成唯識論述記 5.99

5.99

述曰：曾住，現在將滅之時，已能為依，於今時，識為開導故。彼設若不去，後不得生故。由前理、教故，知不假異類之識為開導依。

The Commentary states: Having once abided, when it was in the present and on the verge of cessation, it was already capable of serving as a basis, opening and guiding the consciousness of the present time. If that had not departed, the subsequent could not arise. Through the preceding reasoning and teaching, we know that there is no reliance on a consciousness of a different category to serve as an opening-and-guiding basis.