大乘起信論廣釋卷第三．四．五 5.70

5.70

論。亦不得至以心除心　釋曰。下除失念。於中有二。初舉非總制。後示除方便。此初也。亦不得隨心等者。此令不起外失念心。謂修止者。於前遠離能取所取無所得中常住作害。當初離於倒境倒心。後亦不隨心失念於外境。故有說言。具化心□不□相心即其義也。後以心除心者。此今不起內失念相。前心失念。念著境時既依止。心而捨於境。後心失念。念著內心。即復以其安住止心除其失念內取之心故新論言。前心依境以捨於境。後念依心復捨於心失念相也。

Treatise: "Nor should one... use the mind to eliminate the mind."
Commentary: Below is removing loss of mindfulness. In this there are two: first, citing what is wrong to provide a general prohibition; second, showing the expedient means for elimination. This is the first.
"Nor should one follow the mind..." and so forth: this instructs one not to give rise to an external mind of lost mindfulness. That is, the practitioner of calming constantly abides and applies effort within the previously mentioned state of being detached from both grasper and grasped, having nothing to attain. At first, one separates from inverted objects and the inverted mind, and afterwards one also does not follow the mind to lose mindfulness regarding external objects. Therefore some say, "having a transforming mind [...] a mind of non-[...] signs" is precisely this meaning.
Next, "use the mind to eliminate the mind": this instructs one not to give rise to internal signs of lost mindfulness. When the preceding mind lost mindfulness and, clinging in thought to an object, one then abandoned the object based on calming the mind; when the subsequent mind loses mindfulness and clings in thought to the internal mind, one then again uses that mind settled in calming to eliminate its internal grasping mind of lost mindfulness. Therefore the new translation says: "the preceding mind relies on the object to abandon the object; the subsequent thought relies on the mind to further abandon the mind's signs of lost mindfulness."