The ideogram “無”, transcribed “MU” in English has become much the same kind of hallmark-mantra of Zen as the sacred syllable “ॐ” (“ŌM”) has become for Hinduism.

This comes from a famous kōan, the first of the Mumonkan, telling of how master Jōshū answered to a monk who asked “has even a dog the Buddha-nature?”: Jōshū’s answer was “MU”.

We could waste tedious hours trying to explain this MU. Basically, the Chinese ideogram 無 means “no”, “not”, “none”, “have no”, “lack”, “not to exist”; this is a pretty vague semantic field, but it seems anyhow that Jōshū is not merely stating that a dog does not have the Buddha-nature: rather, his reply is more something like “nothing exists”, or, even more closely, a denial of the dualism of “is” and “is-not” (or “has” and “has-not”).

According to Mumon’s comment on this kōan, Jōshū’s MU is the very barrier set up by the patriarch, which to go through one must uproot all the normal working of the mind: it is the gateless gate (and the MU ideogram is the first ideogram in 無門關).

Now the Hackers, following Douglas Hofstadter, have made MU into something rather different (or, if not different, differently interpreted and less sacred). Namely, answering “mu” to a question means refusing to be trapped in the answers “yes” or “no” (which is certainly what Jōshū did, in refusing their dualism). In the words of Hofstadter, answering “mu” is to “unask” the question. Sometimes, as a pun, the Greek letter μ (mu) is used for this MU.”