In perhaps the most famous koan of all time, a monk asks Chao-chou, a Chinese Chan master, “Does a dog have buddhanature or not?” Joshu’s reply, “Mu,” is not so easy to translate. It’s most often a prefix showing the absence of something, but it’s also the “emptiness” heard over and over in the Heart Sutra. Either way, it’s most definitely not “Yes!”
The real meaning of this mu has had Zen students spinning for centuries. A commentary by Wumen, a dharma descendant of Chao-chou, ends with this verse:
Has a dog buddhanature?
This is the most serious question of all.
If you say yes or no
you lose your own buddhanature.
