The Science of Human Nature | Page 4

sensitive and has the
power of action, the power of moving himself. The table is not sensitive,
nor can it move itself. If the pin is thrust into one's own leg, one has

pain. Human beings, then, are sensitive, conscious, acting beings. And
the study of sensitivity, action, and consciousness is the field of
psychology. These three characteristics are not peculiar to man. Many,
perhaps all, animals possess them. There is, therefore, an animal
psychology 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