大般涅槃經義記 8.168

8.168

問曰：何故性淨之因直名為因、方便之因名為因因，及至果中方便之果直名為果、性淨之果名為果果？釋言：互從皆得無傷，但彼因中從本起末，因緣之理是行根本，故直名因，方便之因從前起故名曰因因。至果之時攝末歸本，方便菩提對前滿足，已受果稱，德成歸本，故名性淨以為果果。

Question: Why is the cause of natural purity directly called "cause" while the cause of expedient means is called "cause-of-cause", and when it comes to the effects, the effect of expedient means is directly called "effect" while the effect of natural purity is called "effect-of-effect"? Answer: Deriving them reciprocally from either is entirely permissible and without flaw. However, in the cause, the branches arise from the root; the principle of dependent origination is the root of practice, and hence is directly called "cause", while the cause of expedient means arises from the preceding and is therefore called "cause-of-cause". When arriving at the effects, the branches are gathered back to the root; bodhi as expedient means is complete relative to the preceding and has already received the designation "effect", and when virtues are perfected they return to the root, so that natural purity is called "effect-of-effect".