An explanation of the Koan answer
A philosophy Koan. Ummon seems to ask his monks to interpret the routine and ritual of their monastic life. To make it worse, whatever you say is as right as is the opposite.
Ummon wants us to understand Zen is not about discussions, but about reality. Put on a shirt, that's something real.
The background story of this Koan
Ummon asked: "The world is such a wide world, why do you answer a bell and do you put on ceremonial robes?"
The bell organises the daily life in a monastery. Everybody has to comply with it.
The (ceremonial) robes of Buddhist monks are sewn together out of seven long strips of sewn out fabric to show the monks selflessness.
Ummons question is: Why are these monks doing these kinds of stupid things instead of living a good life? But that's just a rhetorical question. Ummon knows very well, most monks submit themselves to the discipline of a monastery to become enlightened.
How stupid you are, Ummon says, challenging his monks "holy" behaviour.
Traditional Commentaries and .... Poems (Gata)
When one studies Zen one need not follow sound or colour or form. Even though some have attained insight when hearing a voice or seeing a colour or a form, this is a very common way.
It is not true Zen. The real Zen student controls sound, colour, 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.
Meeting Chan is greatly ordinary. In studying the Way be sure to avoid following sound and proceeding to colour.
Even if you use hearing sound to awaken to the Way or see colour to understand the Heart-mind, still this is ordinary.
Especially, don’t you know the patch-robed monk’s family rides sound and covers up in colour. The top of the head is transcends understanding; wearing appearances transcends the mysterious.
So, although you indeed listen, just say, does sound come to the ear’s boundary path or does the ear go toward the sound’s border frontier?
Even though there is only loudness and silence, forget both. Arriving here to listen, how do you speak of merging?
Going the way of having ear and hearing meet, it is difficult to merge. At the time you manage to hear sound with the eye, only then are you intimate.