八識規矩補註 2.15

2.15

This means that the sixth consciousness designates this seventh [consciousness] as the basis of defilement and purity. For, because this consciousness is contaminated and constantly grasps a self internally, it causes the sixth consciousness to become defiled from moment to moment; because this consciousness becomes uncontaminated and constantly contemplates selflessness, it causes the sixth consciousness to become pure from moment to moment. Thus, whether the sixth becomes defiled or becomes pure is entirely due to the seventh.