When you look up at the clock, you instinctively feel that there is nothing interposed between it and your mind that is conscious of it. You seem to feel that your mind reaches out and envelops it. As a matter of fact, your sense impression of that bit of furniture must filter through a great number of intervening physical agencies before you can become conscious of it. Direct perception of an outside reality is impossible. [Sidenote: _Second-Hand Knowledge_] Before you can become aware of any object there must first arise between it and your mind a chain of countless distinct physical events. Modern science tells us that light is due to undulations or wave-like vibrations of the ether, sound to those of the air, etc. These vibrations are transmitted from one particle of ether or air to another, and so from the thing perceived to the body of man. Think, then, what crisscross of air currents and confusion of ether vibrations, what myriad of physical events, must intervene between any distant object and your own body before sensations come and bring a consciousness of that object's existence! Nor can you be sure, even after any particular vibration has reached the surface of your body, that it will reach your mind unaltered and intact! [Sidenote: _Etheric Vibrations as Causing Sensations_] What goes on in the body itself is made clear by your knowledge of the cellular structure of man. You know that you have a system of nerves centering in the brain and with countless ramifications throughout the structural tissues of the body. You know that part of these nerves are sensory nerves and part of them are motor nerves. You know that the sensory nerves convey to the brain the impressions received from the outer world and that the motor nerves relay this information to the rest of the body coupled with commands for appropriate muscular action. [Illustration: DIAGRAM SHOWING THE FOUR CHIEF ASSOCIATION CENTERS OF THE HUMAN BRAIN] [Sidenote: _The Road to Perception_] The outer end of every sensory nerve exposes a sensitive bit of gray matter. These sensitive, impression-receiving ends constitute together what is called the "sensorium" of the body. When vibrations of light or sound impinge upon the sensorium, they are relayed from nerve cell to nerve cell until they reach the central brain. Then it is, and not until then, that sensations and perceptions occur. Consider, now, the infinitesimal size of a nerve cell and you will have some conception of the number of hands through which the message must pass before it is received by the central office. Many of our sensations, especially those of touch, seem to occur on the periphery of the body--that is to say, at that part of the exposed surface of the body which is apparently affected. If your finger is crushed in a door, the sensation of the blow and the pain all seem to occur in the finger itself. [Sidenote: _The Place Where Sensation Occurs_]