This is from The Gateless Barrier (Wu-Men Kuan), translated and with a Commentary by Robert Aitken.
A monk asked Chao-chou, “Has the dog Buddha nature or not?”
Chao-chou said, “Mu.”
For the practice of Zen it is imperative that you pass through the barrier set up by the Ancestral Teachers. For subtle realization it is of the utmost importance that you cut off the mind road. If you do not pass the barrier of the ancestors, if you do not cut off the mind road, then you are a ghost clinging to bushes and grasses.
What is the barrier of the ancestral Teachers? It is just this one word “Mu” – the one barrier of our faith. We call it the Gateless Barrier of the Zen tradition. When you pass through this barrier, you will not only interview Chao-chou intimately, you will walk hand in hand with all the Ancestral Teachers in the successive generations of our lineage – the hair of your eyebrows entangled with theirs, seeing with the same eyes, hearing with the same ears. Won’t that be fulfilling? Is there anyone who would not want to pass this barrier?
So, then, make your whole body a mass of doubt, and with your three hundred and sixty bones and joints and your eighty-four thousand hair follicles concentrate on this one word “Mu.” Day and night, keep digging into it. Don’t consider it to be nothingness. Don’t think in terms of “has” and “has not.” It is like swallowing a red-hot iron ball. You try to vomit it out, but you can’t.
Gradually you purify yourself, eliminating mistaken knowledge and attitudes you have held from the past. Inside and outside become one. You’re like a mute person who has had a dream–you know it for yourself alone.
Suddenly Mu breaks open. The heavens are astonished, the earth is shaken. It is as though you have snatched the great sword of General Kuan. When you meet the Buddha, you kill the Buddha. When you meet Bodhidharma, you kill Bodhidharma. At the very cliff edge of birth-and-death, you find the Great Freedom. In the Six Worlds and the Four Modes of Birth, you enjoy a samadhi of frolic and play.
How, then, should you work with it? Exhaust all you life energy on this one word “Mu.” If you do not falter, then it’s done! A single spark lights your Dharma candle.
Dog, buddha nature–
the full presentation of the whole;
with a bit of “has” or “has not”
body is lost, life is lost.