Yuelin gave Wumen the koan, a spiritual question, of "Zhaozhou’s dog", with which Wu-men struggled for six years before he attained realization.
[1]: 4 Wumen compiled and commentated the 48-koan collection The Gateless Barrier when he was the head monk of Longxiang (Wade-Giles: Lung-hsiang; Japanese: Ryusho) monastery.
[3]: 6, n2 His teachings, as revealed in his comments in Gate of Emptiness, closely followed those of Dahui Zonggao (大慧宗杲; Wade-Giles: Ta-hui Tsung-kao; Japanese: Daei Sōkō) (1089–1163).
"[3]: xlii In his comment on Case 1, Zhaozhou's dog, he called mu (無) "a red-hot iron ball which you have gulped down and which you try to vomit up, but cannot".
Whatever activity a student proposed, Wumen rejected: "If you follow regulations, keeping the rules, you tie yourself without rope, but if you act any which way without inhibition you're a heretical demon.