Koan

A kōan (公案; ; Korean: 화두; công án) is a story, dialogue, question, or statement from the Chinese Chan-lore, supplemented with commentaries, that is used in Zen Buddhist practice in different ways. The main goal of kōan practice in Zen is to achieve kenshō ("seeing one's true nature").

Extended study of kōan literature as well as meditation (zazen) on a kōan is a major feature of modern Rinzai Zen. They are also studied in the Sōtō school of Zen to a lesser extent. In Chinese Chan and Korean Seon, meditating on a huatou, a key phrase of a kōan, is also major Zen meditation method.

Etymology
The Japanese term kōan is the Sino-Japanese reading of the Chinese word gong'an. The term is a compound word, consisting of the characters 公 "public; official; governmental; common; collective; fair; equitable" and 案 "table; desk; (law) case; record; file; plan; proposal."

According to the Yuan dynasty Zen master Zhongfeng Mingben (中峰明本 1263–1323), gōng'àn originated as an abbreviation of gōngfǔ zhī àndú (公府之案牘, Japanese kōfu no antoku—literally the àndú "official correspondence; documents; files" of a gōngfǔ "government post"), which referred to a "public record" or the "case records of a public law court" in Tang dynasty China. Kōan/gong'an thus serves as a metaphor for principles of reality beyond the private opinion of one person, and a teacher may test the student's ability to recognize and understand that principle.

Commentaries in kōan collections bear some similarity to judicial decisions that cite and sometimes modify precedents. An article by T. Griffith Foulk claims "...Its literal meaning is the 'table' or 'bench' an of a 'magistrate' or 'judge' kung."

Gong'an was itself originally a metonym—an article of furniture involved in setting legal precedents came to stand for such precedents. For example, Di Gong'an (狄公案) is the original title of Celebrated Cases of Judge Dee, the famous Chinese detective novel based on a historical Tang dynasty judge. Similarly, Zen kōan collections are public records of the notable sayings and actions of Zen masters and disciples attempting to pass on their teachings.

Doctrinal background
The popular western understanding sees kōan as referring to an unanswerable question or a meaningless or absurd statement. However, in Zen practice, a kōan is not meaningless, and not a riddle or a puzzle. Teachers do expect students to present an appropriate response when asked about a kōan. According to Hori, a central theme of many koans is the 'identity of opposites':

"[K]oan after koan explores the theme of nonduality. Hakuin's well-known koan, 'Two hands clap and there is a sound, what is the sound of one hand?' is clearly about two and one. The koan asks, you know what duality is, now what is nonduality? In 'What is your original face before your mother and father were born?' the phrase 'father and mother' alludes to duality. This is obvious to someone versed in the Chinese tradition, where so much philosophical thought is presented in the imagery of paired opposites. The phrase 'your original face' alludes to the original nonduality."

Comparable statements are: "Look at the flower and the flower also looks"; "Guest and host interchange". Koans are also understood as pointers to an unmediated "Pure Consciousness", devoid of cognitive activity. Victor Hori criticizes this understanding: "[A] pure consciousness without concepts, if there could be such a thing, would be a booming, buzzing confusion, a sensory field of flashes of light, unidentifiable sounds, ambiguous shapes, color patches without significance. This is not the consciousness of the enlightened Zen master."

The "public cases" of the old masters
Gōng'àn literature developed at some point in between the late Tang dynasty (10th century) to the Song dynasty (960-1279), though the details are unclear. They arose out of the collections of the recorded sayings of Chán masters and "transmission" texts like the Transmission of the Lamp. These sources contained numerous stories which contain many stories of famous past Chán masters which were used to educate Zen students. According to Morten Schlütter "it is not clear exactly when the practice of commenting on old gongan cases started, but the earliest Chan masters to have such commentaries included in the recorded sayings attributed to them appear to be Yunmen Wenyan and Fenyang Shanzhao (947–1024)."

According to Robert Buswell, the gōng'àn tradition “can be viewed as the products of an internal dynamic within Chan that began in the t’ang and climaxed in the Sung.” By the beginning of the Song era, Chan masters were known to use these stories in their sermons, as well as to comment on them and to use them to challenge their students.

Schlütter also writes: "much of the material in the recorded sayings collections of individual Song Chan masters consists of the master quoting (“raising”; ju) a story about a famous past Chan figure’s encounter with disciples or other interlocutors and then offering his own comments on it. the stories held up for comment came to be referred to as gongan, “public cases,” or guze, “old model cases,” both terms borrowed, it would seem, from the language of law."Originally, such a story was only considered a "gōng'àn" when it was commented upon by another Chán master, i.e. when it was used as a "case" study for enlightenment. This practice of commenting on the words and deeds of past masters also served to confirm the master's position as an awakened master in a lineage of awakened masters of the past.

According to Schlütter, these stories were also used "to challenge Chan students to demonstrate their insights: a Chan master would cite a story about a famous master and then demand that his students comment." Later on, certain questions (like: “Why did Bodhidharma come from the West?”) developed independently from the traditional stories and were used in the same fashion. Schlütter also notes that "most commonly used gongan in the Song originally came from the influential Transmission of the Lamp, although the subsequent transmission histories also became sources of gongan."

Over time, a whole literary genre of gōng'àn collection and commentary developed which was influenced by "educated literati" of the Song era. These collections included quotations of encounter-dialogue passages (the "cases", gōng'àns) with a master's comment on the case attached. When a prose comment was added, the genre was called niangu ("picking up the old ones"), and when poems were used to comment, the genre was termed songgu ("eulogizing the old ones"). Further commentaries would then be written by later figures on these initial comments, leading to quite complex and layered texts.

