"Hakuin Ekaku, Self-portrait, ink on silk, 1764. Hakuin was one of the most important figures in Japanese Zen Buddhism. He revitalized and redefined Rinzai, one the three main schools of Japanese Zen Buddhism. He established meditation on a koan as a practice at the heart of Zen teaching. He also invented new koans, including the best-known of them all: "What is the sound of one hand clapping?""