The flood of emotion does not itself greatly affect the mental body, although for a time it may render it almost impossible for any activity from that mental body to come through into the physical brain. That is not because that body itself is affected, but because the astral body, which acts as a bridge between it and the physical brain, is vibrating so entirely at one rate as to be incapable of conveying any undulation which is not in harmony with that. 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 æther 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.