The style of these Song era Zen texts was influenced by many Chinese literary conventions and the style of "literary games" (competitions involving improvised poetry). Common literary devices included: "#The extensive use of allusions, which create a feeling of disconnection with the main theme;
 * 1) Indirect references, such as titling a poem with one topic and composing a verse that seems on the surface to be totally unrelated;
 * 2) Inventive wordplay based on the fact that hanzi (Chinese characters) are homophonic and convey multiple, often complementary or contradictory meanings;
 * 3) Linking the verses in a sustained string based on hidden points of connection or continuity, such as seasonal imagery or references to myths and legends."There were dangers involved in such a highly literary approach, such as ascribing specific meanings to the cases, or become too involved in book learning. Dahui Zonggao is even said to have burned the woodblocks of the Blue Cliff Record, for the hindrance it had become to study of Chán by his students.

"Observing the phrase"
During the late Song (11th - 12th century) the practice of assigning specific gōng'àns to students for contemplation had become quite common and some sources contain examples of Zen masters (e.g. Touzi Yiqing) who became enlightened through contemplating a gōng'àn. Thus, by the time of Dahui Zonggao (1089–1163), this practice was well established. Dahui promoted and popularized the practice extensively, under the name of "observing the phrase zen" (kanhua chan). In this practice, students were to observe (kan) or concentrate on a single word or phrase (huatou), such as the famous mu of the mu-kōan, and develop a sense of "great doubt" within until this ball of doubt "shattered", leading to enlightenment. Dahui's invention was aimed at balancing the insight developed by reflection on the teachings with developing samatha, calmness of mind.

This idea of observing a key phrase or word was Dahui's unique contribution, since the earlier method of gongan contemplation never taught the focusing on a single word, nor did it teach to develop a "ball of doubt that builds up before finally shattering." According to Wright, instead of focusing on the full narrative of a koan, Dahui promoted "intense focus on one critical phrase, generally one word or element at the climax of the koan."

Dahui also taught that meditation on just one huatou of a single gongan was enough to achieve enlightenment, since penetrating one gongan was penetrating into all of them. He went even further, arguing that this new meditation technique was the only way of achieving enlightenment for Chan practitioners of his day. Thus, Schlütter writes that "in this insistence, he was unusual among the Song Chan masters, who generally tended to take a rather inclusive view of Buddhist practice. it is therefore fair to say that Dahui not only developed a new contemplative technique, he also invented a whole new kind of Chan in the process." Whatever the case, Dahui was extremely influential in shaping the development of the Linji school in the Song.

Dale S. Wright also writes that Dahui:

maintained that the hua-t'ou had no meaning and that any intellectualization, any conceptual thinking at all, would obstruct the possibility of break- through. As a corollary to this, Ta-hui warned that the intellectuals who in his day were the ones most interested in koan meditation would be the least likely to succeed at it, given their tendency to think. His advice to them, therefore, was to cease completely any effort to resolve the koan and "to give up the conceit that they have the intellectual tools that would allow them to understand it." The primary effort required in this enterprise was a negative one, "nonconceptualization,"... As Robert Buswell explains, this emphasis on non-conceptual meditation on a gongan meant that "there is nothing that need be developed; all the student must do is simply renounce both the hope that there is something that can be achieved through the practice as well as the conceit that he will achieve that result."

Wright argues that since "the narrative structure of the koan was eliminated in the focus on a single point", that is the hua-t'ou (which was said to have no meaning), such a practice became a samatha-like zazen practice (which even resembles Caodong silent illumination), even if this was never acknowledged by the masters of the Linji school in the Song. Furthermore, Wright also argues that this practice was anti-intellectual since all learning was to be renounced in the practice of kanhua chan. According to Wright, this development left Chinese Chán vulnerable to criticisms by a resurgent neo-Confucianism.

The Chan master's role
According to Kasulis, the rise of gōng'àn contemplation in Song era Zen led to a greater emphasis on the interaction between master and student, which came to be identified as the essence of enlightenment, since "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."

This mutual inquiry of past cases gave Zen students a role model and a sense of belonging to a spiritual family since "one looked at the enlightened activities of one's lineal forebears in order to understand one's own identity." The practice also served to confirm an individual's enlightenment and authority in a specific lineage or school. This formal authorization or confirmation (印可, Ch: yìn kě, Jp: inka, K: inga) was given by their teacher and was often part of a process of "dharma transmission" (Chinese: 傳法 chuán fǎ) in a specific lineage. This formal act placed the "confirmed" Chan master in a special unique position as an interpreter and guide to the gōng'àns.

The importance of the teacher student relationship is seen in modern Japanese kōan training which always requires an authorized teacher (rōshi or oshō) in a specific lineage who has the ability to judge a disciple's understanding and expression of a gōng'àn. In the Rinzai Zen school, which uses kōans extensively, the teacher certification process includes an appraisal of proficiency in using that school's extensive kōan curriculum. According to Barbara O'Brien, the practice of going to a private interview with one's Zen master (sanzen) where one has to prove one's understanding of kōans "is the real point of the whole exercise".

