Koan (Zen riddles) (公案)

Koan can have either of the following meanings:
In China, koan means a document issued by governments between ancient and early-modern times. Protocols, records of trials and judicial precedents were included among them.

Connected to the above meaning, it came to mean a question given to a trainee in Buddhism in the Zen sect as one of the tasks to achieve enlightenment. The majority of these koan are considered to be muriewa (irrational dialogs). Generally, they are called 'Zen mondo' or Zen riddles. Famous koan are such as 'Sekishu no koe' (the sound of one hand), 'Kushibussho' (whether a dog have the Buddha-nature) and 'Soshi Seiraii' (why the Dharma came from the west). It is known generally that koan were used to control the religious system by the Edo bakufu (Japanese feudal government headed by a shogun) in ways such as that where a priest could not become chief priest unless he could solve a certain amount of koan, or that of koan being used to demonstrate the experience of a priest along with horo (the number of years from the time when a person became a priest and received the religious precepts). It was described in detail in an article about the early times of the split of the sect in section six of "Kurinohana" (flower of chestnuts) which was spoken by the machi-shu (towns people) of Miya-machi in Azuchi.

Principal collections of koan

"Mumonkan" (Wu-wen kuan: The Gateless Gate)
"Hekiganroku" (Pi-yen-lu: The Blue Cliff Record)
"Shoyoroku" (Ts'ung-jung lu: The Book of Serenity).

[Original Japanese]