The “mu” kōan, often called “Jōshū’s Dog,” arises from a brief yet piercing exchange: a monk asks Master Jōshū whether a dog has Buddha‑nature, and Jōshū replies with a single word, “mu” (無) – “no,” “not,” or “without.” This response stands in deliberate tension with the Mahāyāna teaching that all beings possess Buddha‑nature, and thus it refuses to settle into a neat doctrinal answer. Rather than clarifying the issue conceptually, “mu” throws the questioner into a space where ordinary reasoning cannot gain a foothold. Under Hakuin Ekaku’s reform of Rinzai Zen, this kōan came to be treated as the fundamental or gateway kōan, the starting point for many students entering formal kōan training. In this way, “mu” functions less as a riddle to be solved and more as a doorway into a radically different mode of seeing.

In actual practice, the student is instructed to concentrate single‑mindedly on “mu,” allowing it to permeate body and mind until it is no longer just a word but an all‑encompassing question. This intense focus is meant to exhaust discursive thinking and to generate what the tradition calls “great doubt,” a profound existential questioning that cannot be appeased by clever interpretations or doctrinal explanations. As this doubt deepens, the habitual reliance on dualistic distinctions—yes and no, self and other, Buddha‑nature and its absence—begins to crack. The kōan thereby serves to shatter dependence on logical thought and conceptual understanding, pushing the practitioner toward a direct, non‑dual experience of reality that Rinzai Zen names kenshō, an initial awakening to one’s true nature.

Within this training, “mu” is both a meditation object and a testing ground. Through sustained zazen and repeated encounters with the teacher, the practitioner’s engagement with “mu” is examined as a measure of spiritual ripeness. The teacher looks not for an ingenious verbal answer, but for a response that reveals whether the student has genuinely broken through the net of discriminating mind. Over time, this single syllable becomes a crucible in which concentrated spiritual power is forged, as all energies are gathered into the living question it embodies. When realization does occur, it is said to resolve the apparent paradox of Buddha‑nature not by argument, but by revealing a dimension where such oppositions no longer hold sway.