成唯識論述記 4.855

4.855

述曰：若五根五識種，第六根應意識種。《攝論》第一以五色根為同法故，第六有別根。五識亦應有根，五識既以種子為根。意識應爾，何須別立？若立六識有現根者，五為同法，例亦應然。

The Commentary states: If the five faculties were seeds of the five consciousnesses, the sixth faculty should be the seed of mental consciousness. Fascicle 1 of the Mahāyānasaṃgraha uses the five physical faculties as analogous dharmas; therefore the sixth has a separate faculty. The five consciousnesses should also have faculties; since the five consciousnesses take seeds as faculties, mental consciousness should do so as well, so why establish a separate one? If one establishes that the sixth consciousness has an actual faculty, with the five as analogous dharmas, by analogy it should also be so.