A monk once asked Joshu, “Has a dog the Buddha-nature?” Joshu answered, “Mu!” (Mu is the negative symbol in Chinese, meaning `No-thing’ or `Nay’.)
For the practical study of Zen, you must pass the barriers set up by the masters of Zen. The attainment of this mysterious illumination means cutting off the workings of the ordinary mind completely. If you have not done this and passed the barrier, you are a phantom among the undergrowth and weeds. Now what is this barrier? It is simply “Mu”, the Barrier of the gate of Zen and this is why it is called “The Gateless Barrier of the Zen sect.”
Those who have passed the barrier are able not only to have an intimate understanding of Joshu, but also of the whole historic line of Zen masters, to walk hand in hand with them, and to enter into the closest relation with them. You see everything with the same eye that they saw with, hear everything with the same ear. Is not this a blessed condition? Wouldn’t you like to pass this barrier? Then concentrate your whole body, with its three hundred and sixty bones and joints, and eighty four thousand hair-holes, into this question; day and night, without ceasing, hold it before you. But do not take it as nothingness, nor as the relative “not”, of “is” and “is not.” It must be like a red-hot iron ball which you have gulped down and which you try to vomit, but cannot.
All the useless knowledge, all the wrong things you have learned up to the present, –throw them away! After a certain period of time, this striving will come to fruition naturally, in a state of internal and external unity. As with a dumb man who has a dream, you will know it for yourself, and yourself only. Suddenly your whole activity is put into motion and you can astonish the heavens above and shake the earth beneath. You are just as if you had got hold of the great sword of Kan-u. You meet a Buddha? You kill him! A master of Zen? You kill him!
Though you stand on the brink of life and death, you have the Great Freedom. In the four modes of the six rebirths you are in a state of peace and truth. Once more, how are you to concentrate on this Mu? Every ounce of energy you have must be expended on it; and if you do not give up on the way, another torch of the law will be lighted.
The dog! The Buddha-Nature!
The perfect manifestation, the command of truth.
If, for a moment, you fall into relativity,
You are a dead man!
As translated by R.H. Blyth