Koans are dialogues that stand at the center of Zen Buddhist literature and are often used to provoke the "great doubt" in testing a trainee's progress. The Mu Koan consists of a brief conversation in which a monk asks Master Zhaozhou whether or not a dog has Buddha-nature. According to the main version, the reply is "Mu" literally, "No," but implying the philosophical notion of nothingness. This case is widely considered to be the single best- known...