Our whole knowledge of the world is, in one sense, self-knowledge. For knowing is a translation of external events into bodily processes, and especially into states of the nervous system and the brain: we know the world in terms of the body, and in accordance with its structure. Surgical alterations of the nervous system, or, in all probability, sense-organs of a different structure than ours, give different types of perception—just as the microscope and telescope change the vision of the naked eye. Bees and other insects have, for example, polaroid eyes which enable them to tell the position of the sun by observing any patch of blue sky. In other words, because of the different structure of their eyes, the sky that they see is not the sky that we see. Bats and homing pigeons have sensory equipment analogous to radar, and in this respect see more “reality” than we do without our special instruments. From the viewpoint of your eyes your own head seems to be an invisible blank, neither dark nor light, standing immediately behind the nearest thing you can see. But in fact the whole field of vision “out there in front” is a sensation in the lower back of your head, where the optical centers of the brain are located. What you see out there is, immediately, how the inside of your head “looks” or “feels.” So, too, everything that you hear, touch, taste, and smell is some kind of vibration interacting with your brain, which translates that vibration into what you know as light, color, sound, hardness, roughness, saltiness, heaviness, or pungence. Apart from your brain, all these vibrations would be like the sound of one hand clapping, or of sticks playing on a skinless drum. Apart from your brain, or some brain, the world is devoid of light, heat, weight, solidity, motion, space, time, or any other imaginable feature. All these phenomena are interactions, or transactions, of vibrations with a certain arrangement of neurons. Thus vibrations of light and heat from the sun do not actually become light or heat until they interact with a living organism, just as no light-beams are visible in space unless reflected by particles of atmosphere or dust. In other words, it “takes two” to make anything happen. As we saw, a single ball in space has no motion, whereas two balls give the possibility of linear motion, three balls motion in a plane, and four balls motion in three dimensions.