Behind
We do not see what is behind us. In this way, it is placed in darkness and remains unknown (compare with Eye), and likely represents something we are unaware of. Jung writes: "... if you look down at the ground in front of you, you are in the sphere of consciousness, but if you look back, you are gazing into the realm of your unconscious, which is always where the eyes of consciousness are not looking." (1984, p. 18.)
The back shares this symbolic value. We do not see what happens behind our back, or what happens to our back. In dreams, it almost always relates to the shadow or unconsciousness in general.