成唯識論演祕 7.143

7.143

言無常行緣初、後性，此能緣行非是十六行中無常，是十六外緣無常心。何所以者？十六行中無常但緣三諦為境；初性非諦，後性無為，非無常故。

Saying that the impermanence ākāra takes the first and last natures as ālambana: this apprehending ākāra is not the impermanence among the sixteen ākāras, but a mind apprehending impermanence outside the sixteen. What is the reason? Impermanence among the sixteen ākāras only takes the three truths as its realm; the first nature is not a truth, and the last nature is unconditioned and not impermanent.