The definition of the One Vehicle 一乗 is a key issue in the East Asian Buddhism. Many scholar-monks discussed the ten principles 十義 explained in a verse of the Mahāyāna­saṃgraha 摂大乗論, the ten point explanation of for whom the Buddha taught and of the Buddhaʼs intentions in preaching the One Vehicle.

This paper examines how the interpretations of the ten principles have developed in the Sanron school 三論宗. Among the monks of this school, the ten principles were traditionally discussed according to the thesis of the Yisheng foxing jiujing lun 一乗仏性究竟論 written by Fabao 法宝 （fl. 7-8c）. The Ichijō Busshō enichi shō一乗仏性慧日抄 written by Enshū 円宗 （fl. 9c） has almost the same interpretation of the ten principles as Fabaoʼs theory. In the medieval period, Chinkai 珍海 （1091-1152） also took up the ten principles using the Yisheng foxing jiujing lun as a starting point for his discussion in the Sanron genshō mongiyō三論玄疏文義要. However, he set Jizangʼs 吉蔵 （549-623） interpretation of the One Vehicle as his theoretical pillar to elaborate his exegesis.

Hakkyō 八教,the famous doctrinal classification (教判論) of the Tendai school, constitutes the eight classifications of the Buddha’s teaching, which are divided into two categories; four kinds of teaching content (化法四教), and four methods of teaching (化儀四教). The former category consists of Hīnayāna teachings (蔵教), Common teachings (通教), Distinct teachings (別教), and Perfect teachings (円教). The latter category consists of Sudden teachings (頓教), Gradual teachings (漸教), Secret teachings (秘密教), and Indeterminate teachings (不定教). However, Saichō最澄 (767–822), the patriarch of the Japanese Tendai School, deviated from the traditional usage of these teachings and incorporated them with the kehō no ton 化法之頓 (content of Sudden teachings) and kehō no zen 化法之漸 (content of Gradual teachings). Kehō no ton and kehō no zen are synonymous with kyōton 教頓 and kyōzen 教漸,which are terms also used in Saichō’s works.

This paper examines the ideological source of Saichō’s unique usage and his motivation for introducing these phraseologies. I first give an overview of the genealogy of the doctrinal classification of the Sudden and Gradual teachings employed by Chinese Tiantai scholars before Saichō. On one hand, the concept of kyōton is based on Zhanran’s 湛然 (711–782) exegesis of Zhiyi’s 智顗 (538–597) doctrinal classification. In contrast, the concept of kyōzen is newly introduced by Saichō. Second, to clarify how Saichō introduced kyōzen, I focus on the context and the usage of kehō no zen in the first volume of the Shugo kokkaishō 守護国界章 (Essay on Protecting the Nation). In this work, Saichō disputed Tokuitsu’s 徳一 (8–9c) criticism of the Gradual teachings. This indicates that the Gradual teachings are the key issue for Saichō in maintaining his doctrinal legitimacy. Therefore kyōzen is newly introduced by Saichō.

In the middle volume of the Shugo kokkaishō (守護国界章, Essays on Protecting the Nation), written by Saichō 最澄 (767–822) in 818 as part of a doctrinal dispute with Tokuitsu 徳一 (d.u.), the question of the interpretation of the Lotus sūtra and Saddharmapuṇḍarīkopadeśa is taken up. In previous studies, it has been pointed out that this discussion is deeply linked to Saichō’s Hokkeron kamon 法華論科文 (Textual Organization of the Saddharmapuṇḍarīkopadeśa), and that the Hokkeron kamon may have been a personal study conducted in preparation for the Shugo kokkaishō. The present paper compares the explanation in the Hokkeron kamon with the argument found in the Shugo kokkaishō concerning “the five section paragraphs” 五分科 of the Skillful Means Chapter 方便品 of the Saddharmapuṇḍarīkopadeśa as paragraphs on the Skillful Means Chapter in the Lotus sūtra. I first confirm the definition of “the five section paragraphs” of the Skillful Means Chapter in the Saddharmapuṇḍarīkopadeśa. Secondly, I give an overview of genealogy, applying it to paragraphs on the Skillful Means Chapter in the Lotus sūtra in Zhanran’s 湛然 (711–782) Fahua wenju ji 法華文句記, Kuiji’s 窺基 (632–682) Fahua xuanzan 法華玄賛 and Saichō’s Hokkeron kamon. Finally, I examine the discussion of the middle volume of the Shugo kokkaishō and consider the relationship of the Hokkeron kamon with the discussion of the Shugo kokkaishō.