成唯識論述記 1.296

1.296

述曰：汝我體應是轉變、無常；作用或有不起時故；如許大等。若不約用難，令體亦轉變，即無同喻；彼不許有滅無常故。若直難用，彼思慮用，有時不起故，犯相符過。雖自性體常，用是無常，無不定失；非共許故。又以體例，用亦應無常，相即亦得。

The Commentary states: The substance of your self should be transformational and impermanent; because there are times when its activity does not arise; like the accepted mahat and so on. If one does not challenge it based on function to cause its substance to also transform, then there would be no positive example, because they do not accept impermanence as cessation. If one directly challenges the function, since their function of deliberation sometimes does not arise, one would commit the fallacy of proving what is already accepted (siddhasādhana). Although the substance of its nature is permanent, its function is impermanent, so there is no fallacy of inconclusiveness, because this is not mutually accepted. Furthermore, taking substance as a parallel, function should also be impermanent; taking them as identical is also acceptable.