I have argued flying saucers with lots of people. … I was interested in this: they keep arguing that it is possible. And that’s true. It is possible. They do not appreciate that the problem is not to demonstrate whether it’s possible or not, but whether it’s going on or not. Whether it’s probably occurring or not, not whether it could occur.
Richard Feynman (1918-1988) American physicist The Meaning of It All (1998)
(Source)