Whatever you know or think you know, of the external world comes to you through some one of your five primary senses, sight, hearing, touch, taste and smell, or some one of the secondary senses, such as the muscular sense and the sense of heat and cold.

The impressions you receive in this way may be true or they may be false. They may constitute absolute knowledge or they may be merely mistaken impressions. Yet, such as they are, they constitute all the information you have or can have concerning the world about you.

Philosophers have been wrangling for some thousands of years as to whether we have any real and absolute knowledge, as to whether matter actually does or does not exist, as to the reliability or unreliability of the impressions we receive through the senses. But there is one thing that all scientific men are agreed upon, and that is that such knowledge as we do possess comes to us by way of perception through the organs of sense.

If you have never given much thought to this subject, you have naturally assumed that you have direct knowledge of all the material things that you seem to perceive about you. It has never occurred to you that there are intervening physical agencies that you ought to take into account.

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.

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!

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.

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.

As a matter of fact, this is not the case, for if one of your arms should be amputated, you would still feel a tingling in the fingers of the amputated arm. Thus has arisen a superstition that leads many people to bury any part of the body lost in this way, thinking that they will never be entirely relieved of pain until the absent member is finally at rest.

Of course, the fact is that you would only seem to have feeling in the amputated arm. The sensation would really occur in the central brain tissue as the organ of the governing intelligence, the organ of consciousness.

And you may set it down as an established principle that all states of consciousness, whether seemingly localized on the surface of the body or not, are connected with the brain as the dominant center.

The facts we have been recounting have been established by the experiments of physiological psychology. Thus, the work of the laboratory has shown that between the moment when a sense vibration reaches the body and the moment when sensation occurs a measurable interval of time intervenes.

If your eyes were to be blindfolded and your hand unexpectedly pricked with a white-hot needle, the time that would elapse before you could jerk your hand away could be readily measured in fractions of a second with appropriate instruments.

This interval is known as reaction-time. It varies greatly with different persons. During this reaction-time, the cell or cells attacked upon the surface of the hand have conveyed news of the assault through numberless intermediate sensory nerve cells to the brain. The brain in turn has sent out its mandate through the appropriate motor nerve cells to all the muscle and other cells surrounding the injured cell, commanding them to remove it from the point of danger.

The work of the nervous system in dealing with the ether vibrations that are constantly impinging upon the surface of the body has been likened to that of the transmitter, connecting wire and receiver of a telephone. Air-waves striking against the transmitter of the telephone awaken a similar vibratory movement in the transmitter itself. This movement is passed along the wire to the receiver, which vibrates responsively and imparts a corresponding wave-like motion to the air.

These air-waves when heard are what we call sound.

In the same way, air-waves striking the ear are communicated by the auditory nerve to the brain, where they awaken a corresponding sensation of sound. But these waves must be vibrating at between 30 and 20,000 times a second. If they are vibrating so slowly or so rapidly as not to come within this range, we cannot hear them.

This process is by no means a mechanical affair. On the contrary, it is a series of mental acts. Every cell in the living telegraph must receive the message and transmit it. Every cell must exercise a form of intelligence, from the auditory cell reporting a sound-wave or the skin cell reporting an injury to the muscle cells that ultimately receive and understand a message directing them to remove the part from danger.

Reaction-time, so called, is thus occupied by cellular action in the form of mental processes intervening between the nerve-ends and the brain center, in much the same way that light and sound vibrations intervene between the object perceived and the surface of the body.

For even the simplest of sense-perceptions we have, then, this sequence of events: first, the object perceived; second, the series of vibrations of ether particles intervening between the object and the body; third, the impression upon the surface of the body; fourth, the series of mental processes, cell after cell, in the nerve filaments leading to the brain; fifth, when these impressions or messages have reached the brain, a determination of what is to be done; and, sixth, a transmission by cellular action of a new message that will awaken some response in the muscular tissues.

This process is completely carried out, however, in only comparatively few instances. The vast majority of sense-impressions awaken no reaction. They are registered in the mind, but they are not perceived. We are not conscious of them. They form a part, not of consciousness, but of subconsciousness. They are messages that reach the mind but are laid aside like unopened mail because they possess no present interest.

Wherever and however you may be placed, you are always and everywhere immersed in a flood of etheric vibrations. Light, sound and tactual vibrations press upon you from every side. At a busy corner of a city street these vibrations rise to a tumultuous fortissimo; in the hush of a night upon the plains they sink to pianissimo. Yet at every moment of your day or night they are there in greater or less degree, titillating the unsleeping nerve-ends of the sensorium.

Your mind cannot take time to make all these sense-impressions the subject of conscious thought. It can trouble itself only with those that bear in some way upon your interests in life.

Your mind is like the receiving apparatus of the wireless telegraph which picks from the air those particular vibrations to which it is attuned. Your mind is selective. It is discriminating. It seizes upon those few sensory images that are related to your interests in life and thrusts them forward to be consciously perceived and acted upon. All others it diverts into a subconscious reservoir of temporary oblivion.

You will have a clearer understanding of the sense-perceptive processes and a more vital realization of the practical significance of these facts when you consider how they affect your knowledge of material things and your conception of the external world.

This subject possesses two distinct aspects.

One aspect has to do with the inability of the sense-organs to record the facts of the outer world with perfect precision. These organs are the result of untold ages of evolution, and, generally speaking, have become wonderfully efficient, but they display surprising inaccuracies. These inaccuracies are called Sensory Illusions.

The other aspect of the Sense-Perceptive Process has to do with the mental interpretation of environment.

Both these aspects are distinctly practical.

You should know something of the weaknesses and deficiencies of the sense-perceptive organs, because all your efforts at influencing other men are directed at their organs of sense.

You should understand the relationship between your mind and your environment, since they are the two principal factors in your working life.