大般涅槃經疏 31.15

31.15

「虛空無故非是過去」下，破外道執中次結同異。所言同者，涅槃、虛空同無三世。所言異者，涅槃是常，虛空是無。又三：初明空非三世；次舉兔角亦非；「是故我說」下，三結同異。

From "Because space is non-existent, it is not of the past" downwards, in refuting the attachments of the non-Buddhists, it next concludes with their similarities and differences. What is meant by similarity is that nirvāṇa and space are alike in lacking the three periods of time. What is meant by difference is that nirvāṇa is permanent, while space is non-existent. There are again three parts: first, clarifying that space is not of the three periods of time; next, citing the rabbit's horns as also not [of the three periods of time]; from "Therefore I say" downwards, third, concluding with the similarities and differences.