"Sometimes I ain't so sho who's got him a right to say a man is crazy and when he ain't. Sometimes I think there ain't none of us pure crazy and ain't none of us pure sane until the balance of us talks him that-a-way. It's like it ain't so much what a fellow does, but it's the way the majority of folks is looking at him when he does it."
                                       ---  from As I Lay Dying, by William Faulkner

What does he mean?

"The Earth Men" from The Martian Chronicles by Ray Bradbury.
Think of a time from history when there was a clash of two very different cultures. What usually happens?
If someone unusual-looking came to your door claiming to be from another planet, what would you do? What would you need as proof?