Chinese gōng'àn collections
Some of the key Song era gōng'àn collections are:

These texts mostly draw and develop stories which are found in other sources, mainly the Anthology of the Patriarchal Hall (Chinese Zǔtángjí, mid-10th century), and the hagiographical Jingde Record of the Transmission of the Lamp (Chinese Jǐngdé Chuándēnglù, early 11th century).
 * The Blue Cliff Record (Chinese: 碧巖錄 Bìyán Lù; Japanese: Hekiganroku) is a collection of 100 kōans compiled in 1125 by Yuanwu Keqin (圜悟克勤 1063–1135), mostly drawn from earlier "transmission" stories.
 * The Book of Equanimity or Book of Serenity (Chinese: 從容録 Cóngróng lù; Japanese: 従容録 Shōyōroku) is a collection of 100 Kōans by Hongzhi Zhengjue (Chinese: 宏智正覺; Japanese: Wanshi Shōgaku) (1091–1157), compiled with commentaries by Wansong Xingxiu (1166–1246).
 * The Gateless Gate (Chinese: 無門關 Wumenguan; Japanese: Mumonkan) is a collection of 48 kōans and commentaries published in 1228 by Chinese monk Wumen (無門 c,1183–1260). The title may be more accurately rendered as Gateless Barrier or Gateless Checkpoint. Five kōans in the collection derive from the sayings and doings of Zhaozhou Congshen (transliterated as Chao-chou in Wade-Giles and pronounced Jōshū in Japanese).
 * The Zhengfayan zang (正法眼藏, "Treasury of the true dharma eye", J.: Shōbōgenzō) is a collection of koans and dialogues compiled between 1147 and 1150 by Dahui Zonggao. Dahui's Treasury is composed of three scrolls prefaced by three short introductory pieces.
 * The Zongmen liandeng huiyao (宗門聯燈會要) was compiled in 1183 by Huiweng Wuming (晦翁悟明) (n.d.), three generations after Dahui in the same line; the sermon is found in zh 20 (x 79: 173a).

After the Song era
Zhongfeng Mingben (1263–1323), a Chinese Chán-master who lived at the beginning of the Yuan Dynasty, revitalized the Chinese Linji school. Zhongfeng put a strong emphasis on the use of gōng'àns, seeing them as a "work of literature [that] should be used as objective, universal standards to test the insight of monks who aspired to be recognized as Ch'an masters". He also promoted Dahui's famous kanhua chan method of meditating on a huatou and influenced several Japanese Rinzai masters of the time who came to China to study with him, including Kosen Ingen, Kohō Kakumyō, Jakushitsu Genkō (1290–1367).

According to Zhongfeng: "The koans do not represent the private opinion of a single man, but rather the hundreds and thousands of bodhisattvas of the three realms and ten directions. This principle accords with the spiritual source, tallies with the mysterious meaning, destroys birth-and-death, and transcends the passions. It cannot be understood by logic; it cannot be transmitted in words; it cannot be explained in writing; it cannot be measured by reason. It is like the poisoned drum that kills all who hear it, or like a great fire that consumes all who come near it. What is called 'the special transmission of the Vulture Peak' was the transmission of this; what is called the 'direct pointing of Bodhidharma at Shao-lin-ssu' is this." In later periods like the Ming dynasty, Chinese Chan developed in different directions, such as incorporating Pure Land elements and the re-introduction of an emphasis on the study of scripture.

However, gōng'àn meditation was still practiced in the Linji lineages. During the Ming, Miyun Yuanwu (1566–1642) was a successful promoter of the Linji school's gōng'àn methods, emphasizing vigorous master disciple encounters which made use of shouting and beating in imitation of classic gōng'àn stories. Yuanwu's efforts to revive the Linji tradition were so successful that according to Marcus Bingenheimer "Miyun’s Tiantong branch 天童派 of the Linji School became the dominant Chan lineage in China and beyond" (c. 17th century). He led numerous communities of thousands of monks and confirmed twelve dharma heirs.

His teachings also influenced Japanese Zen since his student Yinyuan Longqi (Japanese: Ingen Ryūki, 1592–1673) later founded the Ōbaku school in Japan. This lineage also spread the Linji gōng'àn teachings to Vietnam, mainly through the efforts of Yuanshao 元韶 (1648–1728).

Modern Chinese Chán and Korean Seon
In the modern period, the practice of meditating on the critical phrase (huatou) of a gōng'àn is still taught and some Chinese Chan figures like Sheng Yen and Xuyun taught the practice and wrote on it.

Modern Chinese Chán and Korean Seon generally follow the method taught by Dahui, which emphasizes meditation on a huatou ("critical phrase", "word head"). In this method one repeats the phrase over and over again and inquires into it while in meditation (sitting or walking) as well as in daily activities. In this mainland tradition of huatou practice, also called kanhua, "reflection on the koan", a fragment of a koan, such as "mu", or a "what is"-question is used by focusing on this fragment and repeating it over and over again.

In this tradition one generally contemplates one such phrase for an extended period of time, going deeper and deeper into it, instead of going through an extended curriculum as in Japanese Rinzai. A student may be assigned only one hua-tou for their whole life. The focus of this contemplation is on generating the sense of "great doubt" and on having faith in the Dharma and the practice. According to Ford the huatou "becomes a touchstone of our practice: it is a place to put our doubt, to cultivate great doubt, to allow the revelation of great faith, and to focus our great energy."

Xuyun wrote:"The important thing is to stick to Hua Tou at all times, when walking, lying, or standing. From morning to night observing Hua Tou vividly and clearly, until it appears in your mind like the autumn moon reflected limpidly in quiet water. If you practice this way, you can be assured of reaching the state of Enlightenment."Examples of huatous which are used in meditation include: "What is this?"; "What was the original face before my father and mother were born?"; "Who is dragging this corpse about?"; and "Who am I?"

