Before proceeding further it will be necessary first to obtain a clear idea of the function of a nervous tissue and its characteristics; secondly the manner, in which the nervous impulse is propagated; and lastly, we have to discover some compulsive force by which the impulse may be intensified or inhibited during transit. The nerve circuit may be liked to an electric circuit, and invisible impulse bringing about response in the indicator, be it the brain or the galvanometer. In the electric circuit the conducting power of the metallic wire is constant, and the intensity of the electric impulse depends on the intensity of the electric force applied. If the conducting power of the nerve were constant then the intensity of the nervous impulse and its resulting sensation would depend inevitably on the intensity of the shock from outside which starts the impulse. In that case the possibility of the modification of our sensation would be an impossibility. But there may be a likelihood that the power of conduction possessed by a nerve is not constant but capable of change. Should this surmise prove to be correct then we arrive at the momentous conclusion that sensation itself is modifiable, whatever the external stimulus. For the modification of nervous impulse there remains only one alternative; namely, some power to render the vehicle a very much better conductor or a non-conductor according to particular requirements. We require the nervous path to the supra-conducting to have the impulse due to feeble stimulus brought to sensory prominence. When the external blow is too violent we would block the painful impulse by rendering the nerve a non-conductor. Under narcotic the nerve becomes paralysed and we can by its use save ourselves from pain. But such heroic measures are to be resorted to in extreme cases, as when we are under the surgeon's knife. In actual life we are confronted with unpleasantness without notice. A telephone subscriber has an evident advantage, for he can switch off the connection when the message begins to be unpleasant. Statesmen or politicians have been known to cultivate convenient deafness; but that is a mere pretence. The unpleasant things heard, would still continue to rankle. It is not every one that has the courage of Mr. Herbert Spencer who openly resorted to his ear plugs whenever his visitor became tedious.