成唯識論述記 5.244

5.244

述曰：此即外人乘前起問。所以者何？此意因中恒時染污，欲明惑本，先明前四。遍行五數，諸識定有；別境等法，或隨有無，故在後門。

The Commentary states: This is the opponent raising a question following the preceding [discussion]. Why is that? In the causal state, this manas is constantly defiled; wishing to clarify the root of defilement, one first clarified the preceding four. The five omnipresent factors are definitely present in all consciousnesses; factors such as the particular [mental factors] may be present or absent, and therefore are placed in the subsequent section.