Chi-tsang is generally considered a great synthesizer of the San-lun school, that means he faithfully and adequately interprets the teachings of "San-lun" (Three Treatises). However, as he lived at a time when the Lotus Sutra and the Nirvana Sutra were immensely popular and the schools of Tilun and She-lun prosperous and influential, Chi-tsang could not avoid borrowing so heavily from the teachings of there schools and scriptures, that his works can no loner be confined within the limits of the San-lun school. The shift indicates breakthrough in Chi-tsang`s own intellectual development, yet it also has negative impact on the circulation of the San-lun teachings. The present paper attempts to show, through a study of his theory of Buddhahood and of Mind-Essence, how Chitsang combines and integrates the doctrine of the middle-path in the Three Treatises with the concept of "One Vehicle" in the Lotus Sutra, of "Buddhahood" in the Nirvana Sutra, and of "Tathagata-Garbha" in Ti-lun school and She-lun school. It also endeavors to show what new elements have been added, through this integration, to the doctrine of the middle path of the Three Treatises. It is believed that the study proves that Chi-tsang`s philosophy has gone beyond the boundaries of the San-lun school.