Kōan practice

At the beginning of the Song Dynasty, practice with the kōan method became popular, whereas others practiced "silent illumination."^[117] This became the source of some differences in practice between the Linji and Caodong traditions.
A kōan, literally "public case", is a story or dialogue, describing an interaction between a Zen master and a student. These anecdotes give a demonstration of the master's insight. Koans emphasize the non-conceptional insight that the Buddhist teachings are pointing to. Koans can be used to provoke the "great doubt", and test a student's progress in Zen practice.
Responding to a kōan requires a student to let go of conceptual thinking and of the logical way we order the world, so that insight into sunyata arises naturally and spontaneously in the mind. But this does not mean that words are useless, as is demonstrated already by the mere fact that koans are words:

[T]he way to Satori is not through dependence upon words, even if they be words of the Buddha or past Masters; however, one should not reject words, for, imperfect as they are, they are the only means we have of attaining enlightenment. They should use the words and ideas contained in the koans to reach satori, but they should never confuse the two. Conceptualizations, words, logic and reason are means whereby one attains enlightenment, but they must not be equated [w]ith enlightenment.^{[web 16]}

Kōans and their study developed in China within the context of the open questions and answers of teaching sessions conducted by the Chinese Zen masters. The recorded encounter dialogues, and the koan collections which derived from this genre, mark a shift from solitary practice to interaction between master and student:

The essence of enlightenment came to be identified with the interaction between masters and students. Whatever insight dhyana might bring, its verification was always interpersonal. In effect, enlightenment came to be understood not so much as an insight, but as a way of acting in the world with other people^[118]

This mutual enquiry of the meaning of the encounters of masters and students of the past gave students a role model:

One looked at the enlightened activities of one's lineal forebears in order to understand one's own identity [...] taking the role of the participants and engaging in their dialogues instead^[119][h]

Today, the Zen student's mastery of a given kōan is presented to the teacher in a private interview (referred to in Japanese as dokusan (独参), daisan (代参), or sanzen (参禅)). Zen teachers advise that the problem posed by a kōan is to be taken quite seriously, and to be approached as literally a matter of life and death. While there is no unique answer to a kōan, practitioners are expected to demonstrate their understanding of the kōan and of Zen through their responses. The teacher may approve or disapprove of the answer and guide the student in the right direction. There are also various commentaries on kōans, written by experienced teachers, that can serve as a guide. These commentaries are also of great value to modern scholarship on the subject. Kōan-inquiry may be practiced during sitting meditation (zazen), walking meditation (kinhin), and throughout all the activities of daily life. Kōan practice is particularly emphasized by the Japanese Rinzai school, but it also occurs in other schools or branches of Zen depending on the teaching line.