At the current stage of development, the human being leaves his body during sleep. The soul moves, sometimes far away from the physical body, into the astral world and meets with other creatures of the astral world and exchanges thoughts with them. However, when the person wakes up again, he or she does not remember these wanderings. This lack of remembrance is due to humanity’s current stage of development.
However, the student can develop ever higher and higher. The student, who is under the guidance of a so-called master, can gradually turn his consciousness into a continuous consciousness. Then he will be able to bring the experiences of the night into his consciousness during his state of waking. When the student has achieved this continuity of consciousness, he remembers what he has received in the astral world.
This knowledge is not acquired in the physical-sensory world, but it is experienced and transmitted in the astral world and brought into physical life. That is what Plato meant when he talked about the memory of higher states of the soul.
Source (German): Rudolf Steiner – GA 88 – Über die astrale Welt und das Devachan – Berlin, November 18, 1903 (page 62-63)
Translated by Nesta Carsten-Krüger