大乘起信論義疏 1.46

1.46

自下第六釋名。論主何故名真如者？是心法中無所妄故，名之為真。故言「此真如體無有可遣」。妄法可遣，諸法皆真故無可遣，故與真名。是心法中，恒沙佛法無差別相，故名為如。非如生死彼此，差別定性也，故言「亦無可立」。前明真故無有可遣，然應可存，釋此疑故言亦無可立。何故無可立者？以一切法皆同如故也。

From here below, the sixth [topic] explains the name. Why does the treatise author call it tathatā? Because within this mind-dharma there is nothing delusive, it is called "true" (tatha). Therefore it says: "This essence of tathatā has nothing that can be dispelled." Delusive dharmas can be dispelled, but all dharmas are true and thus there is nothing to be dispelled, hence it is given the name "true." Within this mind-dharma, the buddha-dharmas [as numerous as] the sands of the Ganges lack differentiating characteristics, and are therefore called "such" (tathā). They are not like the differentiating, determinate nature of self and other in saṃsāra, and therefore it says: "Nor is there anything that can be established." Previously, it clarified that being true, there is nothing that can be dispelled, yet it might be preserved; to resolve this doubt, it says that there is also nothing that can be established. Why is there nothing that can be established? Because all dharmas are identically such.