=The Science of Psychology.= Now, let us ask, what is the science of
psychology? What kind of problems does it try to solve? What aspect of
the world has it taken for its field of investigation?

We have said that each science undertakes to describe some particular
aspect of the world. Human psychology is the science of human nature.
But human nature has many aspects. To some extent, our bodies are the
subject matter for physiology, anatomy, zoölogy, physics, and chemistry.
Our bodies may be studied in the same way that a rock or a table might
be studied. But a human being presents certain problems that a rock or
table does not present. If we consider 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