had the benefit of as a youngster.
Witness accounts are about as solid a proof as we could hope for, on whether or not extraterestrials exist.
It is actually infinitely easier to prove existence, than non existence of anything. All it needs is one confirmed demonstration, to prove existence of something, whereas the proof of it not existing requires that no such demo will ever happen, so by definition we could wait forever for proof of impossibilty.
Interesting choice of story, shame Drake recently passed away removing all chances of him contributing still to discussions, but thanks for posting.