Stand in front of a mirror and touch your right eyebrow with your left hand; in the mirror, you will see your right hand touching your left eyebrow. A mirror is a very interesting thing because it reveals certain aspects of the invisible world.
In relation to the physical plane, objects on the astral plane are seen reversed, like in a mirror.
Numbers, dates, everything is back to front.
A future event appears as though it were in the past and vice-versa. This is why the astral plane is the world of illusions.
In order to know the truth, you have to venture beyond it to the higher mental or causal plane. This is what we learn in the Teaching.
If you succeed in solving the question of the mirror, you will solve the greatest problems of psychic life.