In its most significant philosophical context, derived from Zen Buddhism, "mu" () functions as a noun representing a state of emptiness or nothingness that transcends conventional dualistic categories. Grammatically, when used as a direct response to a kan, it can be interpreted as an interjection. Its core meaning, however, is not a simple negation but an indication that the premise of a question is fundamentally flawed, thereby rendering a dualistic answer (such as 'yes' or 'no') invalid. It points to a voidness that is not a lack, but a dynamic and boundless potentiality.
The concept is famously illustrated in the first case of the kan collection The Gateless Gate (, Mumonkan). A monk asks the master Zhaozhou Congshen (Jsh Jshin), "Does a dog have Buddha-nature?" Zhaozhou replies, "Mu." This answer is not intended to mean "no" in the ordinary sense. Both a "yes" and a "no" would affirm the question's underlying assumptions and trap the student in conceptual thinking. "Mu" serves as a tool to dismantle this intellectual framework. It is a direct, experiential pointer challenging the student to abandon logical analysis and confront the nature of reality beyond ingrained categories of existence and non-existence, assertion and denial.
The practical application of "mu" is central to Rinzai Zen meditation practice. Students are often instructed to concentrate on this single syllable, not to intellectually analyze it, but to merge their consciousness with it until the discriminating mind collapses. The purpose is to exhaust the intellect and facilitate a breakthrough to a non-dualistic state of awareness, or kensh (enlightenment). In a broader sense, the concept has been adopted in fields like logic and computer science to signify an answer that un-asks the question, representing a state outside the defined logical system, similar to returning a NULL value for an invalid query.
