Of the Mahayana shortcuts to enlightenment, the Zen traditions have, over time, gravitated toward two in particular: meditation (zazen) and koans. The Rinzai tradition, associated with koan practice, has taken an interesting approach, one that might seem uncharacteristic at first. If the Dharma transmission that first propagated Zen, with its emphasis on nonverbal expression, is the true embodiment of the Zen practices, a tradition encouraging verbal exercises might seem incongruous. However, koan practice is a verbal exercise designed to excise the rationalizing mind from the Buddha-mind, serving as a kind of meditative exercise. This leads to an experience of emptiness.

Koans are not “solved.” The application of logical reasoning is unhelpful and unnecessary for koan study. However, the response to a loan is important. Indeed, the answerer is expected to demonstrate an understanding of nonduality. Koan study might first elicit a reaction in student of discursive understanding. However, koan study should ultimately lead to the end of thought and a leap into a realm of pure intuition. The word or expression into which a koan expresses its essential insight is called a wato (in Japanese; hua-tou in Chinese). 

The three classic koan texts are the Blue Cliff Record, the Book of Equanimity, and the Gateless Gate. There are about 1,700 koans, although present-day Zen masters only use about 500 or 600.1

1Shambhala Dictionary of Buddhism and Zen, 1991, 117.