The Gateless Gate, by Ekai, called Mu-mon, tr. Nyogen Senzaki and Paul Reps , at sacred-texts.com
Ummon asked: "The world is such a wide world, why do you answer a bell and don ceremonial robes?"
Mumon's comment: When one studies Zen one need not follow sound or color or form. Even though some have attained insight when hearing a voice or seeing a color or a form, this is a very common way. It is not true Zen. The real Zen student controls sound, color, form, and actualizes the truth in his everyday life.
Sound comes to the ear, the ear goes to sound. When you blot out sound and sense, what do you understand? While listening with ears one never can understand. To understand intimately one should see sound.