法華論疏 3.438

3.438

聞香知，此釋聞香能知也。此是知境者，香者是鼻根所知境也。以鼻根知故者，出能知也。又以眼所見境者，以鼻根知，故是互用。

"Knowing through smelling fragrance" explains knowing through smelling fragrance. "This is knowing the cognitive object": fragrance is the cognitive object known by the nose faculty. "Because it is known through the nose faculty" identifies the subject that knows. Moreover, the cognitive object seen by the eye is known by the nose faculty; hence it is interchangeable use.