Another popular practice in Chinese Chan is using the nianfo (repetition of Amitabha Buddha's name) as a gōng'àn practice. This method of "Nianfo Chan" (念佛禪) was promoted in the modern era by Xuyun and relies on repeating the Buddha's name while also asking "who is reciting?". The practice of using nianfo in a gōng'àn like fashion is also found in the Japanese Ōbaku school and was taught by their founding masters (including Yǐnyuán, i.e. Ingen), indicating that this method dates at least as far back as the Ming dynasty.

The modern Korean master Seung Sahn developed his own curriculum of multiple kōans in his Kwan Um School of Zen, but this was a modern development unheard of in Korean Seon.

The introduction of kōans
When the Chán tradition was established in Japan in the 12th century, both Rinzai and Sōtō, took over the use of kōan study and commenting. In Sōtō-Zen, kōan commentary was not linked to seated meditation. Japanese monks had to master the Chinese language and specific expressions used in the kōan training. The desired "spontaneity" expressed by enlightened masters required a thorough study of Chinese language and poetry. Japanese Zen imitated the Chinese "syntax and stereotyped norms".

During the Kamakura period, the officially recognized Rinzai monasteries belonging to the Gozan (Five Mountain System) where key centers for the study of koans. Senior monks in these monasteries were supposed to compose Chinese verse in a complex style of matched counterpoints known as bienli wen. It took a lot of literary and intellectual skills for a monk to succeed in this system.

The Rinka monasteries, the provincial temples which were under less direct state control, laid less stress on the correct command of Chinese verse. These monasteries developed "more accessible methods of kōan instruction". It had three features:
 * 1) A standardized kōan curriculum;
 * 2) A standardized set of answers based on stereotypes Chinese sayings;
 * 3) A standardized method of secretly guiding students through the curriculum of kōan and answers.

By standardizing the kōan curriculum every generation of students proceeded to the same series of kōans. Students had to memorize a set number of stereotyped sayings, agyō, "appended words". The proper series of responses for each kōan were taught by the master in private instruction sessions to selected individual students who would inherit the dharma lineage.

The development of Rinzai koan curriculums occurred in various stages. According to Eshin Nishimura, Japanese Rinzai-masters like Enni-bennen (圓爾辨圓) (1202-1280) and Nampo-jyoumain (南浦紹明) (1235-1308) had already divided the Chinese koans into three groups namely richi ("ultimate truth"), kikan ("skillful method") and koujyon ("non-attachment"). Musō Soseki (1275–1351) further developed the use of koans. Despite belonging to the Rinzai-school, Musō Soseki also made extensive use of richi (teaching), explaining the sutras, instead of kikan (koan). According to Musō Soseki, both are upaya, "skillful means" meant to educate students. Musō Soseki called both shōkogyu, "little jewels", tools to help the student to attain satori.

In the 18th century, the Rinzai school became dominated by the legacy of Hakuin, who laid a strong emphasis on kōan study as a means to gain kensho, but also not to get stuck in this initial insight and develop a compassionate, selfless attitude. After Hakuin, most Rinzai monasteries followed the teachings of his lineage on koan practice. Koan-study was also further systematized in a standard sequence of koans that the student had to pass and work through step by step. There are two curricula used in Rinzai, derived from two dharma-heirs of Gasan: the Takuju curriculum, and the Inzan curriculum. Both curricula have standardized answers.

Rinzai school
Kōan practice is particularly important among Japanese practitioners of the Rinzai school. Japanese Rinzai uses extensive koan-curricula, checking questions, and jakogo ("capping phrases", quotations from Chinese poetry) in its use of koans,

Koan practice starts with the shokan, or "first barrier", usually the mu-koan or the question "What is the sound of one hand?" After having attained kensho, students continue their practice investigating subsequent koans. In the Takuju-school, after breakthrough students work through the Gateless Gate (Mumonkan), the Blue Cliff Record (Hekigan-roku), the Entangling Vines (Shumon Kattoshu), and the Collection of Wings of the Blackbird (鴆羽集, Chin'u shū). The Inzan-school uses its own internally generated list of koans.

Koan curricula
In Rinzai a gradual succession of koans is studied. There are two general branches of curricula used within Rinzai, the Takuju curriculum, and the Inzan curriculum. However, there are a number of sub-branches of these, and additional variations of curriculum often exist between individual teaching lines which can reflect the recorded experiences of a particular lineage's members. Koan curricula are, in fact, subject to continued accretion and evolution over time, and thus are best considered living traditions of practice rather than set programs of study.

While Hakuin only refers to break-through koans, and "difficult to pass" koans to sharpen and refine the initial insight and foster compassion, Hakuin's descendants developed a fivefold classification system:
 * 1) Hosshin, dharma-body koans, are used to awaken the first insight into sunyata. They reveal the dharmakaya, or Fundamental. They introduce "the undifferentitated and the unconditional".
 * 2) Kikan, dynamic action koans, help to understand the phenomenal world as seen from the awakened point of view; Where hosshin koans represent tai, substance, kikan koans represent yu, function.
 * 3) Gonsen, explication of word koans, aid to the understanding of the recorded sayings of the old masters. They show how the Fundamental, though not depending on words, is nevertheless expressed in words, without getting stuck to words.
 * 4) Hachi Nanto, eight "difficult to pass" koans. There are various explanations for this category, one being that these koans cut off clinging to the previous attainment. They create another Great Doubt, which shatters the self attained through satori. It is uncertain which are exactly those eight koans. Hori gives various sources, which altogether give ten hachi nanto koans:
 * 5) *Miura and Sasaki:
 * 6) **Nansen's Flower (Hekigan-roku Case 40)
 * 7) **A Buffalo Passes the Window (Mumonkan Case 38)
 * 8) **Sōzan's Memorial Tower (Kattō-shō Case 140)
 * 9) **Suigan's Eyebrows (Hekigan-roku Case 8)
 * 10) **Enkan's Rhinoceros Fan (Hekigan-roku Case 91)
 * 11) *Shimano:
 * 12) **The Old Woman Burns the Hut (Kattō-shō Case 162)
 * 13) *Asahina Sōgen:
 * 14) **Goso Hōen's "Hakuun Said 'Not Yet'" (Kattō-shō Case 269)
 * 15) **Shuzan's Main Cable (Kattō-shō Case 280).
 * 16) *Akizuki:
 * 17) **Nansen Has Died (Kattō-shō Case 282)
 * 18) **Kenpō's Three Illnesses (Kattō-shō Case 17).
 * 19) Goi jujukin koans, the Five Ranks of Tozan and the Ten Grave Precepts.

