I had a conversation about flying saucers 
some years ago with a layman
because I am scientific I know all about flying saucers!
So I said "I don't think there are flying saucers".
So my antagonist said,
"Is it impossible that there are flying saucers?
Can you prove that it's impossible?"
"No, I can't prove it's impossible.
It's just very unlikely".
At that he said, "You are very unscientific.
If you can't prove it impossible
then how can you say that it's unlikely?" 
But that is the way that is scientific. 
It is scientific only to say what is more likely and less likely,
and not to be proving all the time the possible and impossible.
To define what I mean, I finally said to him. "Listen
 I mean that from my knowledge 
of the world that I see around me,
I think that it is much more likely
that the reports of flying saucers
are the results of the known irrational
 characteristics of terrestrial intelligence
rather than of the unknown rational efforts
of extra-terrestrial intelligence."
