俱舍論疏 24.30

24.30

論「言上流者至方般涅槃」，釋上流也。於中有二：一總釋、二別釋。此文總釋。言上流者，是上行義。前生般者雖生上界，即彼生處而般涅槃更不上生，無上行義，故生般等不名上流。若生色界要轉生上，皆名上流，有上行故。

On the treatise passage, "As for the one who goes upstream... only then attains parinirvāṇa": This explains the one who goes upstream (ūrdhvasrotas). Herein there are two parts: first, a general explanation; second, a specific explanation. This text is the general explanation. "Goes upstream" means "proceeding upward." As for the former who attains parinirvāṇa upon rebirth, although they are reborn in an upper realm, they attain parinirvāṇa right in that place of rebirth and do not take further rebirth higher up; they lack the meaning of proceeding upward, which is why those who attain parinirvāṇa upon rebirth and so forth are not called "upstream." If, having been reborn in the form realm, one must successively take birth higher up, all such are called "upstream," because there is upward progression.