When we use the body and mind to look at forms, and when we use the body and mind to listen to sounds, even though we are sensing them directly, it is not like a mirror’s reflection of an image, and not like water and the moon. While we are experiencing one side, we are blind to the other side.
Dogen, speaking quite straightforwardly now, tells us that when we perceive the so-called external world, we are still not separate from that external world, like a mirror's image is separate from what's being reflected, or the reflection of the moon on the water is a separate thing from the moon itself. The words that you're reading right now in this post are not "out there" somewhere separate from you yourself. It's all potential experiencing potential.
While we are experiencing the world of the relative, the world of various forms and objects that our discriminating mind divides from limitless potential, we are blind to the fact that those various forms and objects also include the one experiencing them. In the world of the absolute, both subject and object, the perceiver and the perceived, are both nothing but potential. But while we experience the relative, we are blind to the absolute.
This implies that if we stop using the body and mind to sense forms and sounds, the blindness may fall away. To do this (stop using mind and body), sit quietly somewhere, etc. . . .