Bassui Tokusho, The Sermon On One Mind

 At work, at rest, never stop trying to realize who it is that hears. Even though your questioning becomes almost unconscious, you won't find the one who hears, and all your efforts will come to naught.

Yet sounds can be heard, so question yourself to an even profounder level. At last, every vestige of self-awareness will disappear and you will feel like a cloudless sky. Within yourself you will find no "I," nor will you discover anyone who hears. This Mind is like the void, yet it hasn't a single spot that can be called empty. This state is often mistaken for Self-realization, but continue to ask yourself even more intensely, " Now who is it that hears?"

If you bore and bore into this question, oblivious to anything else, even this feeling of voidness will vanish and you will be unaware of anything- total darkness will prevail. (Don't stop here, but) keep asking with all your strength, "What is it that hears?"

Only when you have completely exhausted the questoning will the question burst; now you will feel like a man come back from the dead. This is true realization. You will see the Buddhas of all the universes face to face and the Patriarchs past and present.