成唯識論述記 8.439

8.439

述曰：此論主答。初總文也，謂識等五支正在因位，差別之相難以了知，遂依當生之位，別立因支為五。

The Commentary states: This is the author's answer. First is the general text, which means that while the five branches, namely consciousness and so forth, are precisely in their causal stage, their differentiated marks are difficult to know; thus, based on their stages of future rebirth, the causal branches are separately established as five.