A monk once asked master Chao-chou, "Does a dog have Buddha-nature or not?"

Chao-chou said, "Mu"

Chao-chou Ts'ung-shen (Zhaozhou, Japanese, Joshu) was a leading Ch'an/Zen master in China and a Dharma successor of Nan-chüan P'u-yüan (Japanese, Nansen Fugan). He had a profound experience of Enlightenment was he was eighteen, which simply indicated to him that there was a way worth pursuing further (i.e. Enlightenment is not an end, but a step on a path). After forty years of training with Nan-chüan, he wandered in China, seeking other Ch'an masters. At the age of eighty, he settled in Chao-chou, gathering pupils around him. He instructed gently and quietly, but in very sharp and short ways. Among his koans, his famous Mu koan is often given still to to Zen pupils as their first koan: 'A monk once asked master Chao-chou, "Does a dog have Buddha-nature [or, buddhata] or not?" Chao-chou said, "Wu" (Japanese, Mu).

Chao-chou was especially important in showing how Ch'an and Tao relate together, opening the way to creative co-existence. When he asked Nan-chüan about the Tao, Nan-chüan replied, 'Ordinary mind is Tao.' Chao-chou asked how he should move toward it. Nan-chüan answered, 'If you try to move toward it, you go away from it.' Chao-chou said, 'But if we do not try, how do we know that it is Tao?' Nan-chüan replied, 'Tao does not belong to "knowing" or "not-knowing": knowing is illusion, not-knowing is blank emptiness. If you really attain to the Tao of no-doubt, it is like the vast abyss, limitless, boundless. How, then, can there be a right and wrong in the Tao?' At these words, Chao-chou was suddenly Enlightened, and his Enlightenment is known as funi daido, 'the non-duality of the great Tao'---which is a near synonym for the buddha-nature empty of self, no-self Anatta and differentiation.

The character "MU" can be translated as "has not", "is without", "without", "lack of", "absence". 

In the following excerpt, Master Dogen Zenji explores various layers of meaning of the famous expression, recorded as a dialog  between Master Chao chou and one of his students.

A monk once asked master Chao-chou, "Does a dog have Buddha-nature or not?"

Chao-chou said, "Mu"

We should clarify the meaning of this question.

A dog is a dog. The question does not ask whether the Buddha-nature can or cannot exist in the dog; it asks whether even an iron man learns the truth. To happen upon such a poison hand may be a matter for deep regret, and at the same time the scene recalls the meeting, after thirty years, with half a sacred person.

Chao chou says, "It is without.” (MU)

When we hear this expression, there are concrete paths by which to learn it: the "being without” with which the Buddha-nature describes itself may  be expressed like this;  the "not having "  which describes the dog itself may be expressed like this; and "there is nothing," as exclaimed by an onlooker, may be expressed like this. There may come a day when this "being without" becomes merely the grinding away of a stone.  

The monk says, "All living beings totally have the Buddha-nature. Why is the dog without?" The intention here is as follows: "If all living beings did not exist, then the Buddha-nature would not exist and the dog would not exist. How about this point? Why should the dog's Buddha-nature depend on 'non-existence’.?"

Chao chou says, "Because it has Karmic consciousness.”

The intention of this expression is that even though the reason it exists is Karmic consciousness and to have Karmic consciousness is the reason it exists, the dog is without anything, and the Buddha-nature is without anything. Karmic consciousness never understands intellectually what the dog is, so how could the dog meet the Buddha-nature? Whether we cast away duality or take up both sides, the state is just the constant working of Karmic consciousness.

A monk asks Chao chou, "Does the Buddha –nature exist even in a dog or not?"

This question may be the fact that this monk is able to stand up to Chao chou. Thus, assertions and questions about the Buddha-nature are the everyday tea and meals of Buddhist patriarchs.

Chao chou says, "It exists."

The situation of this "It exists" is beyond the "existence" of scholastic commentary teachers and the like, and beyond the dogmatic "existence" of the Existence School. We should move ahead and learn the Buddha's Existence. The Buddha's Existence is Chao chou's "It exists." Chao chou's "it exists" is "the dog exists," and "the dog exists" is "the Buddha-nature exists."

The monk says, "It exists already—then why does it forcibly enter this concrete bag of skin?"

This monk's expression of the truth poses the question of whether it is present existence, whether it is past existence, or whether it is Existence already; and although Existence already resembles the other "existences" Existence already clearly stands alone. Does Existence already need to force its way in? Or does Existence already not need to force its way in? The action of forcibly entering this concrete bag of skin does not accommodate idle heedless consideration.

Chao chou says “Because it knowingly commits a deliberate violation”

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Fundamentally, our experience as experienced is not different from the Zen master's. Where
we differ is that we place a fog, a particular kind of conceptual overlay onto that experience
and then make an emotional investment in that overlay, taking it to be "real" in and of itself.

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REGARDING MU

MAJOR HISTORICAL BUDDHIST MASTERS, ZEN ANCESTORS AND ZEN PATRIARCHS

Bodhidharma, Hui'ko, Hui Shen, Hui Neng, Shih-t'ou Hsi-ch'ien, Zhaozhou, Moshan Liaoran,
Mugai Nyodai, Nagarjuna, Ganapati Muni, Kuan Yin, Tung-Shan, Te Shan, Dogen

Based on Master Dogen's SHOBOGENZO Book 2, "Bussho" translated by Gudo Nishijima & Chodo Cross, Windbell Publications