Fascinating frailty of the human mind

Can you please explain in other words "frality of the mind"?

Now, there's fascinating frailty of the human mind, that psychologists know all about. And it's called argument from ignorance. And this is how it goes. Ready? Somebody sees lights flashing in the sky. They've never seen it before. They don't understand what it is. They say: A UFO! The U stands for unidentified.