The Gateless Gate (Wumenguan)

A collection of 48 koans compiled by the Chinese Chan master Wumen Huikai in 1228 and published the following year. Each case presents a short exchange or story about earlier Chan masters, followed by Wumen's brief comment and a short verse. The first and most famous case is Zhaozhou's dog: asked whether a dog has Buddha-nature, Zhaozhou answers 'Wu' ('no' or 'nothing'), and Wumen urges students to concentrate on this one word with their whole being until their doubt breaks open. Other well-known cases include Baizhang and the fox, Juzhi raising one finger, the Buddha holding up a flower and Mahākāśyapa smiling, Nanquan cutting the cat, 'ordinary mind is the Way', and 'not the wind, not the flag'. The aim is not to solve the koans as riddles but to use them to break through habitual thinking. In Japan, where it is called the Mumonkan, it became a basic text of Rinzai Zen practice.

It is the shortest and most widely used koan collection, and it shows how Zen tries to bring about insight through paradox and concentrated doubt rather than argument.