俱舍論記 20.18

20.18

「又已謝業至有現因在」者，釋有果故。又已落謝過去世業，有當果故，顯有二世。謂若實無過去體者，善、惡二業其體應無。由業無故，未來當果亦應無有。非現在果生時有現因在，以異熟果非因俱故。又解：非未來果生時有現因在，以異熟果非與因無間故。又解：非現、未果生時有現因在，以異熟果非與因俱及無間故。

Regarding "Furthermore, ceased karma ... there is a present cause existing": this explains "because karma has a result". Furthermore, karma that has already ceased in the past period has a future result, which demonstrates that the two periods exist. That is, if past entities were substantially non-existent, the entities of wholesome and unwholesome karma would be non-existent. Because karma would not exist, the future result would also not exist. It is not the case that when a present result arises there is a present cause existing, because a fruition result (vipāka-phala) is not simultaneous with its cause. Another explanation: it is not the case that when a future result arises there is a present cause existing, because a fruition result is not immediate to its cause. Another explanation: it is not the case that when present and future results arise there is a present cause existing, because a fruition result is neither simultaneous with nor immediately contiguous to its cause.