What Is the Sound of One Hand Clapping?

Q.  What is the sound of one hand clapping?

A.  According to the book The Sound of the One Hand: 281 Zen Koans With Answers, translated by Yoel Hoffman, the answer is an action, thrusting out one's hand, followed by a dialogue with the teacher.   According to the book, the dialogue might go something like this:

Master:  In clapping both hands a sound is heard: what is the sound of the one hand?
Student:  The pupil faces his master, takes a correct posture, and without a word, thrusts one hand forward.
Master:  If you've heard the sound of the one hand, prove it.
Student:  Without a word, the pupil thrusts one hand forward.
<<The pupil is not taken in by "prove it."  He evades explanations by simply implying "that's it.">>
Master:  It's said that if one hears the sound of the one hand, one becomes a Buddha.  Well then, how will you do it?
Student:  Without a word, the pupil thrusts one hand forward.
<<The pupil is not taken in by "enlightenment-non-enlightenment."  His answer implies "here, now.">>
Master:  After you've become ashes, how will you hear it?
Student:  Without a word, the pupil thrusts one hand forward.
Master:  What if the one hand is cut by the Suimo Sword (the sharpest of all swords)?
Student:  It can't be.
or
Student:  If it can, let me see you do it.
or
Student:  Without a word, the pupil thrusts one hand forward.
<<There is nothing to be cut.  Cut nothing and you still have nothing.>>
Master:  Why can't it cut the one hand?
Student:  Because the one hand pervades the universe.
Master:  Then show me something that contains the universe.
Student:  Without a word, the pupil thrusts one hand forward.
Master:  The before-birth-one hand, what is it like?
Student:  Without a word, the pupil thrusts one hand forward.
<<The pupil is not taken in by "life-death."  The notion of "before life" is artificial and can be entertained only while alive.>>
Master:  The Mt. Fuji-summit-one-hand, what is it like?
Student:  The pupil shading his eyes with one hand, takes the pose of looking down from the summit of Mt. Fuji and says, "What a splendid view."
Master:  Attach a quote to the Mt. Fuji-summit-one-hand.
Student:   (quote)  Floating clouds connected the sea and the mountain, And white flat plains spread into the states of Sei and Jo.
<<There is no need to speculate too much about the meaning of quotes.  The pupil responds to "summit" in a natural way by describing the view from the summit.>>
Master:  Did you hear the sound of the one hand from the back or from the front?
Student:  Extending one hand, the pupil repeatedly says, "Whether it's from the front or from the back, you can hear it as you please"
<<The question is a trap.  The master tests whether the pupil is taken in by the distinction or not.  The "sound of the one hand" is not to be located spatially.  Nevertheless it is not unrelated to space.>>
Master:  Now that you've heard the sound of the one hand, what are you going to do?
Student:  I'll pull weeds, scrub the floor, and if you're tired, give you a massage.
<<The student answers according to his own situation.>>
Master:  If it's a convenient thing, let me hear it too!
Student:  Without a word, the pupil slaps his master's face.
<<By slapping he implies that the master should not underestimate his understanding of the koan.>>
Master:  The one hand--how far will it reach?
Student:  The pupil places his hand on the floor and says, "This is how far it goes."
<<Another trap question.>>
Master:  The before-the-fifteenth-day-one-hand, the after-the-fifteenth-day-one-hand, what's it like?
Student:  The pupil extends his right hand and says, "This is the before-the-fifteenth-day-one-hand."  Extending his left hand he says, "This is the after-the fifteenth-day-one-hand."  Bringing his hands together he says, "This is the fifteenth-day-one-hand."
Master:  The sublime-sound-of-the-one-hand, what is it like?
Student:  The pupil immediately imitates the sound he happens to hear when sitting in front of his master.  That is, if it happens to be raining outside, he imitates the sound of rain, if at that moment a bird happens to call, he imitates a bird's call.
Master:  The soundless-voice-of-the-one-hand, what is it like?
Student:  Without a word, the pupil abruptly stands up, then sits down again, bowing in front of his master.
Master:  The true sphere of the one hand, what's it like?
Student:  "I take it to be as fleeting as a dream or phantom, or as something like an illusory flower.  That's how I think of it."
Master:  The source of the one hand, what is it?
Student:  "On the plain there is not the slightest breeze that stirs the smallest grain of sand.
                (quote)
                All communication with places north of the
                White Wolf River is disconnected,
                And south to the Red Phoenix City,
                autumn nights have grown so long."

        According to Hoffmann, this Koan was composed by a Master Hakuin in the 18th century.   It is the first Koan that a student is given when he enters a temple.  The Koan cannot be explained by logic and can take up to three years to solve.