When Tozan was studying with Isan, he asked Isan about Chu Kukushi’s “Sermons by Insentient Creatures.”
Isan said, “Sermons by insentient creatures are given here for us too, but few can hear them.”

Do you hear the bamboos? These are the sermons referred to, sermons from insentient beings. Once you are silent, even roses start speaking to you.

[The wind is blowing and the bamboos start commenting with their creaking.]

Do you see? – the loud speaking of the bamboos. The deeper your silence, the louder you will be able to hear it.

Tozan said, “I am not yet certain about them, would you please teach me?”
Isan said nothing, but raised his stick straight up.
Tozan said, “I do not understand. Would you explain it to me?”
Isan said, “I would never tell you about this with the mouth given to me by my parents!”
This was his way of teaching.

This mouth, given by your parents is not capable of saying it; but the being is not given to you by your parents. You have come through them, they have been vehicles, but you are not part of them. Your body is made by your parents, the temple is raised by them, but the deity in the temple, the being, comes from eternity. It cannot come from mortal bodies.

Isan then suggested that Tozan visit Ungan who Tozan later succeeded. Coming up to Ungan, Tozan asked, “Who can hear the sermons of insentient creatures?”
“Insentient creatures can hear them,” answered Ungan.
“Why can I not hear them?” asked Tozan.
Ungan raised his staff straight up and said, “Do you hear?”
“No, I don’t,” answered Tozan.
Ungan said, “Don’t you know the sutra says, ‘Birds and trees, all meditate on the Buddha and the Dharma?’”

At this Tozan suddenly became enlightened.
He wrote the following verse:
Wonderful! How wonderful!
Sermons by insentient creatures;
you fail if you listen with your ears;

I repeat:

You fail if you listen with your ears.
Listening with your eyes, you hear them.