According to Akizuki there was an older classification-system, in which the fifth category was Kojo, "Directed upwards". This category too was meant to rid the monk of any "stink of Zen". The very advanced practitioner may also receive the Matsugo no rokan, "The last barrier, and Saigo no ikketsu, "The final confirmation". "The last barrier" is given when one left the training hall, for example "Sum up all of the records of Rinzai in one word!" It is not meant to be solved immediately, but to be carried around in order to keep practising. "The final confirmation" may be another word for the same kind of koan. Shin'ichi Hisamatsu gave "If nothing what you do will do, then what will you do?" as 'unanswerable' question, which keeps nagging on premature certainty.

The breakthrough-koan
In the Rinzai-school, the Sanbo Kyodan, and the White Plum Asanga, koan practice starts with the assignment of a hosshi or "break-through koan", usually the mu-koan or "the sound of one hand". Students are instructed to concentrate on the "word-head", like the phrase "mu". In the Wumenguan (Mumonkan), public case No. 1 ("Zhaozhou's Dog"), Wumen (Mumon) wrote: "... concentrate yourself into this 'Wú' ... making your whole body one great inquiry. Day and night work intently at it. Do not attempt nihilistic or dualistic interpretations.'"

Arousing this great inquiry or "Great Doubt" is an essential element of kōan practice. It builds up "strong internal pressure (gidan), never stopping knocking from within at the door of [the] mind, demanding to be resolved". To illustrate the enormous concentration required in kōan meditation, Zen Master Wumen commented, "It is like swallowing a red-hot iron ball. You try to vomit it out, but you can't."

Analysing the koan for its literal meaning won't lead to insight, though understanding the context from which koans emerged can make them more intelligible. For example, when a monk asked Zhaozhou (Joshu) "does a dog have Buddha-nature or not?", the monk was referring to the understanding of the teachings on Buddha-nature, which were understood in the Chinese context of absolute and relative reality.

Kensho
The continuous pondering of the break-through koan (shokan) or Hua Tou, "word head", leads to kensho, an initial insight into "seeing the (Buddha-)nature.

The aim of the break-through koan is to see the "nonduality of subject and object": "The monk himself in his seeking is the koan. Realization of this is the insight; the response to the koan [...] Subject and object – this is two hands clapping. When the monk realizes that the koan is not merely an object of consciousness but is also he himself as the activity of seeking an answer to the koan, then subject and object are no longer separate and distinct [...] This is one hand clapping (sic)."

Various accounts can be found which describe "becoming one" with the koan and the resulting breakthrough: "I was dead tired. That evening when I tried to settle down to sleep, the instant I laid my head on the pillow, I saw: 'Ah, this outbreath is Mu!' Then: the in-breath too is Mu!' Next breath, too: Mu! Next breath: Mu, Mu! 'Mu, a whole sequence of Mu! Croak, croak; meow, meow – these too are Mu! The bedding, the wall, the column, the sliding-door – these too are Mu! This, that and everything is Mu! Ha ha! Ha ha ha ha Ha! that roshi is a rascal! He's always tricking people with his 'Mu, Mu, Mu'!..."

But the use of the mu-koan has also been criticised. According to Ama Samy, the main aim is merely to "'become one' with the koan". Showing to have 'become one' with the first koan is enough to pass the first koan. According to Samy, this is not equal to prajna: "The one-pointed, non-intellectual concentration on the hua-t'ou (or Mu) is a pressure-cooker tactic, a reduction to a technique which can produce some psychic experiences. These methods and techniques are forced efforts which can even run on auto-pilot. They can produce experiences but not prajna wisdom. Some speak of 'investigating' the hua-t’ou, but it is rather a matter of concentration, which sometimes can provide insights, yet no more than that."

Sassho – Checking questions
Teachers may probe students about their kōan practice using sassho, "checking questions" to validate their satori (understanding) or kensho (seeing the nature). For the mu-koan and the clapping hand-koan there are twenty to a hundred checking questions, depending on the teaching lineage. The checking questions serve to deepen the insight or kyōgai of the student, but also to test his or her understanding.

Standardized answers
Those checking questions, and their answers, are part of a standardised set of questions and answers. Ama Samy states that the "koans and their standard answers are fixed." Isshu Muira Roshi also states, in The Zen Koan: "In the Inzan and Takuju lines, the answers to the koans were more or less standardized for each line respectively." Missanroku and missanchō, "Records of secret instruction" have been preserved for various Rinzai lineages. They contain both the kōan curricula and the standardized answers.

