Tokuitsu 徳一 (c. 760–835, traditional lifedates) was an early-Heian Japanese Hossō 法相 (Yogācāra / Dharmalakṣaṇa) school scholar of the Aizu 會津 region in the eastern Kantō. He is one of the most famous figures of early-Heian Japanese Buddhist scholasticism, principally remembered for his doctrinal-disputational confrontations with the two great founders of the Heian-era Buddhist schools — Saichō 最澄 (founder of Tendai) and Kūkai 空海 (founder of Shingon).
His confrontation with Saichō is the better-documented: the famous Tendai-Hossō one-vehicle / three-vehicle controversy (一乘三乘論爭) of 817–821, in which Tokuitsu argued for the Hossō three-vehicle doctrinal position (that the Buddha’s teaching is variously suited to three different capacities) against Saichō’s Tendai one-vehicle claim. The exchanges produced substantial Tokuitsu treatises (Busshō shō 佛性抄 and others) that are documented in the polemical tradition.
His confrontation with Kūkai is preserved (in part) in the KR6t0164 Zhēnyán zōng wèijué wén 眞言宗未決文 — a one-fascicle Q&A treatise of unresolved doubts about the Shingon school, framed as Tokuitsu’s polemical questioning of the Shingon doctrinal program. Whether the work is genuinely by Tokuitsu, or a later redaction of his arguments by Hossō partisans, is debated in modern scholarship; the work is traditionally attributed to Tokuitsu but bears certain features (later vocabulary, references to subsequent doctrinal developments) suggesting later redaction.
Tokuitsu’s importance lies less in his individual doctrinal positions than in his role as the principal early-Heian apparent-school challenger to the new Esoteric Buddhism of Kūkai and the new Tendai of Saichō. The Tendai-Hossō and Shingon-Hossō polemical exchanges of the 810s–830s established the doctrinal-disputational framework within which all subsequent medieval Japanese Buddhist sectarian-polemical literature operates.
Source: DILA Buddhist Person Authority A001712; standard Japanese Hossō-school and early-Heian sources; Paul Groner, Saichō: The Establishment of the Japanese Tendai School.