The following is from Paul Reps’s book Zen Flesh, Zen Bones: A Collection of Zen & Pre-Zen Writings. Over the last sixty years, I have read this book several times and regard it as a treasure. In fact, it was the second book I read after I moved to Los Angeles late in 1966. The quote consists of a zen koan followed by a commentary and quatrain from Ekai, also known as Mu-mon, written some time in the 13th century.

Two monks were arguing about a flag. One said: “The flag is moving.”

The other said: “The wind is moving.”

The sixth patriarch happened to be passing by. He told them: “Not the wind, not the flag; mind is moving.”

Mumon’s comment: The sixth patriarch said: “Not the wind, not the flag; mind is moving.” What did he mean? If you understand this intimately, you will see the two monks trying to buy iron and gaining gold. The sixth patriarch could not bear to see those two dull heads, so he made such a bargain.

Wind, flag, mind moves,
The same understanding,
When the mouth opens
All are wrong.