大般涅槃經集解 70.47

70.47

案僧亮曰：牛馬互無，則無性無常，不對治無常，不得稱常。涅槃對病為譬也。

Sengliang states: The mutual non-existence of cow and horse means that the absence of their nature is impermanent; it does not counteract impermanence, so it cannot be called permanent. Nirvāṇa is used as an analogy to counter affliction.