大乘義章 1.128

1.128

問曰：經說生死是其無常無樂無我不淨，涅槃是其常樂我淨，云何言空？論言「為破顛倒心故，宣說生死苦無常等，不欲令人著無常等，以實相中無常苦等不可得故。又為破人顛倒心故，宣說涅槃常樂我淨，不欲令人著常樂等，亦以實中常樂淨等不可得故。」良以諸法皆不可得故，故名為如。

Question: Sūtras teach that saṃsāra is impermanent, without bliss, non-self, and impure, while nirvāṇa is permanent, blissful, self, and pure; why say they are empty? The treatise says: 'In order to demolish inverted minds, [the Buddha] taught that saṃsāra is suffering, impermanent, and so forth, not wishing to cause people to become attached to impermanence and so forth, because in the real mark impermanence, suffering, and so forth cannot be found. Furthermore, in order to demolish people's inverted minds, he taught that nirvāṇa is permanent, blissful, self, and pure, not wishing to cause people to become attached to permanence, bliss, and so forth, also because in reality permanence, bliss, purity, and so forth cannot be found.' It is truly because all dharmas cannot be found that it is called tathatā.