首楞嚴義疏注經 3.78

3.78

揀異前五也。逆流，猶返緣也。地，處也。前五但順取外境，不能返緣內塵，名此內塵為緣不及處。此不及處唯意根合，即此合處為意知根。

This distinguishes it from the previous five. 'Flowing against the current' is like conditioning in reverse. 'Place' means location. The previous five only comply with grasping external objects and cannot condition internal dusts in reverse; this internal dust is called the place that conditioning cannot reach. This unreachable place joins only with the mind faculty; precisely this place of joining is the mind-knowing faculty.