佛說盂蘭盆經疏 2.98

2.98

"Four assemblies": monks, nuns, laymen, and laywomen; or it is said to mean humans, devas, dragons, and ghosts. Because there is doubt, both are preserved. However, all living beings rely upon their father and mother; therefore, the kindness of parents equals heaven and earth. Although these [beings] had supreme filial piety, they had not found its gateway; now having received the divine prescription, they faithfully know its efficacious proof, that it can certainly eliminate the tribulations of seven lifetimes and repay the toils of the two parents. Knowing in themselves that their minds had attained their goal, they therefore rejoiced and accepted the command.