e or substance.

Let us make the matter clear by an illustration. Suppose I see a picture
on the wall. My eyes are directed toward the picture. Light from the
picture is refracted within the eyes, forming an image on each retina.
The retina is sensitive to the light. The light produces chemical
changes on the retina. These changes set up an excitation in the optic
nerves, which is conducted to a certain place in the brain, causing an
excitation in the brain. Now the important point is that when this
excitation is going on in the brain, _we are conscious, we see the
picture_.

As far as science can determine, we do not see, nor hear, nor taste, nor
smell, nor have any other sensation unless a sense organ is excited and
produces the excitation in the brain. There can be no doubt about our
primary, sensory experience. By primary, sensory experience is meant our
immediate, direct knowledge of any aspect of the world. In this field of
our conscious life, we are entirely dependent upon sense organs and
nerves and brain. Injuries to the eyes destroying their power to perform
their ordinary work, or injuries to the optic nerve or to the visual
center in the brain, make it impossible for us to see.

These facts are so self-evident that it seems useless to state them. One
has but to hold his hands before his eyes to convince himself that the
mind sees by means of eyes, which are physical sense organs. One has but
to hold his hands tight over his ears to find out that he hears by
means of ears--again, physical sense organs.

But simple and self-evident as the facts are, their acceptance must have
tremendous consequences to our thinking, and to our view of human
nature. If the mind is dependent in every feature on the body with its
sense organs, this must give to this body and its sense organs an
importance in our thought and scheme of things that they did not have
before. This close dependence of mind upon body must give to the body a
place in our scheme of education that it would not have under any other
view of the mind. We wish to emphasize here that this statement of the
close relation of the mind and body is not a theory which one may accept
or not. It is a simple statement of fact. It is a presupposition of
psychology. By "presupposition" is meant a fundamental principle which
the psychologist always has in mind. It is axiomatic, and has the same
place in psychology that axioms have in mathematics. All explanations of
th