r the differences between a human
being and a table, we shall see at once the special field of psychology.
If we stick a pin into a leg of the table, we get no response. If we
stick a pin into a leg of a man, we get a characteristic response. The
man moves, he cries out. This shows two very great differences between a
man and a table. The man is _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