成唯識論述記 3.605

3.605

述曰：下解譬喻，於中有二，先舉法生滅喻，後廣解之，此即初也。一切因果皆非斷、常，故言法爾。

The Commentary states: Below explains the metaphor. Within this there are two parts: first, presenting the metaphor for the arising and cessation of phenomena; second, explaining it extensively. This is the first. All causes and effects are neither discontinuous nor permanent; therefore it is said to be "intrinsically natural."