Mu is the name of a hypothetical lost continent, which is thought to have been located in the Pacific Ocean before it sank beneath the waters, similar to Atlantis and Lemuria, with which it is sometimes identified.

A monk once asked master Chao-chou, "Does a dog have Buddha-nature or not?"

Chao-chou said, "Mu"


At first the answer to the query posed by the monk seems obvious. A central tenet of Buddhist thought is the belief that all sentient beings have Buddha-nature, so to that extent that a dog is sentient, a dog has Buddha-nature. That this answer is so obvious suggest that this is not the response the monk is looking for: The question is not to be interpreted literally and responded to conceptually. In fact, rather than a straightforward question the utterance by the monk to Chao Chou constitutes a Koan

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KO-ANS
The word Koan or Ko-an comes from the Chinese term kung-an, literally "public notice," or "public announcement." There are reported to be some 1,700 Koans in all. There are two major collections of Koans, the first being the Pi-yen lu, that is, the "Blue Cliff Records," consisting of 100 Koans selected and commented on by Yüan-wu, in 1125. The second being the Wumenkuan, also known as the Mumonkan, a collection of 48 Koans compiled in 1228 by the Chinese priest Hui-k\'ai, also known as Wu-men. Both collections derived their Koans from the same orignal source, the much earlier Cheng-te ch\'uan-teng lu texts.
Basically a Koan is a paradoxical utterance used in Zen as a center of concentration in meditation. The paradoxical nature of Koans is essential to their function: The attempt to break down conceptual thought. Koans are constructed so that they do not succumb to conceptual analysis and thereby require a more direct response from the meditator. Interpreted in this way, the question is an appeal to Chao Chou to draw on his own insight into reality. This restricts the possible responses open to Chao Chou. An affirmative answer to the question would fail and Chao Chou could be accused of reliance on traditional teaching, rather than personal understanding. As such, it would fail to meet the challenge a Koan represent. Nevertheless, Chao Chou cannot answer "no" since this would be to deny scripture. This places Chao Chou in a perplexing situation. Both the ordinary conceptual responses are inappropriate: He cannot answer either yes or no. It is this inability to provide a satisfactory conceptual response that constitutes the paradoxical nature of the present Koan. Koans set up paradox situations like this in an attempt to provoke a non-conceptual response from meditators.


When we practice Koans, we often only deal with what is immediately provided by the translator. We rarely investigate other sources and dig below the surface. And there is always a lot more to a Koan, or any barrier for that matter, than first meets the eye.
Often, central parts of the ancient Koans were extracted from other sources. The masters who created Koan collections used source materials that were familiar to the people who were studying these Koans. They were presented within a known cultural and historical matrix. The teachers assumed that listeners had a grounding in basic principles of Buddhism and local folklore. For us, ten centuries later, the challenge is to uncover the full spectrum of the Koan, its breadth and depth.(source)

Chao Chou\'s response is to answer neither yes-nor-no: To answer Mu. Mu is not as unusual as it first seems. There are many everyday questions that we would not want to answer either yes or no to. Consider the question: "Have you stopped beating your dog yet?" Now it is notoriously easy to invent a situation in which either a positive or a negatively answer to this question is misleading. Either answer will mislead if I ask the question of a devoted animal lover, someone who would never mistreat any animal. If I was to demand a yes or no answer from an such a person they would be in a situation equally perplexing as Chao Chou\'s: any response they make will be misleading. A positive answer has the implication that the mis-treatment once took place and has now ceased. Whereas a negative reply implies that this non-existent mistreatment is still continuing.

The difficulty with answering this question for a pet lover is that the question itself set up a misleading picture of things. The question implies the existence of something that has never taken place and any response only seems to place one more firmly within that view of things. The correct response is to question the question: To ask for an alternative way of picture things. This is also implicit in the notion Mu. To answer a question with Mu (to say neither yes-nor-no) is to deny the validity of the question itself. The reason the answer is neither yes-nor-no is because the question sets up misleading categories, similar to Avyaakata in the sutras, that which do not apply to the situation being examined. Mu is a call for the question to be unasked. A call to look beyond the limiting conceptualisation implicit in the question. In fact, Mu is more extreme than this: It is a call to move beyond the limiting perspective of conceptualisation itself and to a directly contact with ultimate reality via pre-reflective awareness.