The wild fox kōan, also known as "Pai-chang 's fox" and "Hyakujō and a Fox", is an influential kōan story in the Zen tradition dating back as early as 1036, when it appeared in the Chinese biographical history T'ien-sheng kuang-teng lu. It was also in The Gateless Gate (Mandarin: 無門關 Wúménguān; Japanese: 無門関 Mumonkan), a 13th-century collection of 48 kōans compiled by the Chinese monk Wumen, as case two.
Overview
The koan tells the story of a monk who, after denying that an enlightened person falls into cause and effect, was turned into a wild fox for five hundred lifetimes. He appears to Zen Master Baizhang (Wade-Giles: Pai-chang; Japanese: Hyakujō) and demands a "turning word," a phrase intended to prompt one to realization, to be freed from his animal form.

After Baizhang tells him not to ignore cause and effect, the monk confirms that he has been released from his wild fox body and asks to be given a monk's funeral rites. Later, when Baizhang's disciple Huangbo (Wade-Giles: Huang-po; Japanese: Ōbaku) asks what would have happened had the monk not denied cause and effect, Baizhang tells Huangbo to come close so he can answer him. Huangbo steps forward and slaps Baizhang, ostensibly in the awareness that Baizhang had intended to strike him. Baizhang laughs approvingly and compares Huangbo to the Indian monk and Zen patriarch Bodhidharma.

The kōan
Main case
Tanahashi gives the following rendering of the koan:

Wumen's commentary and poem
Shibayama gives the following translation of Wumen's commentary and verse:
Interpretation
The meaning of the kōan has been the object of intense debate and scrutiny within Zen due to its complexity and multi-layered themes. It was rated by Zen Master Hakuin (1686–1769) as a nantō kōan, one that is "difficult to pass through" but has the ability to facilitate "postenlightenment cultivation" or "realization beyond realization" (shōtaichōyō). Important themes include causality (karma in Buddhism), the power of language, reincarnation, and the folklore elements involved in the insertion of the fox into the tale.
Traditionally, interpretations since the time of the Mumonkan have stressed the nonduality of the two understandings of causality expressed within the case. Dōgen, regarded by Sōtō adherents as the founder of their practice, recast the kōan in the later part of his life as an affirmation of the fundamental importance of acknowledging cause and effect. The significance of this debate goes to fundamental issues in Buddhist thought, especially the meaning of enlightenment with respect to cause and effect.
Dōgen's interpretation
Dōgen articulated a doctrine called Genjōkōan which stressed “full participation in and through all phenomena” as the kōan bears on the immediate here-and-now. In this way, the stereotypical account of Dōgen as wholly in opposition to the use of kōans is in error. Dōgen drew upon kōans as an arbiter of philosophical authority and credence to grant his own ideas, often rooted in practical considerations, authenticity and importance.
Dōgen's views on the wild-fox kōan underwent a transformation from the conventional interpretation to an embrace of causality, as can be seen in two writings contained in the Shōbōgenzō, a compilation of fascicles written between the years 1231 and 1253. In contrast to his appropriation of other significant kōans, Dōgen initially supported the conventional understanding of the wild-fox kōan and only later recanted his view. The "Daishugyō" fascicle, written in 1244 (less than a year after Dōgen left the capital Kyoto to build a monastery in Echizen), conforms to the conventional view of the nonduality of the two notions of causality:
This view, which is in accordance with Wumen's position, notes the contingency of causality and non-causality and the need to transcend a limited perspective of cause and effect. However, Dōgen turns against this interpretation later in his life and stakes out a position in opposition to the conventional reading.





