俱舍論記 8.48

8.48

「三界無邊至猶若虛空」者，答。三界無邊，所以度生難盡。若依化地部，即有始起有情不從業、惑生，第二身已去方從惑、業生。由數生故，佛度難盡。

"The three realms are boundless [...] just like empty space": the reply. The three realms are boundless, which is why liberating sentient beings is difficult to exhaust. If one relies on the Mahīśāsakas, then there are sentient beings who newly begin without arising from karma and defilement; only from the second life onward do they arise from defilement and karma. Because they are repeatedly born, it is difficult for the Buddhas to liberate them exhaustively.