=Dependence of Mind on Nerves and Brain.= We have seen how in a general way the mind is dependent on the body. We have seen how in a more intimate way it is dependent on the special sense organs. But the part of the body to which the mind is most directly and intimately related is the nervous system. The sense organs themselves are merely modifications of the nerve ends together with certain mechanisms for enabling stimuli to act on the nerve ends. The eye is merely the optic nerve spread out to form the retina and modified in certain ways to make it sensitive to ether vibrations. In addition to this, there is, of course, the focusing mechanism of the eye. So for all the sense organs; they are, each of them, some sort of modification of nerve-endings which makes them sensitive to some particular force 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.