大乘起信論略述 1.148

1.148

論。名為意識至分別事識　述曰。此制立其名也。名意識者。既緣六塵。應名六識。而今就其一意識義故。不別出眼等五識。但名意識。故涅槃云。譬如一識分別說六。即此意識依於六根別取六塵故云分離。又能分別去來內外種種事相故復說名分別事識。

Treatise: It is named mental consciousness... consciousness of discriminating phenomenal objects.
Commentary: This establishes its name. As for being named "mental consciousness": since it conditions the six dusts, it should be called the six consciousnesses; yet here, in terms of the meaning of a single mental consciousness, it does not separately bring out the five consciousnesses of the eye and so on, but is only named "mental consciousness." Therefore the Mahāparinirvāṇa-sūtra says: "For example, a single consciousness is discriminatorily explained as six." Because this mental consciousness relies on the six sense-bases to separately apprehend the six dusts, it is said to "separate"; and because it can discriminate past and future, internal and external, various phenomenal marks, it is further named the "consciousness of discriminating phenomenal objects".