Porcupine came by Tallspruce one day to see if he could just hang out with Raven. He found her in consultation with Woodpecker, so he made himself scarce until Raven was free. Then he said, "You're an experienced master now. Do you have a general set of guidelines for your teaching of the Way?"*"What is Mu?" derives from the story that Master Zhaozhou was asked, "Has a dog Buddha nature?" and replied "Mu."
Raven said, "Don't give away the nest."
Porcupine said, "You seem to be saying that you can only give hints."
Raven said, "I don't hint. I say it directly."
Porcupine said, "OK, what is Mu?"*
Raven said, "Stillpond is an analogy, not a metaphor."
Porcupine said, "What's the metaphor?"
Raven said, "No longer a stranger to your inheritance."
Porcupine said, "Doesn't sound so direct to me.
Raven said, "OK, what is Mu?" (adapted from Robert Aitken, Zen Master Raven)