大般涅槃經集解 55.44

55.44

案僧亮曰：云何受身者，有二種：有自在、不自在，知如來自在也。何故受身者，知如來為眾生故受也。為誰受身者，為中有等不等，明如來等眾生也。僧亮曰：云何而受是身者，乃知感應之道，而不能知所以爾耶，謂以大悲之心故受身也。

Sengliang comments: Regarding "how [the Tathāgata] receives a body": there are two kinds [of receiving a body], being autonomous and not autonomous; this is to know that the Tathāgata is autonomous. Regarding "for what reason he receives a body": this is to know that the Tathāgata receives it for the sake of sentient beings. Regarding "for whose sake he receives a body": among those [beings] there are equals and unequals; this explains that the Tathāgata treats sentient beings equally. Sengliang says: Regarding "how he receives this body": one may know the way of sympathetic resonance, yet fail to know the reason why it is so; it means that he receives a body out of a heart of great compassion.