In the physical world we always have to take into consideration a revealed side, the phenomena, and a hidden side, the forces. When we are active on the physical plane, in the first place we bring about phenomena, but every action does in fact reach up also into the Arupa plane (the higher regions of the spiritual world) and has there its counter-action. Deeds on the physical plane impress themselves into the Arupa plane, like a monogram into a seal and there remain. The substance of the Arupa plane is delicate, soft and enduring; it is Akasha and human actions remain inscribed there.
Source: Rudolf Steiner – GA 93a – THE FOUNDATIONS OF ESOTERICISM: Lecture XXI – Berlin, 19th October 1905
Translated by Vera and Judith Compton-Burnett