Scholars disagree about when the dispute over the precepts began. One scholar has suggested that Saichō may have been influenced by some of Dōchū’s disciples who, following Chien-chen’s teachings, called for a more open Buddhist order. Others have suggested that on his trips to Kyushu and Tōgoku, Saichō may have gone to see the kaidan (precepts platforms) in these two areas, and thus may have been considering reforms as early as 814. Neither of these theories has been proven.

The dispute began in earnest with Saichō’s efforts to have the court designate his monastery, Hieizanji, a Mahāyāna temple. On the seventh day of the second month of Konin 9 (late 817) Kōjō reports that Saichö told him that he wanted “to establish a Mahāyāna temple in order to propagate the (Tendai) School.” Kōjō seems to have been surprised by Saichō’s statement. According to Kōjō’s Denjutsu isshinkaimon, the following conversation took place:

The disciple (Kōjō) said, “There are no Mahāyāna temples in Japan. Why must we now suddenly establish a One-vehicle temple (ichijōji)?”

Our late teacher (Saichō) said, “I will bestow on you the title ‘One-vehicle (ichijō)’.”

His disciple (Kōjō) said, “Since there are no Mahāyāna temples in Japan yet, I will not take the One-vehicle title. Please bestow it on me after we establish a Mahāyāna temple.”

Our late teacher said, “In India there are purely Mahāyāna temples, purely Hinayāna temples and mixed (Mahāyāna and Hinayāna) temples.”

His disciple said, “If these three types of temples exist, then I ought to take the title. Temples are, after all, only places where monks reside.”

At this time, no mention was made of precepts. Exactly what the term ‘Mahāyāna temple’ or ‘One-vehicle temple’ designated and how such an institution would differ from the temples in Japan at that time is not specified in the conversation. Probably it was the beginning of an attempt by Saichō to develop a monastic institution which would embody, in a practical way, the issues being debated with Tokuitsu. If the court had recognized Saichō’s claim that Hieizanji was a Mahāyāna temple, it certainly would have enhanced the reputation of the Tendai School at the expense of the Hossō and Sanron schools by implying that they did not have purely Mahāyāna temples.

The Buddha-nature was a controversial topic in India, China and Japan. In his classic study of the long history of these disputes, Tokiwa Daijō (1870-1945) argued that the debate between Saichō and Tokuitsu represented the culmination of the disputes over the Buddha nature. The issues were probably more clearly demarcated and discussed by both men than at any other time in East Asian Buddhist history. Many new issues were introduced into the debate and their relation to the subject of the Buddha-nature investigated. Saichō displayed considerable ingenuity and an impressive command of Buddhist literature in drawing upon a variety of sources to formulate and defend his position. At the same time, the debate with Tokuitsu prepared the ground for the controversy over the bodhisattva precepts (bosatsukai) by indicating that the Tendai and Hossō positions were irreconcilable. Saichō’s petitions concerning the bodhisattva precepts … represent his attempt to formulate practices which would reflect the more theoretical statements which he had advanced in his works criticizing Tokuitsu.

[Page 90-91] The turning point in Saichō’s career began with a trip to Tōgoku (Eastern provinces) in 817. … During his travels in Tōgoku, Saichō probably heard about the Hossō Monk Takuitsu. … [Page 96] The dispute between Saichō and Takuitsu covered a wide range of problems, including the proper method of classifying the Buddha’s teachings, the evaluation of Tendai methods of exegesis of the Lotus Sūtra, and the criticism of various Tendai doctrines and meditations. Both men offered detailed and penetrating arguments in support of their positions. …

[Page 105-106] In its later stages, the debate focused on the exegesis of the Lotus Sūtra, the most authoritative scripture for the Tendai School.

Saichō often referred to his own school as the Tendai Hokkeshū, the Tendai School of Lotus Sūtra interpretation. It was not the only school which interpreted the Lotus Sūtra, since both the Hossō and Sanron schools had long traditions of exegesis of the Lotus Sūtra. Thus Saichō’s task was to show that the Tendai interpretation was the most authoritative one.

Tokuitsu followed the orthodox Hossō interpretation, as it was presented in Tz’u-en’s (632-682) commentary, the Fa hua hsüan tsan. According to Tz’u-en’s writings, the Buddha had a hidden purpose in preaching the Lotus Sūtra. He wanted to encourage people of undetermined nature to strive to become bodhisattvas, rather than being content with merely becoming or striving to become arhats or pratyekabuddhas. The One-vehicle doctrine was an expedient teaching (gonkyō) directed toward those of undetermined nature. The claims of the Lotus Sūtra that it was the ultimate teaching (jitsukyō) were an expedient designed to encourage this particular class of practitioner and were not to be considered as teachings which were universally valid. Predictions in sūtras that certain people would attain Buddhahood were said to refer to the individual’s possession of the gyōbusshō and to not have any general significance. Statements that all beings had the Buddha nature, such as that by the bodhisattva Jōbukyō (Never-disparaging), were said to refer only to the inactive ribusshō.

In contrast to the Hossō position presented above, Saichō believed that the Lotus Sūtra was the Buddha’s ultimate teaching, a direct revelation of the Buddha’s enlightenment, which was valid for all men and not just intended for one particular group. Saichō supported his arguments by referring to passages in the Lotus Sūtra that stated that the sūtra was indeed the Buddha’s ultimate teaching, by defending the Tendai classification of Buddhist doctrine, and by attacking the Hossō classification of Buddhist doctrine. The defense and exegesis of the The teachings of the One-vehicle, the universality of salvation, and the speedy realization of salvation were eventually accepted as standard Buddhist teachings throughout Japan. The new schools of the Kamakura reformation developed out of the Tendai School and adopted the positions that Saichō had defended, though not without changes. Even some Hossō monks such as Ryōhen (1194-1252) tried to reconcile the differences between Hossō and Tendai by devising positions which allowed for universal salvation and the quick attainment of salvation.