Mu (kōan)
無 (mu)
Jōshū’s *Mu*—無, “no / not / nothing”—is the answer the Chinese master Zhaozhou (Jap. Jōshū) gives the monk who asks whether a dog has *buddha-nature*. The first of the forty-eight cases in the *Mumonkan* (“The Gateless Barrier”), it is the pivotal *kōan* of Zen: not a riddle to solve, but a wall raised before discursive thought. “Has” and “has not” shatter equally against it; *Mu* is neither affirmation nor negation, but what exhausts the mind that would decide, until the breakthrough.
A monk in all seriousness asked Joshu: “Has a dog Buddha-nature or not?” Joshu retorted “Mu!”
The first case of the Mumonkan consists of two sentences: “A monk in all seriousness asked Joshu: ‘Has a dog Buddha-nature or not?’ Joshu retorted, ‘Mu!’” — A monk earnestly asks Jōshū: “Does a dog have Buddha-nature, or not?” Jōshū replies: “Mu!” The character 無 literally means “no,” “not-having,” “nothing.” But this is not the answer to the question.
For the question is a trap. The Nirvāṇa Sūtra teaches that every sentient being—human, animal, plant—possesses Buddha-nature. If Jōshū answers “it has it,” he recites catechism; if he answers “it does not,” he contradicts the Dharma. The expositor Yasutani-roshi, in his commentary (teisho) quoted by Philip Kapleau, puts it bluntly: whatever the answer, “either answer would be absurd,” for “everything being Buddha-nature,” to decide yes or no is to miss the point. Thus, Mu has “nothing to do with the existence or non-existence of Buddha-nature but is itself Buddha-nature”: it does not concern the presence or absence of Buddha-nature—it is Buddha-nature.
Hence Mumon (Wu-men), compiler of the collection, warns (as cited by Kapleau): “Do not construe Mu as nothingness and do not conceive it in terms of existence or non-existence.” Do not reduce Mu to void, nor think of it as being or non-being. It must be borne “like a red-hot iron ball one can neither swallow nor spit out” until inside and outside become one. This is the barrier: “It is Mu, the one barrier of the supreme teaching.” A kōan which, unlike the sound of one hand, neither stirs the intellect nor the imagination—it “stands coldly apart” from both. Reason finds no purchase on it; it wears itself out in vain, and it is in this deadlock that the breakthrough or awakening may open. Mu works the same nerve as no-mind: it forces the mind to cease grasping itself through discourse.
Mu-kōan ≠ Taoist wu. Same character—無—but a different operation. Laozi’s wu is a thesis on being: the fertile void, the hub’s hollow from which function arises, the absence that gives rise to use. Jōshū’s Mu asserts nothing about reality; it is a wall thrown up before thought that would seek to conclude.
Mu-kōan ≠ logical riddle. A riddle has a solution that sharper reasoning eventually reaches. Mu is “utterly impervious to logic and reason”: it has no answer to uncover; it is designed for discursive thought to exhaust itself and let go.
Mu-kōan ≠ nihilistic negation. Mu is not a “nothing” that cancels, nor a declaration that the dog lacks Buddha-nature—Mumon explicitly warns against taking it for void. It does not erase the world: it strips Buddha-nature bare by ceasing to treat it as “a particular thing” one either has or lacks. Here, it closely resembles emptiness, which is not nothingness but the absence of inherent nature.