"What is the sound of one hand clapping?" is a koan from this school of Mahayana Buddhism
Zen sayings or koans belong to a school of Mahayana Buddhism (
though... [DOC] [TLE] Soto Zen and Koans  Empty Field Zendo [PAR] The Mahayana is characterized by its investigation of shunyata, equally 
translated as ... and the understanding of Buddha as an all pervading universal 
principle.