Kukai (774 to 835), founder of the Shingon school upon his return from studies in Tang China, taught that an ordinary person could become a buddha in this very body (sokushinjobutsu) rather than across countless future lifetimes, and the three mysteries (sanmitsu) are the mechanism his own writings give for how that happens. His treatise on attaining buddhahood in this body, collected among his major works, sets out the teaching that a buddha's own body, speech and mind are themselves the ultimate reality of all things, present but concealed in ordinary experience, and that a practitioner who forms the buddha's hand gesture (the mudra, the mystery of body), recites the buddha's true word (the mantra, the mystery of speech) and holds the buddha's mind through contemplation (the mystery of mind) draws the practitioner's own three activities into correspondence with the buddha's, so that the two come into what Kukai calls mutual empowerment (kaji) rather than one simply absorbing the other. Scholarship on Kukai's esoteric system reads this triad against the wider Mahayana teaching of buddha-nature: where exoteric Buddhism generally treats buddhahood as a distant attainment, Kukai's own argument is that the universal buddha Mahavairocana's activity is already identical with body, speech and mind as such, and ritual practice does not create that identity but discloses it. The doctrine itself is described, not withheld, in Kukai's published writings and in the academic study built on them.