(Yakusan Igen) 751-834
Yaoshan Waiyan was first ordained by a Vinaya master. However, he tired of the focus of the Vinaya on rules and sought out a new teacher, finding Shitou Xiqian. According to tradition, Shitou sent him to Mazu Daoyi (709-788) for instruction where he obtained enlightenment.
This story about Yaoshan is told by Dōgen in the Zazenshin fascicle of Shōbōgenzō (The Treasury of the True Dharma Eye):
When Great Master Yakusan is sitting, a monk asks him “What are you thinking of in the still-still state?”
The Master says, “Thinking the concrete state of not thinking.”
The monk says, “How can the state of not thinking be thought.”
The Master says, “It is non-thinking.”