法華論疏 3.129

3.129

根未熟令熟故者，即是狂子未敢服藥，欲令其服也。示現涅槃量，如父欲令服藥故現死。佛今亦爾，欲令眾生了悟，示現涅槃。

"Because their faculties are not yet mature, causing them to mature" is just like the delirious sons not daring to take the medicine, wishing to make them take it. "Manifesting the measure of nirvāṇa" is like the father feigning death because he wants them to take the medicine. The Buddha is now also like this; wishing to make sentient beings understand and awaken, he manifests nirvāṇa.