The Mumonkan (Wu-wen kuan; Gateless Gate) books of the Zen sect (無門関)

Mumonkan (written as 無門関 or 無門關 in Chinese characters) is a collection of koan (questions that a master gives to a person who seeks the secret of Zen to avoid deviating from the correct way) compiled by Mumon Ekai in the Sung period in China.

In Mumonkan, as many as 48 koan were selected by Mumon Ekai from various sources, with sho (words of praise) and annotations added to each of them. This is a text that is always used in Kanna Zen (koan-based Zen). In particular, the first koan, 狗子仏性 (趙州無字), is simply as follows, literally 'Is a dog provided with bussho (the Buddhist nature)?', to which the answer is 'None'. However, this is the first problem given to a Zen priest, and it is said to be so difficult that it takes at least three years to solve the problem.

Although having not attracted much attention in the medieval period, Mumonkan came to attract attention in the Edo Period. Even now its use is advocated strongly.