Jap., lit., “public notice”; the Chinese kung-an originally meant a legal case constituting a precedent. In Zen a kōan is a phrase from a sūtra or teaching on Zen realization, an episode from the life of an ancient master, a mondō or a hossen—whatever the source, each points to the nature of ultimate reality. Essential to a kōan is paradox, i.e., that which is “beyond” (Gk., para) “thinking” (Gk., dokein), which transcends the logical or conceptual. Thus, since it cannot be solved by reason, a kōan is not a riddle. Solving a kōan requires a leap to another level of comprehension.~
Kōans have been used in Zen as a systematic means of training since around the middle of the 10th century. Since the kōan eludes solution by means of discursive understanding, it makes clear to students the limitations of thought and eventually forces them to transcend it in an intuitive leap, which takes them into a world beyond logical contradictions and dualistic modes of thought. On the basis of this experience, students can demonstrate their own solution of the kōan to the master in a dokusan spontaneously and without recourse to preconceived notions. The word or expression into which a kōan resolves itself when one struggles with it as a means of spiritual training is called the wato (Chin., hua-tou). It is the “punch line” of the kōan. In the famous kōan “Chao-chou, Dog,” for example, mu is the wato. Many longer kōans have several watos.
In general, kōan practice is associated with the Rinzai school, however kōans have also been used, both in China and Japan, in the Sōtō school. To begin with, kōan practice prevents a student from falling back after a first enlightenment experience into “everyman’s consciousness”; beyond that, it helps the student to deepen and extend his or her realization.