According to the Five-period classification that prevailed during the Northern and Southern Dynasties, the Lotus sūtra does not expound the doctrine of the Buddha-nature or the eternal abiding of the Buddha. In this point the Lotus sūtra was regarded as inferior to the Nirvāṇa sūtra. Zhiyi and Jizang criticize this Five-period classification, arguing instead that the Lotus sūtra, like the Nirvana sūtra, also expounds the doctrine of the Buddha-nature or the eternal abiding of the Buddha. If their argument is to be accepted, why did the Buddha preach the Nirvāṇa sūtra after the Lotus sūtra? This is the problem taken up for consideration by Zhiyi and Jizang.