=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
thinking. Individual differences in human capacity make necessary a
treatment of the different types and grades of intelligence, and the
compilation of tests for determining these differences. We must also
treat of the application of psychology to those fields where a knowledge
of human nature is necessary.

=Applied Psychology.= At the beginning of a subject it is legitimate to
inquire concerning the possibility of applying the principles studied to
practical uses, and it is very proper to make this inquiry concerning
psychology. Psychology, being the science of human nature, ought to be
of use in all fields where one needs to know the causes of human action.
And psychology is applicable in these fields to the extent that the
psychologist is able to work out the laws and principles of human
action.

In education, for example, we wish to influence children, and we must go