無, the koan's answer, carries an attested English reading, the word “no”. Blyth prints the bare Japanese sound “Mu”, discards the parenthetical “(No)”, and then construes the sound as the very nothingness he himself translated the koan as forbidding.

The confrontation

R. H. Blyth

Joshu answered “Mu!” (No).

Zen and Zen Classics, Volume 4: Mumonkan, Hokuseido Press, Tokyo, 1966, p. 22

The canonical source

趙州和尚因僧問。狗子還有佛性。也無。州云無

A monk asked Zhaozhou: does a dog have the Buddha-nature, or not? Zhaozhou said: “no” (無).

Zhaozhou's dog (趙州狗子) · ReadZen · T48n2005 0292c23

狗子 a dog 佛性 Buddha-nature no / without

And where he construes the word, it leaves the band further

the state of Mu, the blessed state of the poor in spirit. It is a condition of the soul which we see unmistakably in Buddha and Christ. (p. 30)
The dog is Mu, the Buddha-Nature is Mu, No-dog is Mu, the devil-nature is Mu, all is Mu! (p. 38)

The dictionary's band

Attested band verbatim from the dictionary entry 無字

  • the word 'no'
  • Zhaozhou's 'no'
  • the 'no' case

Entry Explanation (verbatim)

…taken up as the object of inquiry, not bare negation or wordlessness … explicitly rejecting both nihilistic void and ordinary has-versus-has-not interpretation.

KWIC [Wumen Huikai, T48n2005]

參箇無字。晝夜提撕。莫作虛無會。莫作有無會。

Blyth knew better

Blyth knew better

But do not take it as nothingness, nor as the relative “not”, of “is” and “is not.”

Zen and Zen Classics, Volume 4: Mumonkan, p. 32

This is Blyth's own faithful rendering of the entry's occurrence-line, 莫作虛無會 and 莫作有無會. He translates the prohibition on page 32, which sits between the two construals, and construes 無 as exactly the nothingness it forbids on either side of it, pages 30 and 38. The self-conviction is on the page: he prints the rule, then breaks it on both sides. And there is a second self-conviction across his own oeuvre. A quarter-century earlier, in Zen in English Literature and Oriental Classics (1942), he rendered the very same answer into plain English: “A monk said to Joshu, ‘Has this dog the Buddha-nature or not?’ Joshu replied ‘No!’” (p.23). He had the English word in his own hand in 1942, and by the 1966 Mumonkan he left it the Japanese sound “Mu.” The strongest knew-better on the site is the same translator, the same koan, the plain “No!” become “Mu.”

The literal reading

無 is Zhaozhou's answer to whether a dog has the Buddha-nature. The dictionary attests it as an English word, the word “no”: a flat negation, taken up by Wumen as the object of inquiry (參箇無字), to be worn day and night. The entry's own occurrence-line forbids two readings by name: 莫作虛無會, do not make it a nothingness; 莫作有無會, do not make it a matter of has against has-not.

The alteration

Blyth leaves the answer as the bare Japanese sound “Mu.” Three things follow. The parenthetical “(No)”, the dictionary's attested reading, is dropped as the rendering, and the koan's answer is never re-Englished; an untranslated transliteration, where the entry attests an English word, is outside the band. The cipher is the Japanese sound of a Chinese graph in a Chinese text, so it is load-bearing, construal-level, not mere apparatus. And he then mystifies the sound into “all is Mu,” a blessed state read into 無 that the entry's own KWIC forbids by name. The koan the whole book is named after is delivered as a Japanese cipher and then filled with the mystical void it was built to refuse.