A monk asked Zen master Zhaozhou, “Does a dog have buddha-nature?” Zhaozhou replied, “Mu!”
“Mu!” is one of a handful of Zen stories from ancient China that have become famous. This koan has served as the “Gateless Gate” into Zen for countless students in China, Japan, and elsewhere. Though it is often attributed to Zen master Zhaozhou (Japanese: “Joshu”), the story’s origins stretch further back into history. As I point out in my book Tracking Bodhidharma, there is some evidence that the story goes back to the nominal founder of Chinese Zen, the first ancestor Bodhidharma himself.
The connection between Bodhidharma and the "Mu!" koan can be found in an old Chinese ditty of unknown origin that goes