首楞嚴義疏注經 8.486

8.486

地獄治情，鬼中治想，情想既盡，故云成空。然所空者，即依情、想所發之業也。二道之業既亡，却為畜生酬其宿債，駝、驢、牛、馬身命償他。若在餘類，隨應受對。

Hell disciplines emotion, and the ghost realm disciplines thought. Emotion and thought having been exhausted, it is therefore said that they 'become empty.' However, what is emptied is the karma engendered in reliance upon emotion and thought. When the karma of these two destinies has vanished, they turn into animals to repay their past debts, compensating others with their lives and bodies as camels, donkeys, oxen, and horses. If born among other kinds, it corresponds to what they ought to pay.