金光明經文句記 2.378

2.378

初、古釋可知故無常。言此語者，唯除如來句也。壽量既為智之所知，是可思法，驗是無常。

First, the ancient explanation that being knowable means impermanence. Saying 'this phrase' refers to the line 'except only the Tathāgata'. Since the lifespan is known by wisdom, it is a conceivable dharma, which proves it is impermanent.