I hold in my hand a lens. This lens, in its shape, resembles a certain other lens through which I look in examining it. It was, indeed, modeled after the other, which is a part of my organ of vision. I place the glass lens in a microscope, and a hitherto unknown world is revealed to me. It was there before, but I could not see it. Do I see it now _with the lens_? It is evident that the lens is merely an aid to vision, since the lens in my eye is also necessary to convey the picture to my mind. But now another question: Do I see with the lens which is a part of my eye? Is not that also merely an aid to vision? Let us consider. Since I have two eyes, I may lose one of them without losing the power to see. If I am so unfortunate as to lose one, then, if the eye is not merely an aid to vision, but part of the vision itself, it would naturally follow that I should see only half as well as before; but this, very evidently, is not true. I can read as well as ever. For the examination of anything on a flat surface, one eye is as good as two. Notice, also, that the lens of the eye and the glass lens are not only alike in shape and transparency, but that both are composed of material substances that can be analyzed, and that both are used to acquire knowledge of such substances and the relations existing between them. The glass lens is merely a supplement to the lens of the eye. It is one step further removed from the vision, but even the lens of the eye itself is not the seeing power. That lies back of all. Take now the ear-trumpet, a contrivance to concentrate sound to a given point. It is intended as an aid to hearing, but it is not inseparably associated with the power to hear. A person with normal senses does very well without it. How about the ear itself? Does that constitute a part of the hearing power of a man? If it does, what is the necessity of the auditory nerve? If the hearing and the ear were one and the same, there would be no need of this connecting link with the brain. The external and the internal ear, like the ear-trumpet, are purely material, and by means of them we are able to cognize those material emanations called sound. I speak of sound as a material emanation, because whatever sound comes to us through the ear comes from some material source. The ear, being material, is adapted to convey such emanations to the brain, through which the mind becomes conscious of their existence. The sense of touch, also, is exclusively adapted to the acquainting of its owner with still another aspect of things material. Hardness, softness, smoothness, roughness, heat, cold, and other attributes of matter become known through this sense, and it may be considered a rule without exception that when the sense of touch is excited, some material object is responsible. The same thing is true of the senses of smell and taste, but as their field of action is comparatively limited, I will allow the first three named to represent the whole number.