In Sōtō-Zen they are called monsan, an abbreviation of monto hissan, "secret instructions of the lineage". The monsan follow a standard question-and-answer format. A series of questions is given, to be asked by the master. The answers are also given by the master, to be memorized by the student.

According to critics, students are learning a "ritual performance", learning how to behave and respond in specific ways, learning "clever repartees, ritualized language and gestures and be submissive to the master's diktat and arbitration."

In 1916 Tominaga Shūho, using the pseudonym "Hau Hōō", published a critique of the Rinzai kōan system, Gendai sōjizen no hyōron, which also contained a translation of a missanroku. The missanroku part has been translated by Yoel Hoffmann as "The Sound of the One Hand" (see ) and.

Jakugo – Capping phrases
In the Rinzai-school, passing a koan and the checking questions has to be supplemented by jakugo, "capping phrases", citations of Chinese poetry to demonstrate the insight. Students can use collections of those citations, instead of composing poetry themselves.

Post-satori practice
After the initial insight further practice is necessary, to deepen the insight and learn to integrate it in daily life. In Chinese Chán and Korean Seon, this further practice consists of further pondering of the same Hua Tou. In Rinzai-Zen, this further practice is undertaken by further koan-study, for which elaborate curricula exist. In Sōtō-Zen, Shikantaza is the main practice for deepening insight.

Real-life integration
After completing the koan-training, Gogo no shugyo is necessary: "[I]t would take 10 years to solve all the kōans [...] in the sōdō. After the student has solved all koans, he can leave the sōdō and live on his own, but he is still not considered a roshi. For this he has to complete another ten years of training, called 'go-go-no-shugyō' in Japanese. Literally, this means 'practice after satori/enlightenment', but Fukushima preferred the translation 'special practice'. Fukushima would explain that the student builds up a 'religious personality' during this decade. It is a kind of period that functions to test if the student is actually able to live in regular society and apply his koan understanding to daily life, after he has lived in an environment that can be quite surreal and detached from the lives of the rest of humanity. Usually, the student lives in small parish temple during this decade, not in a formal training monastery."

Completing the koan-curriculum in the Rinzai-schools traditionally also led to a mastery of Chinese poetry and literary skills: "[D]isciples today are expected to spend a dozen or more years with a master to complete a full course of training in koan commentary. Only when a master is satisfied that a disciple can comment appropriately on a wide range of old cases will he recognize the latter as a dharma heir and give him formal 'proof of transmission' (J. inka shomei). Thus, in reality, a lot more than satori is required for one to be recognized as a master (J. shike, roshi) in the Rinzai school of Zen at present. The accepted proof of satori is a set of literary and rhetorical skills that takes many years to acquire."

Breathing practices
Hakuin Ekaku, the 17th century revitalizer of the Rinzai school, taught several practices which serve to correct physical and mental imbalances arising from, among other things, incorrect or excessive koan practice. The "soft-butter" method (nanso no ho) and "introspection method" (naikan no ho) involve cultivation of ki centered on the tanden (Chinese:dantian). These practices are described in Hakuin's works Orategama and Yasen Kanna, and are still taught in some Rinzai lineages today.

In Japanese Sōtō
Few Sōtō Zen practitioners concentrate on kōans during meditation, but the Sōtō sect has a strong historical connection with kōans, since many kōan collections were compiled by Sōtō priests. During the 13th century, Dōgen, founder of the Sōtō sect in Japan, quoted 580 kōans in his teachings. He compiled some 300 kōans in the volumes known as the Greater Shōbōgenzō. Dōgen wrote of Genjokōan, which points out that everyday life experience and indeed, the whole universe in this moment, is the "fundamental kōan", which does not refer to any ancient Zen story, but to the "heart of the matter", the question of life and death.

Over time, Sōtō sect adopted various koan meditation methods from other schools like Rinzai, including the method of observing a koan in meditation and koan curriculums. By the 15th century, Sōtō temples were publishing koan texts, and Sōtō monks often studied at Rinzai temples and passed on Rinzai koan practice lineages (and vice versa). Sōtō teachers continued to write and collect kōan texts throughout the medieval period. Later kōan collections compiled and annotated by Sōtō priests include The Iron Flute (Tetteki Tōsui) by Genrō Ōryū in 1783 and Verses and Commentaries on One Hundred Old Cases of Tenchian (Tenchian hyakusoku hyoju) compiled by Tetsumon in 1771.

However, during the late 18th and 19th century, the Sōtō tradition of kōan commentary and practice became criticized and suppressed in the Sōtō school, due to a reform movement that sought to return to the teaching of Dōgen and standardise the procedures for dharma transmission. An important figure in this development was Gentō Sokuchū (1729-1807), who sought to remove Rinzai and Obaku influences on Sōtō and focus strictly on Dōgen's teachings and writings.

Another reason for suppressing the kōan tradition in the Sōtō school may have been to highlight the differences with the Rinzai school, and create a clear Sōtō identity. This reform movement had started to venerate Dōgen as the founding teacher of the Sōtō school and they sought to make Dōgen's teachings the main standard for the Sōtō school. While Dōgen himself made extensive use of kōan commentary in his works, it is clear he emphasized shikantaza ("just sitting") without an object, instead of the koan introspection method.

In Sanbo Kyodan and White Plum Asanga
The Sanbo Kyodan school of the former Sōtō-priest Hakuun Yasutani, and the White Plum Asanga of Taizan Maezumi and the many groups that derive from him, incorporate koan-study. The Sanbo kyodan places great emphasis on kensho, initial insight into one's true nature, as a start of real practice. It follows the so-called Harada-Yasutani koan-curriculum, which is derived from Hakuin's student Takuju. It is a shortened koan-curriculum, in which the so-called "capping phrases" are removed. The curriculum takes considerably less time to study than the Takuju-curriculum of Rinzai.

