On negative thinking :

When someone is convinced that things can’t be done, he will cling to that conviction in the face of the most obvious contradiction.  At the time that Robert Fulton gave the first public demonstration of his steamboat, one of those “can’t be done fellows stood in the crowd along the shore repeating, “he can’t start her.”

Suddenly there was a belch of steam and the boat began to move.  Startled, the man stared for a moment and then began to chant, “He can’t stop her.”