佛說盂蘭盆經疏 2.20

2.20

觀求生處，是天眼通。繇證道而得，故云道眼。世間有二，謂三界是器世間，六道是有情世間。然尊者喪親之日猶是凡夫，不知父母生于何道。今成聖果，力可追求，故以天眼上下觀視，于三界處尋六道身。得通便觀，故云即也。

Viewing and searching for the place of rebirth is the superknowledge of the divine eye. Because it was obtained through realizing the path, it is called the 'eye of the path'. As for 'the world', there are two kinds: the three realms are the world of vessels (bhājana-loka), and the six destinies are the world of sentient beings (sattva-loka). However, on the day the Venerable One lost his parents, he was still an ordinary person and did not know into which destiny his parents had been born. Now, having attained the noble fruit, his power was sufficient to seek them out; therefore, with the divine eye, he viewed upward and downward, searching for their bodies among the six destinies within the three realms. Because he viewed immediately upon obtaining the superknowledge, the text says 'immediately'.