The Japanese and Korean term mu (Japanese: 無; Korean: 무) or Chinese wu (traditional Chinese: 無; simplified Chinese: 无), meaning "not have; without", is a key word in Buddhism, especially Zen traditions.

Old Chinese *ma 無 is cognate with the Proto-Tibeto-Burman *ma "not". This reconstructed root is widely represented in Tibeto-Burman languages; for instance, ma means "not" in both Written Tibetan and Written Burmese.^[1]

The Standard Chinese pronunciation of wú 無 "not; nothing" historically derives from (c. 7th century CE) Middle Chinese mju, (c. 3rd century CE) Late Han Chinese muɑ, and reconstructed (c. 6th century BCE) Old Chinese *ma.^[2]

Other varieties of Chinese have differing pronunciations of Chinese: 無. Compare Cantonese mou4; and Southern Min IPA: [bo˧˥] (Quanzhou) and IPA: [bə˧˥] (Zhangzhou).

The common Chinese word wú 無 was adopted in the Sino-Japanese, Sino-Korean, and Sino-Vietnamese vocabularies. The Japanese kanji 無 has on'yomi readings of mu or bu, and a kun'yomi (Japanese reading) of na. The Korean hanja 無 is read mu (in Revised, McCune–Reischauer, and Yale romanization systems). The Vietnamese Hán-Việt pronunciation is vô or mô.

In modern Chinese, Japanese and Korean it is commonly used in combination words as a prefix to indicate the absence of something, e.g., Chinese: 无线; pinyin: wúxiàn / musen (無線) / museon (무선 ) for "wireless".^[6] In Classical Chinese, it is an impersonal existential verb meaning "not have".^[7]

The same character is also used in Classical Chinese as a prohibitive particle, though in this case it is more properly written Chinese: 毋; pinyin: wú.^[8]

In traditional Chinese character classification, the uncommon class of phonetic loan characters involved borrowing the character for one word to write another near-homophone. For instance, the character 其 originally depicted a ji "winnowing basket", and scribes used it as a graphic loan for qi 其 "his; her; its", which resulted in a new character ji 箕 (clarified with the bamboo radical ⺮) to specify the basket.

The character wu 無 originally meant "dance" and was later used as a graphic loan for wu "not". The earliest graphs for 無 pictured a person with outstretched arms holding something (possibly sleeves, tassels, ornaments) and represented the word wu "dance; dancer". After wu 無 "dance" was borrowed as a loan for wu "not; without", the original meaning was elucidated with the 舛 "opposite feet" at the bottom of wu 舞 "dance".

The Gateless Gate, which is a 13th-century collection of Chan or Zen kōans, uses the word wu or mu in its title (Wumenguan or Mumonkan 無門關) and first kōan case ("Zhao Zhou's Dog" 趙州狗子). Chinese Chan calls the word mu 無 "the gate to enlightenment".^[9] The Japanese Rinzai school classifies the Mu Kōan as hosshin 発心 "resolve to attain enlightenment", that is, appropriate for beginners seeking kenshō "to see the Buddha-nature"'.^[10]

The koan originally comes from the Zhaozhou Zhenji Chanshi Yulu (Chinese: 趙州真際禪師語錄), The Recorded Sayings of Zen Master Zhao Zhou, koan 132:

The Book of Serenity Chinese: 從容録; pinyin: cóngrónglù, also known as the Book of Equanimity or more formally the Hóngzhì Chánshī Guǎnglù Chinese: 宏智禪師廣錄, has a longer version of this koan, which adds the following to the start of the version given in the Zhaozhou Zhenji Chanshi Yulu.

