成唯識論述記 4.731

4.731

述曰：釋前所以。後煩惱等由初斷道，皆已無因，種子無故，永不生故，便成無學。有我本識，雖前起道斷隨應惑，後煩惱起持煩惱種。得初道時不成無學，後斷煩惱而得斷果。若無此識持煩惱種，初道起位，惑種皆無，應初道起即成無學。

The Commentary states: This explains the reason for the above. Because subsequent defilements and the like would all already be without cause due to the initial path of abandonment, since their seeds would be absent and they would never arise again, one would immediately become an aśaikṣa. With our root consciousness, although the path arises earlier and abandons the corresponding defilements, when subsequent defilements arise, it sustains the seeds of defilements. When attaining the initial path, one does not become an aśaikṣa; only after subsequently abandoning defilements does one attain the fruition of abandonment. If this consciousness did not exist to sustain the seeds of defilement, then at the time the initial path arose, the seeds of defilement would all be absent, and one should become an aśaikṣa as soon as the initial path arose.