What is the Sound of the Single Hand? When you clap together both hands a sharp sound is heard; when you raise the one hand there is neither sound nor smell.
Is this the High Heaven of which Confucius speaks? Or is it the essentials of what Yamamba describes in these words: “The echo of the completely empty valley bears tidings heard from the soundless sound?” This is something that can by no means be heard with the ear. If conceptions and discriminations are not mixed within it and it is quite apart from seeing, hearing, perceiving, and knowing, and if, while walking, standing, sitting, and reclining, you proceed straightforwardly without interruption in the study of this koan, you will suddenly pluck out the karmic root of birth and death and break down the cave of ignorance. Thus you will attain to a peace in which the phoenix has left the golden net and the crane has been set free of the basket. At this time the basis of mind, consciousness, and emotion is suddenly shattered; the realm of illusion with its endless sinking in the cycle of birth and death is overturned. The treasure accumulation of the Three Bodies and the Four Wisdoms is taken away, and the miraculous realms of the Six Supernatural Powers and Three Insights is transcended.
From p. 164, Yabukoji, in The Zen Master Hakuin: Selected Writings, Translated by Philip B. Yampolsky, Columbia University Press, New York and London, 1971.