During zazen, neither loathe nor be charmed by any of your thoughts. With your mind turned inward, look steadily into their source and the delusive feelings and perceptions in which they are rooted will evaporate.
This is not yet Self-realization, however, even though your mind becomes bright and empty like the sky, you have awareness of neither inner nor outer, and all the ten quarters seem clear and luminous. To take this for realization is to mistake a mirage for reality. Now even more intensely search this mind of yours which hears:
Your physical body, composed of the four basic elements, is like a phantom, without reality, yet apart from this body, there is no mind. The empty-space of ten quarters can neither see nor hear; still, something within you does hear and distinguish sounds. Who or what is it?
When this question totally ignites you, distinctions of good and evil, awareness of being or emptiness, vanish like a light extinguished on a dark night. Though you are no longer consciously aware of yourself, still you can hear and know you exist. Try as you will to discover the subject hearing, your efforts will fail and you will find yourself at an impasse.
All at once, your mind will burst into great enlightenment and you will feel as though you have risen from the dead, laughing loudly and clapping your hands in delight. Now for the first time, you will know that Mind itself is Buddha.
Were someone to ask, “What does one’s Buddha-mind look like?”
I would answer: “In the trees fish play, in the deep sea birds are flying.”
What does this mean? If you don’t understand it, look into your own Mind and ask yourself: “What is he, this master who sees and hears?”
Make the most of time: it waits for no one!
BASSUI’S LETTER TO LORD NAKAMURA, GOVERNOR OF AKI PROVINCE