瑜伽論記 13.28

13.28

有之因緣者，此有第八之所以也，雖言說有，未知有之所以，既知所以，有故第八存之，其與現數受等俱轉。

Regarding 'the causes and conditions of its existence': this is the reason why the eighth consciousness exists; although its existence is spoken of, the reason for its existence is not known; once the reason is known, because it exists, the eighth consciousness is maintained, and it operates together with active mental factors such as feeling.