In the original text, the question is used as a conventional beginning to a question-and-answer exchange (mondo). The reference is to the Mahāyāna Mahāparinirvāṇa Sūtra^[14] which says for example:

In this light, the undisclosed store of the Tathagata is proclaimed: "All beings have the Buddha-Nature".^[15]

Koan 363 in the Zhaozhou Zhenji Chanshi Yulu shares the same beginning question.^[16]

This koan is one of several traditionally used by Rinzai school to initiate students into Zen study,^[3] and interpretations of it vary widely. Hakuun Yasutani of the Sanbo Kyodan maintained that

The koan is not about whether a dog does or does not have a Buddha-nature because everything is Buddha-nature, and either a positive or negative answer is absurd because there is no particular thing called Buddha-nature.^[17]

This koan is discussed in Part 1 of Hau Hoo's The Sound of the One Hand: 281 Zen Koans with Answers. In it, the answer of "negative", mu, is clarified as although all beings have potential Buddha-nature, beings who do not have the capacity to see it and develop it essentially do not have it. The purpose of this primary koan to a student is to free the mind from analytic thinking and into intuitive knowing. A student who understands the nature of his question would understand the importance of awareness of potential to begin developing it.^[18]

The Japanese scholar Iriya Yoshitaka [ja] made the following comment on the two versions of the koan:

I have held doubts for some time even with regard to the way the so-called "Chao-chou's Word No" has been previously dealt with. To the question "Does a dog have the Buddha-nature?", on the one hand Monk Chao-chou replied affirmatively, but on the other hand he replied negatively. However, Zen adherents in Japan have rendered the koan exclusively in terms of his negative response, and completely ignored the affirmative one. Moreover, it has been the custom from the outset to reject the affirmative response as superficial compared to the negative one. It seems that the Wu-men kuan is responsible for this peculiarity.^[19]

The common approach espoused [...] emphasizes a particular understanding of the role of the koan based on the “head-word” or “critical phrase” method developed by the prominent twelfth century Chinese master, Daie. This approach takes the “Mu” response in a non-literal way to express a transcendental negation that becomes the topic of an intensive contemplative experience, during which any and all thoughts or uses of reason and words are to be cut off and discarded for good rather than investigated for their expressive nuances and ramifications. Yet, historical studies demonstrate quite persuasively that an overemphasis on this single approach to one version of the kōan is somewhat misleading.^[20]

The term is often used or translated to mean that the question itself must be "unasked": no answer can exist in the terms provided. Zhaozhou's answer, which literally means that dogs do not have Buddha nature, has been interpreted by Robert Pirsig and Douglas Hofstadter to mean that such categorical thinking is a delusion, that yes and no are both correct and incorrect.

In Robert M. Pirsig's 1974 novel Zen and the Art of Motorcycle Maintenance, mu is translated as "no thing", saying that it meant "unask the question". He offered the example of a computer circuit using the binary numeral system, in effect using mu to represent high impedance:

For example, it's stated over and over again that computer circuits exhibit only two states, a voltage for "one" and a voltage for "zero." That's silly! Any computer-electronics technician knows otherwise. Try to find a voltage representing one or zero when the power is off! The circuits are in a mu state.^[21]

The word features prominently with a similar meaning in Douglas Hofstadter's 1979 book, Gödel, Escher, Bach. It is used fancifully in discussions of symbolic logic, particularly Gödel's incompleteness theorems, to indicate a question whose "answer" is to

"Mu" may be used similarly to "N/A" or "not applicable," a term often used to indicate the question cannot be answered because the conditions of the question do not match the reality. A layperson's example of this concept is often invoked by the loaded question "Have you stopped beating your wife?",^[23] to which "mu" would be the only respectable response.^[24]

Because of this meaning, programming language Raku uses "Mu" for the root of its type hierarchy.^[25]

In the Japanese manga series, Death Note, ends in a thematic conclusion which contains mu's usage as "not applicable": "All humans, without exception, will eventually die. The place they go after is MU (無, nothingness)".^[26]

Tsugumi Ohba, the writer of Death Note, explained in the postmortem follow-up volume that the meaning behind "MU after death" was that "the dead person should never come back to life, and it's cheating to revive dead characters as manga. So it became MU". Ohba further explained that "all humans die someday, and when we die, we can never come back to life, so let's do our best while we are at it" was the single most important theme Ohba wanted to express in writing the series.^[27]