大乘起信論義記 2.270

2.270

第四、智識者，是事識內細分別，謂不了前心所現境故，起染淨微細分別，故云智也。

The fourth, the cognitive consciousness, is the subtle differentiation within the empirical consciousness; meaning that because of failing to comprehend the sense-objects manifested by the preceding mind, it gives rise to subtle discriminations of defilement and purity, and is therefore called "cognitive".