"Makes sense. ...If another boat did arrive, it might be possible to prove it. If someone saw it and said they saw it, then you're done. But it's impossible to prove that such a boat `didn't arrive'. Even if everyone says that no such boat arrived--that they didn't see it arrive--you can't prove that it didn't `arrive secretly'. It's that thing they call a `#fefefeDevil's Proof<white>'."