The Science of Human Nature | Page 4

as well as human psychology.
A study of the human body shows us that the body-surface and many parts within the body are filled with sensitive nerve-ends. These sensitive nerve-ends are the sense organs, and on them the substances and forces of the world are constantly acting. In the sense organs, the nerve-ends are so modified or changed as to be affected by some particular kind of force or substance. Vibrations of ether affect the eye. Vibrations of air affect the ear. Liquids and solutions affect the sense of taste. Certain substances affect the sense of smell. Certain organs in the skin are affected by low temperatures; others, by high temperatures; others, by mechanical pressure. Similarly, each sense organ in the body is affected by a definite kind of force or substance.
This affecting of a sense organ is known technically as stimulation, and that which affects the organ is known as the stimulus.
Two important consequences ordinarily follow the stimulation of a sense organ. One of these is movement. The purpose of stimulation is to bring about movement. To be alive is to respond to stimulation. When one ceases to respond to stimulation, he is dead. If we are to continue alive, we must constantly adjust ourselves to the forces of the world in which we live. Generally speaking, we may say that every nerve has one end in a sense organ and the other in a muscle. This arrangement of the nerves and muscles shows that man is essentially a sensitive-action machine. The problems connected with sensitivity and action and the relation of each to the other constitute a large part of the field of psychology.
We said just now, that a nerve begins in a sense organ and ends in a muscle. This statement represents the general scheme well enough, but leaves out an important detail. The nerve does not extend directly to a muscle, but ordinarily goes by way of the brain. The brain is merely a great group of nerve cells and fibers which have developed as a central organ where a stimulation may pass from almost any sense organ to almost any muscle.
But another importance attaches to the brain. When a sense organ is stimulated and this stimulation passes on to the brain and agitates a cell or group of cells there, we are conscious. Consciousness shifts and changes with every shift and change of the stimulation.
The brain has still another important characteristic. After it has been stimulated through sense organ and nerve, a similar brain activity can be revived later, and this revival is the basis of memory. When the brain is agitated through the medium of a sense organ, we have sensation; when this agitation is revived later, we have a memory idea. A study of consciousness, or mind, the conditions under which it arises, and all the other problems involved, give us the other part of the field of psychology.
We are not merely acting beings; we are conscious acting beings. Psychology must study human nature from both points of view. We must study man not only from the outside; that is, objectively, in the same way that we study a stone or a tree or a frog, but we must study him from the inside or subjectively. It is of importance to know not only how a man acts, but also how he thinks and feels.
It must be clear now, that human action, human behavior, is the main field of psychology. For, even though our main interests in people were in their minds, we could learn of the minds only through the actions. But our interests in other human beings are not in their minds but in what they do. It is true that our interest in ourselves is in our minds, and we can know these minds directly; but we cannot know directly the mind of another person, we can only guess what it is from the person's actions.
=The Problems of Psychology.= Let us now see, in some detail, what the various problems of psychology are. If we are to understand human nature, we must know something of man's past; we must therefore treat of the origin and development of the human race. The relation of one generation to that preceding and to the one following makes necessary a study of heredity. We must find out how our thoughts, feelings, sensations, and ideas are dependent upon a physical body and its organs. A study of human actions shows that some actions are unlearned while others are learned or acquired. The unlearned acts are known as instincts and the acquired acts are known as habits. Our psychology must, therefore, treat of instincts and habits.
How man gets experience, and retains and organizes this experience must be our problem in the chapters on sensations, ideas, memory, and