瑜伽師地論略纂 4.57

4.57

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.