Saichō’s discussions of the rapid realization of buddhahood are found in his polemical writings criticizing the Hossō school and defending Tendai teachings. He argued that Tendai teachings were superior because they led to buddhahood more rapidly than Hossō practices. Tendai practices could benefit everyone, but Hossō practices would not result in buddhahood for anybody in Japan because nobody could follow them. Saichō argued that the perfect religious faculties (enki) of the Japanese had already matured so that they need not bother with lesser teachings.

Saichō introduced and developed the term sokushin jōbutsu [attainment of buddhahood in this very body] in his last written work, Hokke shūku, as a part of his program to demonstrate the superiority of the Lotus Sutra and the Tendai interpretation of it. The power of the Lotus Sutra to lead the practitioner to realization with his current body is introduced as the eighth of ten reasons why the Lotus Sutra is superior to other texts. The following passage demonstrates how Saichō employed the description of the dragon king’s daughter to prove that the Lotus Sutra applied to all sentient beings and would quickly bring them salvation.

This passage (about the dragon king’s daughter) concerns those beings who can realize buddhahood only with difficulty and reveals the power of the Lotus Sutra to help them. She is an animal, (one of lower levels of the) six destinies [realms], obviously the result of bad karma. She is female and clearly has faculties which are not good. She is young and thus has not been practicing religious austerities for a long time. And yet, the wondrous power of the Lotus Sutra endows her with the two adornments of wisdom and merit. Thus we know that the power of the Lotus Sutra reveals it to be the jewel among the scriptures and a rarity in the world.