Muzhou Shuts the Door

KOAN:

Master Muzhou’s one room was usually shut, and it was completely empty. Yunmen went straight to Muzhou’s door and knocked. He does this twice and is turned away. The third time, Master Muzhou asked, “Who is it?”
Yunmen said, “It is me, Yunmen Wenyan.”
Muzhou blocked the entrance and said, “Why do you keep coming?”
Yunmen replied, “I am not clear about myself.”
Muzhou said, “Absolutely useless stuff!” then pushed Yunmen out and shut the door.
In this way Yunmen attained understanding.

—from the Record of Yunmen