To attain kensho, most students are assigned the mu-koan. After breaking through, the student first studies twenty-two "in-house" koans, which are "unpublished and not for the general public", but are nevertheless published and commented upon. There-after, the students goes through the Gateless Gate (Mumonkan), the Blue Cliff Record, the Book of Equanimity, and the Record of Transmitting the Light. The koan-curriculum is completed by the Five ranks of Tozan and the precepts.

Does a dog have Buddha-nature
"A monk asked Zhàozhōu, "Does a dog have Buddha nature, or does he not have Buddha-nature?"

Zhaozhou said, "Wú"."

"Zhaozhou" is rendered as "Chao-chou" in Wade-Giles, and pronounced "Joshu" in Japanese. "Wu" appears as "mu" in Japanese, meaning "no", "not", "nonbeing", or "without" in English. This is a fragment of Case No. 1 of the Wúménguān. However, another koan presents a longer version, in which Zhaozhou answered "yes" in response to the same question asked by a different monk: see Case No. 18 of the Book of Serenity.

The sound of one hand
"Two hands clap and there is a sound. What is the sound of one hand? (隻手声あり、その声を聞け)"

Victor Hori comments: "... in the beginning a monk first thinks a kōan is an inert object upon which to focus attention; after a long period of consecutive repetition, one realizes that the kōan is also a dynamic activity, the very activity of seeking an answer to the kōan. The kōan is both the object being sought and the relentless seeking itself. In a kōan, the self sees the self not directly but under the guise of the kōan ... When one realizes ('makes real') this identity, then two hands have become one. The practitioner becomes the kōan that he or she is trying to understand. That is the sound of one hand."

Yet, Hakuin himself introduced this question with a reference to Kanzeon (Guanyin), bodhisattva of great compassion, who hears the sounds of the suffering ones in the world, and is awakened by hearing these sounds and responding to them. To hear the sound of one hand is to still the sounds of the world, that is, to put an end to all suffering.

Original face
"Huìnéng asked Hui Ming, "Without thinking of good or evil, show me your original face before your mother and father were born.""

This is a fragment of case No. 23 of the Wumenguan.

Killing the Buddha
"If you meet the Buddha, kill him. (逢佛殺佛)"

Other koans
"A student asked Master Yun-Men (949 AD) "Not even a thought has arisen; is there still a sin or not?"

Master replied, "Mount Sumeru!""

"A monk asked Dongshan Shouchu, "What is Buddha?"

Dongshan said, "Three pounds of flax.""

This is a fragment of case No. 18 of the Wumenguan as well as case No. 12 of the Blue Cliff Record.

"A monk asked Ummon, "What is the teaching that transcends the Buddha and patriarchs?"

Ummon said, "A sesame bun.""

- Blue Cliff Record, case no. 77

"A monk asked Zhaozhou, "What is the meaning of the ancestral teacher's (i.e., Bodhidharma's) coming from the west?"

Zhaozhou said, "The cypress tree in front of the hall.""

This is a fragment of case No. 37 of the Wumenguan as well as case No. 47 of the Book of Serenity.

Cultural legacy
Gentō Sokuchū, the 18th century abbot of Dogen's Eihei-ji, aggressively sought to reform Sōtō from all things 'foreign' and associated with Rinzai, including kōans. The unorthodox Zen monk Ikkyū contemplated kōans for years while creating dolls for a merchant in Kyoto, specifically penetrating the case no. 15 from The Gateless Gate and thereafter earning his dharma name Ikkyū.

Facing criticism by Buddhists such as Philip Kapleau and D. T. Suzuki for misunderstanding Zen, Alan Watts claimed that a kōan supported his lack of zazen practice. On the topic, Suzuki claimed: "I regret to say that Mr. Watts did not understand that story."

Douglas Hofstadter's 1979 book Gödel, Escher, Bach: an Eternal Golden Braid discusses Zen kōans in relation to paradoxical questions and perceiving reality outside of one's experience. Inspired by Zen teachings (including kōans), Frank Herbert wrote on the subject of the paradoxical elements of his Dune series:

"What especially pleases me is to see the interwoven themes, the fugue like relationships of images that exactly replay the way Dune took shape. As in an Escher lithograph, I involved myself with recurrent themes that turn into paradox. ... It's like a kōan, a Zen mind breaker."

The 1989 South Korean film Why Has Bodhi-Dharma Left for the East? bases much of its narrative on kōans, with its title deriving from a particular kōan about the founder of Zen, Bodhidharma.

After becoming smitten with Zen (even offering to turn his own house into a zendo), filmmaker Alejandro Jodorowsky meditated and studied koans with the traveling monk Ejo Takata (1928–1997). After the release of The Holy Mountain, Jodorowsky gave a talk at the University of Mexico on the subject of kōans. After this talk, Takata gifted Jodorowsky his keisaku, believing that the filmmaker had mastered the ability to understand kōans.

In the 1958 novel The Dharma Bums, Jack Kerouac paraphrases the Yunmen shit-stick kōan as: "The Buddha is a dried piece of turd". The second volume of the manga Lone Wolf and Cub by Kazuo Koike and Goseki Kojima is titled 'The Gateless Barrier' and revolves around a Linji kōan ("If you meet a Buddha, kill him") as the protagonist is tasked to kill a troublesome "living Buddha".

