In Zen, “mu” (無) literally means “no,” “not,” or “without,” yet in practice it functions as far more than a simple negation. It is most famously encountered in the koan where a monk asks Zhaozhou (Joshu), “Does a dog have Buddha-nature?” and receives the single-word reply, “Mu.” Within the broader Mahayana context, where all beings are said to possess Buddha-nature, this answer is not a doctrinal denial but a deliberate shock to the mind caught in “has” and “has not.” “Mu” thus points beyond the habitual division of reality into opposites such as existence and non-existence, enlightened and unenlightened. It gestures toward emptiness (śūnyatā), the absence of any fixed, graspable essence in things, and exposes the limitations of language and logic in approaching ultimate truth.

In actual Zen training, “mu” is explored most directly through koan practice, especially in the Rinzai tradition. Often given as a first koan, it is not treated as a riddle to be solved by clever reasoning, but as a living question that must permeate one’s whole being. The practitioner may sit in meditation with the word “mu,” or with the question “What is mu?”, allowing conceptual thought to run its course until it exhausts itself. Rather than analyzing, the student is urged to become completely absorbed in “mu,” to the point of “becoming mu” rather than thinking about it from a distance. This total engagement can extend into walking, breathing, and daily activities, so that “mu” is carried into every moment as a cutting word that undercuts clinging to views and fixed ideas.

Through such sustained and wholehearted inquiry, the usual strategies of the discriminating mind begin to fall away. “Mu” serves as a wedge that pries open the tight grip of dualistic thinking, making room for a direct, non-conceptual realization that Zen describes as kenshō or satori. The aim is not an intellectual definition of “mu,” but an experiential insight into a reality that transcends all yes-and-no, all for-and-against. In this way, “mu” becomes both a symbol and a method: a single syllable that simultaneously negates attachment to concepts and points toward the original nature that cannot be captured by any concept at all.