"What is the sound of one hand clapping?" "Does a dog have Buddha-nature?" These cryptic expressions are among the best-known examples of koans, the confusing, often contradictory sayings that form the centerpiece of Zen Buddhist learning and training. Viewed as an ideal method for attaining and transmitting an unimpeded experience of enlightenment, they became the main object of study in Zen meditation, where their contemplation was meant to exhaust the capacity of the rational mind and the expressiveness of speech. Koan compilations, which include elegant poetic and eloquent prose...