瑜伽師地論略纂 4.57

4.57

又復說言：我觀世間有情，不易可得，長夜流轉，不為汝等母父等者，不易者，難也。難可得諸有情不為汝之母父，一切皆曾為父母等義。前為父母更互作義，此皆為故，是二差別。

Furthermore, it is said: 'I observe sentient beings in the world, and it is not easy to find one who, wandering in the long night [of saṃsāra], has not been your mother or father...' 'Not easy' means difficult. It is difficult to find sentient beings who have not been your mother or father, meaning all have been your parents and so forth. The previous [point] was that parents acted reciprocally; here, it is that all have been so; this is the difference between the two.