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Mu (無) is a Japanese word meaning "null", "nothing", "emptiness", "vacuum", "void". It is used as a response to certain koans and other philosophical questions in Zen Buddhism, with the meaning, "Your question is inherently nonsensical and irrelevant."
When the famous Zen master Joshu was asked, "Does a dog have buddha nature?", he replied, "Mu."
Some earlier thinkers had maintained that creatures such as dogs did have the buddha nature; others, that they did not. Joshu sought to avoid this speculation by pointing out that it was a waste of time.
