The Yasen kanna, the best known of all Hakuin’s kana hōgo writings, describes the “Zen sickness” that Hakuin experienced following his initial enlightenments, his visit to the cave of the hermit Hakuyū Sennin deep in the mountains of Kita Shirakawa northeast of Kyoto, and his reception from Hakuyū of the special methods of meditation through which he overcame his illness.

Hakuin’s Zen illness appears to have been a type of chronic, serious pulmonary condition, probably psychosomatic in origin, for which Hakuyu prescibed the contemplative techniques referred to by the hermit as the naikannohō 内観法 (method of introspection) and the nansonohō 軟酥法 (“soft ghee” method). Through assiduous application of these practices Hakuin was fully restored to health. He wrote the Yasen kanna, describing his experiences in narrative form, for the sake of those suffering from similar conditions. It was the most widely read of all Hakin’s kana hōgo works during the Tokugawa period, and remains the most popular of his writings even today. Many reports exist of cures effected through the practice of these techniques.

Despite its popularity, the Yasen kanna cannot be regarded as one of Hakuin’s representative works. In content it is limited to health-promoting techniques, and touches on none of the themes of his religious thought. Its popularity, and the fact that it is the title most often mentioned in association with Hakuin, has in fact had the unfortunate effect of hindering a broader study of Hakuin’s thought.