There is a well-known story of Hui-neng, a well-respected Buddhist monk who later became known as the founder of the Zen school, who one day happened to be passing by two monks.
"Two monks were arguing about the temple flag waving in the wind. One said, "The flag moves." The other said, "The wind moves." They argued back and forth but could not agree.
The Sixth Ancestor said, "Gentlemen! It is not the wind that moves; it is not the flag that moves; it is your mind that moves." The two monks were struck with awe."
- The Mumonkan Case 29, translation by Robert Aitken