Deshan's Enlightenment (GG28)

KOAN:

Deshan visited Longtan and questioned him sincerely far into the night. It grew late and Longtan said, “Why don’t you retire?” Deshan made his bows and lifted the blinds to withdraw, but was met by darkness.

Turning back he said, “It is dark outside.” Longtan lit a paper candle and handed it to Deshan. Deshan was about to take it when Longtan blew it out. At this, Deshan had sudden realization and made bows. Longtan said, “What truth did you discern?” Deshan said, “From now on I will not doubt the words of an old master who is renowned everywhere under the sun.”

—Gateless Gate, Case 28