大般涅槃經義記 9.21

9.21

前明不住，下明欲去。如來於此入大三昧深禪定窟，眾不見故，名入涅槃。此乃寄人顯彰實證，令人學也。問曰：上說如來畢竟常在世間不入涅槃，今以何故言己不住定入涅槃？釋言：佛德有四種門。一共世門，恒在世間未曾暫滅；二住寂門，未曾暫出；第三反寂起用之門，現化隨物；第四息用歸寂之門，捨化歸真。前據初門彰己在世畢竟不滅，今就第四故說不住，入涅槃矣。

The preceding explained non-abiding; below explains the desire to depart. The Tathāgata here enters the cave of the great samādhi and profound meditative absorption; because the multitude does not see him, this is called entering nirvāṇa. This relies on the person to manifest and display true realization, causing people to learn it. Question: Previously it was said that the Tathāgata is ultimately always in the world and does not enter nirvāṇa; why is it now said that he does not abide and definitely enters nirvāṇa? Explanation: The Buddha's virtues have four gates: First, the gate of sharing the world, constantly being in the world without ever ceasing for a moment. Second, the gate of abiding in quiescence, never emerging for a moment. Third, the gate of turning from quiescence to generate functioning, manifesting transformations in accord with beings. Fourth, the gate of ceasing functioning to return to quiescence, relinquishing transformations to return to the real. The former was based on the first gate, manifesting that his presence in the world is ultimately unextinguished; now, relying on the fourth, he speaks of non-abiding and entering nirvāṇa.