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 A kōan (/ˈkoʊæn, -ɑːn/ KOH-a(h)n; Japanese: 公案; Chinese: 公案; pinyin: gōng'àn ; Korean: 화두, romanized: hwadu; Vietnamese: công án) is a story, dialogue, question, or statement from the Chinese Chan-lore, supplemented with commentaries, that is used in Zen practice to provoke the "great doubt" and initial insight of Zen-students. Prolonged koan-study shatters small-minded pride of, and identification with, this initial insight, and spurs further development of insight and compassion, and integration thereof in daily life and character.
 The Japanese term kōan is the Sino-Japanese reading of the Chinese word gong'an (Chinese: 公案; pinyin: gōng'àn; Wade–Giles: kung-an; lit. 'public case'). 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.
 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. Gong'an developed during the Tang dynasty (618–907) from the recorded sayings collections of Chán masters, which quoted many stories of "a famous past Chán figure's encounter with disciples or other interlocutors and then offering his own comment on it". Those stories and the accompanying comments were used to educate students, and broaden their insight into the Buddhist teachings.
 Those stories came to be known as gong'an, "public cases". Such a story was only considered a gongan when it was commented upon by another Chán master. This practice of commenting on the words and deeds of past masters confirmed the master's position as an awakened master in a lineage of awakened masters of the past.
 Kōan practice developed from a literary practice, styling snippets of encounter-dialogue into well-edited stories. It arose in interaction with "educated literati". There were dangers involved in such a literary approach, such as ascribing specific meanings to the cases. 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. Kōan literature was also influenced by the pre-Zen Chinese tradition of the "literary game"—a competition involving improvised poetry.
 The style of writing of Zen texts has been influenced by "a variety of east Asian literary games":
 During the Song dynasty (960–1297) the use of gong'an took a decisive turn. Dahui Zonggao (1089–1163) introduced the use of kanhua, "observing the phrase". 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.
 In the 11th century, this practice had become common. A new literary genre developed from this tradition as well. Collections of such commented cases were compiled, which consisted of the case itself, accompanied by verse or prose commentary.
 Dahui's invention was aimed at balancing the insight developed by reflection on the teachings with developing samatha, calmness of mind. Ironically, this development became in effect silent illumination, a "[re-absorbing] of kōan-study into the "silence" of meditation (ch'an)". It led to a rejection of Buddhist learning:
 Some extent of Buddhist learning could easily have been recognized as a precondition for sudden awakening in Chán. Sung masters, however, tended to take the rejection literally and nondialectically. In effect, what they instituted was a form of Zen fundamentalism: the tradition came to be increasingly anti-intellectual in orientation and, in the process, reduced its complex heritage to simple formulae for which literal interpretations were thought adequate. This development left Chinese Chán vulnerable to criticisms by neo-Confucianism, which developed after the Sung Dynasty. Its anti-intellectual rhetoric was no match for the intellectual discourse of the neo-Confucianists.
 The recorded encounter dialogues, and the kōan 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 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 Kōan training requires a qualified teacher who has the ability to judge a disciple's handling of the tradition. 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, "sanzen is the real point of the whole exercise," where one has to prove one's understanding.
 Some of the earliest texts in which Kōans occur are the Anthology of the Patriarchal Hall (Chinese Zǔtángjí), mid-10th century, and the hagiographical collection The Jingde Record of the Transmission of the Lamp, also rendered into English as The Record of Transmitting the Light (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).
 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 full title is The Record of the Temple of Equanimity With the Classic Odes of Venerable Tiantong Jue and the Responsive Commentary of Old Man Wansong 萬松老評唱天童覺和尚 頌古從容庵錄 (Wansong Laoren Pingchang Tiantong Jue Heshang Songgu Congrong An Lu) (Taisho Tripitaka Vol. 48, No. 2004)
 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) (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).
 Dahui Zonggao (大慧宗杲) (1089–1163) the Zhengfayan zang (正法眼藏), "Treasury of the true dharma eye" (W-G.: Cheng-fa yen-tsang, (J.: Shōbōgenzō) 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).
 In the officially recognized monasteries belonging to the Gozan (Five Mountain System) the Chinese system was fully continued. Senior monks 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 with less control of the state, laid less stress on the correct command of the Chinese cultural idiom. These monasteries developed "more accessible methods of kōan instruction". It had three features:
 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.
 Zhongfeng Mingben (1263–1323), a Chinese Chán-master who lived at the beginning of the Yuan Dynasty, revitalized the Rinzai-tradition, and put a strong emphasis on the use of koans. He saw the kung-ans as "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":
 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. 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".
 Accoridng to Eshin Nishimura, Japanese Rinzai-masters like En’ni-ben’nen (圓爾辨圓) (1202-1280) and Nampo-jyoumain (南浦紹明) (1235-1308) already divided the Chinese koans into three groups, comform the function they assigned to them, namely richi("ultimate truth"), kikan("skillful method") and koujyon ("non-attachment").
 Musō Soseki (1275–1351) relativized 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, koan-study was systematized in a standard sequence and classification of koans. 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.
 During the late 18th and 19th century, the tradition of kōan commentary became suppressed in the Sōtō school, due to a reform movement that sought to standardise the procedures for dharma transmission. One 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 identity. This movement also started to venerate Dogen as the founding teacher of the Sōtō school. His teachings became the standard for the Sōtō teachings, neglecting the fact that Dogen himself made extensive use of kōan commentary.
 In Chinese Chán and Korean Seon, the emphasis is on Hua Tou, "observing the phrase," contemplating one koan throughout one's lifetime, though Seung Sahn used the Rinzai-style of kōan practice in his Kwan Um School of Zen.
 In Chinese Chán and Korean Seon, the primary form of Koan-study is kanhua, "reflection on the koan", also called Hua Tou, "word head". In this practice, a fragment of the koan, such as "mu", or a "what is"-question is used by focusing on this fragment and repeating it over and over again:
 Who is it who now repeats the Buddha's name?
 Who is dragging this corpse about?
What is this?
What is it?
What was the original face before my father and mother were born?
 
Who am I? The student is assigned only one hua-tou for a lifetime. In contrast to the similar-sounding "who am I?" question of Ramana Maharshi, hua-tou involves raising "great doubt":
 This koan 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. Kōan practice is particularly important among Japanese practitioners of the Rinzai sect. The Japanese Rinzai-school 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.
 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:
 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.
 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.
 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. 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.
 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 Hoffmann (1975)) and Bodiford (1993, p. 264 note 29). 
 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.
 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.
 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. 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.
 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 is the fundamental kōan. 
 Other kōan collections compiled and annotated by Sōtō priests include:
 In Japanese Sōtō Zen the use of koans has been abandoned since the late eighteenth and nineteenth century: 
 ...kōan practice was largely expunged from the Sōtō school through the efforts of Gentō Sokuchū (1729–1807), the eleventh abbot of Entsuji, who in 1795 was nominated abbot of Eiheiji. 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.
 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.
 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.
 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.
 If you meet the Buddha, kill him. (逢佛殺佛) 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."
 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.
 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.
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