大乘起信論義記 2.217

2.217

First, as for "the mark of cognition": upon the marks manifested by the preceding manifesting consciousness, failing to understand that they are the manifestation of one's own mind, the mental factor of discernment initially arises to differentiate defilement and purity and grasp them as having fixed natures; hence it says, "based on objects... between the pleasant and the unpleasant."