Southern Barbarians

KOAN:

Yaoshan said, “I understand the scriptures of Buddhism. But I hear that in the South, the Zen practitioners, the barbarians, directly point to the human mind.”
Shitou said, “They see their natures and become Buddhas.”
“This is still not clear to me. I humbly ask you to explain it.”
Shitou said, “This way won’t do, and not this way won’t do.”
He then said, “You ought to go see Mazu.”

Yaoshan asked Mazu, “They see their nature and become Buddhas?”
Mazu said, “Sometimes I make him raise his eyebrows and blink. Sometimes I do not make him raise his eyebrows and blink. Sometimes raising the eyebrows and blinking is alright. Sometimes raising the eyebrows and blinking is not alright. How about you?”
Hearing these words, Yaoshan was greatly awakened.
Mazu said, “What do you see that makes you bow?”
Yaoshan said, “When I was with Shitou, I was like a mosquito biting into an iron ox.”
Mazu said, “Since you’ve realized the truth, guard it well, but still, your master is Shitou.”

Yaoshan went back to Shitou. Then one day, there was a kind of true meeting:
Seeing Yaoshan sitting zazen, Shitou said, “What are you doing here?”
“I’m not doing anything at all,” said Yaoshan.
“In that case, you’re sitting idly,” said the teacher.
“If I were sitting idly, then I’d be doing something.”
Shitou said, “You say you’re not doing anything. What is this not doing?”
Yaoshan said, “Not even the ten thousand sages know.”