One day Guishan was picking tea with Yangshan. Guishan said, "All day long I've heard your voice but haven't seen your form."
  Yangshan shook the tea bush. Guishan commented, "You know how it works but you don't know what it is."
  "You know what it is but you don't know how it works," Yangshan responded.
  "I spare you thirty blows of my staff," said Guishan.

-from Entangling Vines, Thomas Yuho Kirchner (trans)