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 ne