When you see a person today, you may well recognize them tomorrow, or even after a year, because their outer form has remained constant. Similarly with the animal, the plant or mineral. This is not at all the situation with the beings which are incorporated on the astral plane. These possess a continuously changing form, a shape which in many of them, from one moment to the next become another, because the form which can be observed in the astral plane is the exact expression of the inner soul experiences and soul activity of these beings.
Just think about yourself, how you may observe your soul in the morning just after you have received a cheerful letter and how the joyful message filled your soul with delight and pleasure and how this feeling lived in the soul. Then think how your soul will express itself in the direct contrary situation, how different the image will be when you receive news of a death in the afternoon, or you are shaken in rage and fear. Consider how your outer expression changes each time as a result of what took place in the soul, then you have an image of what happens on the astral plane. Hence the bewildering scurrying and continuously changing forms of the astral beings.
Source: Rudolf Steiner – GA 108 – Regarding Higher Worlds – Vienna, 21st November 1908
Translated by Hanna von Maltitz