The permanent colours of the astral body react upon, the mental.  They produce in it their correspondences, several octaves higher, in the same manner as a musical note produces overtones.  The mental body in its turn reacts upon the causal in the same way, and thus all the good qualities expressed in the lower vehicles by degrees establish themselves permanently in the ego.  The evil qualities cannot do so, as the rates of vibrations which express them are impossible for the higher mental matter of which the causal body is constructed.

So far, we have described vehicles which are the expression of the ego in their respective worlds—­vehicles, which he provides for himself; in the physical world we come to a vehicle which is provided for him by Nature under laws which will be later explained—­which though also in some sense an expression of him, is by no means a perfect manifestation.  In ordinary life we see only a small part of this physical body—­only that which is built of the solid and liquid subdivisions of physical matter.  The body contains matter of all the seven subdivisions, and all of them play their part in its life and are of equal importance, to it.

We usually speak of the invisible part of the physical body as the etheric double; “double” because it exactly reproduces the size and shape of the part of the body that we can see, and “etheric” because it is built—­of that finer kind of matter by the vibrations of which light is conveyed to the retina of the eye. (This must not be confused with the true aether of space—­that of which matter is the negation.) This invisible part of the physical body is of great importance to us, since it is the vehicle through which flow the streams of vitality which keep the body alive, and without it, as a bridge to convey undulations of thought and feeling from the astral to the visible denser physical matter, the ego could make no use of the cells of his brain.

The life of a physical body is one of perpetual change and in order that it shall live, it needs constantly to be supplied from three distinct sources.  It must have food for its digestion, air for its breathing, and vitality for its absorption.  This vitality is essentially a force, but when clothed in matter it appears to us as a definite element, which exists in all the worlds of which we have spoken.  At the moment we are concerned with that manifestation of it which we find in the highest subdivision of the physical world.  Just as the blood circulates through the veins, so does the vitality circulate along the nerves; and precisely as any abnormality in the flow of the blood at once affects the physical body, so does the slightest irregularity in the absorption or flow of the vitality affect this higher part of the physical body.