the sound of one hand clapping
A classic Zen kōan (公案) used in Rinzai Zen training. Refers to the paradoxical question 'What is the sound of one hand clapping?' (隻手の声を聞け).
See also: 公案
The kōan 'Hear the sound of one hand clapping' was popularized by Zen master Hakuin.
From 隻手 (one hand) + の (possessive particle) + 声 (sound). The phrase originates from the Zen kōan attributed to Hakuin Ekaku (白隠慧鶴).