I think Hume's 'Of Miracles' (1748) provides a good framework here : https://en.wikipedia.org/wiki/Of_Miracles https://en.wikipedia.org/wiki/Of_Miracles Hume's

I think Hume's 'Of Miracles' (1748) provides a good framework here : https://en.wikipedia.org/wiki/Of_Miracles https://en.wikipedia.org/wiki/Of_Miracles Hume's argument against miracles is based on : 1) The uniformity of nature (the laws of nature are regular and unchanging) 2) The probability of testimony; based on the number and character of witnesses, and circumstances in which the miracle was witnessed. 3)A miracle is defined by Hume to be a violation of a law of nature. Not just very low likelihood events. (Though not explicitly a violation of the laws of nature themselves, Aliens visiting Earth would require violating the laws of nature as we currently understand them) So, Hume argues, the probability of a miracle occurring is very low and the probability of a false report of a miracle occurring is much higher. Therefore, it is more likely that a report of a miracle is false than that it is true in nearly all circumstances. I think this argument pretty much holds for Aliens and UFOs too.