Caoshan's Dharma Body (BS52)

KOAN:

Caoshan asked Elder De, “’The true dharma-body of Buddha is like the empty sky. It manifests form in response to things—just like the moon on the water.’ How do you explain the principle of this response?”
De said, “It is like a donkey looking into a well.”
Caoshan said, “You put it in a nice way, but you were able to say only eighty percent.”
De said, “How about you, Master?”
Caoshan said, “It is like a well looking at a donkey.”

– Book of Serenity, Case 52