kōan

In Zen Buddhism, a superficially nonsensical question or riddle used by a Zen master to help a pupil achieve satori (enlightenment). It is used in the Rinzai school of Zen.

A kōan supposedly cannot be understood through the processes of logic; its solution requires attainment of a higher level of insight. An often repeated example is ‘What is the sound of one hand clapping?’

An answer would be that the word ‘clapping’ by definition involves two hands, and therefore this particular kōan is unanswerable.