Two monks were watching a flag fluttering in the breeze at Tang-era Shaolin Monastery. One said, “The flag is moving.” The other argued, “No, it’s the wind that is moving.” They debated back and forth. Master Huineng (the 6th Patriarch, 638–713) happened to be passing by.
He interjected: “It is neither the wind nor the flag that moves. It is your mind that moves.” Hearing this, the two monks were struck with realization​
—source.