Excerpt from Object Lessons: And How to Give Them; Second Series for Intermediate and Grammar Schools
Our knowledge of the material world is obtained through the senses. The organs of sense are the eye, the ear, the nose, the tongue and palate, and the nerves of touch located in the skin. The special nerves of these organs are acted on by things external to the body; the effect is conveyed to the brain; and mental impressions or ideas are the result. Thus a red colour acting on the retina, the sound from a whistle acting on the auditory nerves, or the smell of an onion on the olfactory nerves produces a definite mental impression. The five sensory organs, then, are so many doors and windows by which knowledge enters the mind.
There is, however, another source of knowledge of material bodies. In this case the mental impressions are derived from within the body, and are due to muscular exertion. It is by muscular feeling that we estimate the amount of force required to overcome resistance. Thus we get ideas of elasticity and weight from the amount of active energy put forth by the muscles to overcome inertia in the one case and gravitation in the other. If a weight is placed in the hand we are conscious of a certain amount of force expended to keep it from falling; if the weight is increased we are conscious of an increased expenditure of muscular energy.
The mental impressions, formed by and through the senses, including muscular feeling, are called sensations.
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