大乘義章 11.67

11.67

對境別者。慈心多緣無樂眾生；悲心多緣有苦眾生；喜心多緣得樂眾生；捨緣究竟解脫眾生，以彼究竟得解脫故心即放捨。又捨多緣怨親及中三品眾生，捨此等故。以斯境別，故分四種。故《涅槃》云「器若有慈，即不得有悲喜捨心，餘亦如是。」故立四種（此三門竟）。

As for the difference in objective sphere: the mind of loving-kindness mostly takes as its object sentient beings devoid of bliss; the mind of compassion mostly takes as its object sentient beings possessing suffering; the mind of sympathetic joy mostly takes as its object sentient beings who have attained bliss; equanimity takes as its object sentient beings who are completely liberated, because since they are completely liberated, the mind simply lets them go. Furthermore, equanimity mostly takes as its object the three grades of sentient beings—enemies, kin, and neutral ones—because it relinquishes these differences. Because of these distinct objective spheres, they are divided into four kinds. Hence the Nirvāṇa [Sūtra] states: 'If a vessel has loving-kindness, it cannot have the minds of compassion, sympathetic joy, or equanimity; the others are also like this.' Therefore, four kinds are established (this completes the third section).