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 the working of the mind must be stated in terms of nerve and brain action, and stimulation of sense organs. Since the sense organs are the primary and fundamental organs through which we get experience, and since the sensations are the elementary experiences out of which all mental life is built, it is necessary for us to have a clear idea of the sense organs, their structure and functions, and of the nature of sensations. =Vision.= _The Visual Sense Organs._ The details of the anatomy of the eye can be looked up in a physiological textbook. The essential principles are very simple. The eye is made on the principle of a photographer's camera. The retina corresponds to the sensitive plate of the camera. The light coming from objects toward which the eyes are directed is focused on the retina, forming there an image of the object. The light thus focused on the retina sets up a chemical change in the delicate nerve tissue; this excitation is transmitted through the optic nerve to the occipital (back) part of the brain, and sets up brain action there. Then we have visual sensation; we see the object. The different colors that we see are dependent upon the vibration frequency of the ether. The higher frequencies give us the colors blue and green, and the lower frequencies give us the colors yellow and red. The intermediate frequencies give us the intermediate colors blue-green and orange. By vibration frequencies is meant the rate at which the ether vibrates, the number of vibrations a second. If the reader wishes to know something about these frequencies, such information can be found in a textbook on physics.