大般涅槃經疏 1.16

1.16

已將五序別屬當竟，更說一序各序五章。夫眾不出八佛，八佛與現佛不異名如，無非佛者故名為是。二施無差故言如，同入祕藏故言是。如來之行故言如，一行遍攝故言是。同一佛性故言如，義體不二故言是。邪惡不二故言如，更無過者故言是。當知如是通序五章。

Having finished assigning the five prefaces respectively, we will further explain how each single preface prefaces all five chapters. The assembly does not go beyond the eight buddhas; the eight buddhas being non-different from the present Buddha is called 'Thus', and there being none who are not buddhas is called 'is so'. The two givings being without difference is called 'Thus', and entering the secret treasury together is called 'is so'. Being the practice of the Tathāgata is called 'Thus', and one practice ubiquitously encompassing [all] is called 'is so'. Being identical in the one buddhadhātu is called 'Thus', and the essence of the meaning being non-dual is called 'is so'. Heterodoxy and evil being non-dual is called 'Thus', and there being none surpassing this is called 'is so'. One should know that 'Thus' commonly prefaces all five chapters.