The Gateless Gate, by Ekai, called Mu-mon, tr. Nyogen Senzaki and Paul Reps , at sacred-texts.com
Once when Hyakujo delivered some Zen lectures an old man attended them, unseen by the monks. At the end of each talk when the monks left so did he. But one day he remained after they had gone, and Hyakujo asked him: "Who are you?"
The old man replied: "I am not a human being, but I was a human being when the Kashapa Buddha preached in this world. I was a Zen master and lived on this mountain. At that time one of my students asked me whether or not the enlightened man is subject to the law of causation. I answered him: 'The enlightened man is not subject to the law of causation.' For this answer evidencing a clinging to absoluteness I became a fox for five hundred rebirths, and I am still a fox. Will you save me from this condition with your Zen words and let me get out of a fox's body? Now may I ask you: Is the enlightened man subject to the law of causation?"
Hyakujo said: "The enlightened man is one with the law of causation."
At the words of Hyakujo the old man was enlightened. "I am emancipated," he said, paying homage with a deep bow. "I am no more a fox, but I have to leave my body in my dwelling place behind this mountain. Please perform my funeral as a monk." Then he disappeared.
The next day Hyakujo gave an order through the chief monk to prepare to attend the funeral of a monk. "No one was sick in the infirmary," wondered the monks. "What does our teacher mean?"
After dinner Hyakujo led the monks out and around the mountain. In a cave, with his staff he poked out the corpse of an old fox and then performed the ceremony of cremation.
That evening Hyakujo gave a talk to the monks and told them this story about the law of causation.
Obaku, upon hearing the story, asked Hyakujo: "I understand that a long time ago because a certain person gave a wrong Zen answer he became a fox for five hundred rebirths. Now I want to ask: If some modern master is asked many questions and he always gives the right answer, what will become of him?"
Hyakujo said: "You come here near me and I will tell you."
Obaku went near Hyakujo and slapped the teacher's face with his hand, for he knew this was the answer his teacher intended to give him.
Hyakujo clapped his hands and laughed at this discernment. "I thought a Persian had a red beard," he said, "and now I know a Persian who has a red beard."
Mumon's comment: "The enlightened man is not subject." How can this answer make the monk a fox?
"The enlightened man is one with the law of causation." How can this answer make the fox emancipated?
To understand this clearly one has to have just one eye.