大方廣圓覺修多羅了義經略疏 2.1339

2.1339

良以推窮迷本，照徹覺源，是以理貫群經，義無不盡。於此若解，則諸教煥然；若不了之，何知正道？故云眼目。

This is precisely because it investigates and exhausts the root of delusion and thoroughly illuminates the source of enlightenment; therefore, its principle threads through all sūtras, and its meaning leaves nothing unexhausted. If one understands this, then all teachings become luminous; if one does not understand it, how could one know the correct path? Hence it is called the "eye."