妙法蓮華經玄義 2.22

2.22

As for 'such force', it is evil efficacy. For example, like a piece of material: although it has not yet been used, by pointing out what it is capable of, one says it is useful. The Mahāparinirvāṇa-sūtra says: 'To build a house one takes wood, not thread; to make cloth one takes thread, not mud or wood.' The hells have the function of ascending knife-trees and sword-mountains; the hungry ghosts have the function of swallowing molten copper and eating red-hot iron; among animals, the strong subdue the weak, fish chew upon one another, and they haul carts and pull heavy loads—these are all evil forces and functions.