A monk asked Joshu, a Chinese Zen master: `Has a dog Buddha-nature or not?'
Joshu answered: 'Mu.' [Mu is the negative symbol in Chinese, meaning `No-thing' or 'Nay'.]
Mumon's comments: To realize Zen one has to pass through the barrier of the patriachs. Enlightenment always comes after the road of thinking is blocked. If you do not pass the barrier of the patriachs or if your thinking road is not blocked, whatever you think, whatever you do, is like a tangling ghost. You may ask: What is a barrier of a patriach? This one word, Mu, is it.
This is the barrier of Zen. If you pass through it you will see Joshu face to face. Then you can work hand in hand with the whole line of patriachs. Is this not a pleasant thing to do?
If you want to pass this barrier, you must work through every bone in your body, through ever pore in your skin, filled with this question: What is Mu? and carry it day and night. Do not believe it is the common negative symbol meaning nothing. It is not nothingness, the opposite of existence. If you really want to pass this barrier, you should feel like drinking a hot iron ball that you can neither swallor nor spit out.
Then your previous lesser knowledge disappears. As a fruit ripening in season, your subjectivity and objectivity naturally become one. It is like a dumb man who has had a dream. He knows about it but cannot tell it.
When he enters this condition his ego-shell is crushed and he can shake the heaven and move the earth. He is like a great warrior with a sharp sword. If a Buddha stands in his way, he will cut him down; if a patriach offers him any obstacle, he will kill him; and he will be free in this way of birth and death. He can enter any world as if it were his own playground. I will tell you how to do this with this koan:
Just concentrate your whole energy into this Mu, and do not allow any discontinuation. When you enter this Mu and there is no discontinuation, your attainment will be as a candle burning and illuminating the whole universe.
Has a dog Buddha-nature?
This is the most serious question of all.
If you say yes or no,
You lose your own Buddha-nature.
(Koan from The Gateless Gate, by Mumon)
So ... what is nothing?