摩訶止觀 6.72

6.72

空門者，《大經》云：「迦毘城空，如來藏空，大涅槃空。」又云：「令諸眾生悉得無色大般涅槃。」涅槃非有，因世俗故名涅槃有。涅槃非色、非聲，云何而言可得見聞？即是空門。

As for the emptiness gate, the *Mahāparinirvāṇa Sūtra* says: "The city of Kapilavastu is empty, the tathāgatagarbha is empty, and great nirvāṇa is empty." It also says: "Causing all sentient beings to attain formless mahāparinirvāṇa." Nirvāṇa is not existent; because of the conventional, nirvāṇa is called existent. Nirvāṇa is neither form nor sound; how could it be said that it can be seen or heard? This is the emptiness gate.