Kōsai

or Kōsai Jōgakubō, was a former monk of the Tendai Buddhist sect and disciple of Hōnen's Pure Land Buddhist lineage. Kōsai, along with another monk called Gyōkū, were famous as advocates of the which proved to be quite popular as well as controversial. Kōsai was eventually exiled to Shikoku during a period of persecution of Hōnen's lineage by the Japanese government.

Kōsai's teaching emphasizes a single faithful moment in which a single recitation of Amitābha's name (nembutsu), would cause a transformative religious experience that was itself sufficient to attain birth in the Pure Land. His teaching emphasizes true faith over practice. This doctrine of "single nembutsu" (ichinen) drew criticism from established Buddhist sects at the time, and even Hōnen's other disciples like Benchō. Kōsai's teaching may have also influenced Shinran (1173–1262), the founder of Jōdo Shinshū, who also emphasizes the primacy of true faith (shinjin) and whose teaching is similar to Kōsai's. '' Along with Gyōkū, Kōsai emphasized faith over practice, though his view did not mean that he rejected practice after the establishment of true faith. Indeed, Blum argues that Kōsai affirmed Hōnen's position on oral recitation, even while emphasizing the mental aspect of nembutsu. Apart from this, it is not known how Hōnen responded to Kōsai's doctrine itself. Some sources from the Chinzei-ha sect of Jōdo-shū, claim that he was expelled from Hōnen's community by Hōnen. According to Blum, since this claim "can be found only in two biographies of Hōnen written by members of this Chinzei lineage" and "is not corroborated by any thirteenth-century source", it is likely that the idea that Kōsai was excommunicated by Hōnen is a later sectarian fabrication of the Chinzei sect.

Figures such as Benchō and Ryōchū, leaders of the Chinzei Pure Land lineage, denounced Kōsai's views as heretical and as a teaching which promotes immorality due to its lack of respect for ethical precepts. However, Mark Blum argues that these critiques as just as likely to be political than moral, and that "the animosity toward those holding the ichinengi position may be more about factional rivalry than moral turpitude."