俱舍論疏 6.130

6.130

論「相應因體至義何差別」，問也。心、心所法是相應因體，亦俱有因體。既是一因應無二，必應於中有其別相。如是二因有何差別？

The treatise says, "What is the difference in meaning [...] between the essential nature of the associated cause?" which is a question. Mind and mental factors are the essential nature of the associated cause, and also the essential nature of the coexistent cause (sahabhū-hetu). Since they are one cause, there should not be two; there must necessarily be distinct characteristics between them. What is the difference between these two causes?