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.