大乘起信論略述 1.135

1.135

論。四者名為至染淨法故　述曰。是事識中細分法執。不了現識所現境界。離心無有。而起染淨微細分別。故言智也。

Treatise: Fourth is named [the knowledge consciousness (pravicaya-vijñāna), because it discriminates defiled and pure phenomena].
Commentary: This is the subtle aspect of attachment to phenomena within the consciousness of phenomenal objects (vastu-prativikalpa-vijñāna). Failing to realize that the realms manifested by the manifesting consciousness do not exist apart from the mind, it gives rise to subtle discriminations of defilement and purity; therefore it is called "knowledge".