In hacker culture, funny short stories concerning computer science developed, named hacker koans. The book Jargon File contains many kōans, including the AI Koans. The Codeless Code is another book about software engineers at big businesses instead of unix hackers, deriving its title from the Gateless Gate.

The song "False Prophet" by Bob Dylan includes the line: "I climbed a mountain of swords on my bare feet", a reference to a Gateless Gate kōan ("You must climb a mountain of swords with bare feet"). British musical artist Brian Eno collaborated with Intermorphic on developing a generative music software system named Koan. In 2009, American composer and multi-instrumentalist Tyshawn Sorey released his second album, Koan.

The 1997 novel The Sound of One Hand Clapping by Richard Flanagan (and its 1998 film adaptation of the same name) derives its title from a kōan by Hakuin Ekaku. The episode of the 2014 first season of Fargo entitled "Eating the Blame" derives its episode title from a koan of the same name from the Shasekishū. Cyriaque Lamar of io9 stated that the approach to technology in Tron: Legacy was reminiscent of kōans.

Criticism
According to Mario Poceski, encounter dialogues (which form the content of koans) are not historically reliable and "have little or nothing to do with the lives, ideas, and teachings of the Tang-era protagonists who are featured in them." Such stories instead reflect the artistic license and religious imagination of mid-tenth century Chan. Poceski further observes that although Zen is often portrayed as promoting spontaneity and freedom, encounter-dialogue exegesis actually points in the opposite direction, namely towards a tradition bound by established parameters of orthodoxy. According to Poceski, on the whole, the encounter-dialogue genre is marked by formulaic repetition and cliché. He says: "In the end, notwithstanding the iconoclastic ethos imputed to them, it is apparent that these textual sources are products of a conservative tradition that, in the course of its growth and transformation during the Tang-Song transition, was keen to promote a particular version of Buddhist orthodoxy and secure its place as the main representative of elite Chinese Buddhism."

Koans have also been criticized from within the Zen tradition at various points throughout history. The Song era master Foyan Qingyuan (1067-1120) rejected the use of koans (public cases) and similar stories, arguing that they did not exist during the time of Bodhidharma and that the true koan is "what is presently coming into being." According to Schlütter, "the practice was common enough to attract criticism" which can be found in Song sources like the Sengbao zhengxu zhuan (True continuation of the chronicles of the saṃgha treasure), where one master called Chanti Weizhao "rails against deluded masters who teach people to contemplate (can) gongan stories."

According to Arthur Braverman, Bassui Tokushō (1327–1387) "was very critical of the Rinzai practice of studying kōans, perhaps because they were becoming more and more formalized, hence losing their original spirit."

The unconventional Rinzai master Bankei Yōtaku (1622-1693) famously criticized the kōan method, seeing it as a hopelessly contrived and artificial technique. Bankei referred to kōans disparagingly as "old wastepaper" and referred to Zen masters who required devices in order to guide people as engaging in "devices Zen." Bankei said: "'Unlike the other masters everywhere, in my teaching I don't set up any particular object, such as realizing enlightenment or studying koans. Nor do I rely on the words of the buddhas and patriarchs. I just point things out directly, so there's nothing to hold onto, and that's why no one will readily accept [what I teach].'"

The Sōtō school emphasizes shikantaza as its main practice, though it does not completely reject the study and use of koans. That being said, some Sōtō figures have criticized the Rinzai style koan method. For example, the famous Sōtō master Kodo Sawaki criticized Rinzai koan practice as "stepladder Zen" and wrote:"From the end of the Song Dynasty to the Yuan [and] Ming dynasties techniques developed, and solving koans was the way monks became respected for having had satoris. Well, today [monks] have satoris, which in certain religious sects allows the monks to be candidates to be head priests of temples. That's the way they think. But they're wrong. Believe in zazen itself, and if you put your whole body into it, that is [true] zazen."

The modern Korean Sŏn master Daehaeng taught that it was not necessary to receive a hwadu (the "word head" of a koan) from others since everyone already has their own "original hwadus." She says:"Daily life is itself a hwadu, so there is no need to receive a hwadu from others or to give a hwadu to others. Your very existence is a hwadu. Thus, if you are continuously holding on to a hwadu someone else gave you, when will you be able to solve your original hwadu? Trying to solve another person's hwadu is like turning empty millstones or spinning a car's wheels without moving forward. Your body itself is a hwadu. Birth itself is a hwadu. Work itself is a hwadu. The vast universe is a hwadu. If you want to add more hwadus to these, when will you be able to taste this infinitely deep world we live in?"

According to Alan Watts, the koan method suffered from two drawbacks. The first is that it can potentially lead to a kind of romanticism for exotic cultural forms. The second was that its method of deliberately rousing great doubt and then breaking through it after an intense period of striving amounted to a kind of psychological trick. Regarding this, Watts says: "The second, and more serious, drawback can arise from the opposition of satori to the intense “feeling of doubt” which some koan exponents so deliberately encourage. For this is to foster a dualistic satori. To say that the depth of the satori is proportional to the intensity of seeking and striving which precede it is to confuse satori with its purely emotional adjuncts. In other words, if one wants to feel exhilaratingly light-footed, it is always possible to go around for some time with lead in one’s shoes–and then take them off. The sense of relief will certainly be proportional to the length of time such shoes have been worn, and to the weight of the lead. This is equivalent to the old trick of religious revivalists who give their followers a tremendous emotional uplift by first implanting an acute sense of sin, and then relieving it through faith in Jesus."