The nature of perception
Take a look around you - around the room or out the window. What do you see as you and your eyes move around? Shapes? Figures? Background? Shadows? Areas of light and dark? Your knowledge of the objects you see and their relative location is extensive, and you have a good idea of what they will feel like even if you have not touched them. If the lighting suddenly changes (if lamps are turned on or off or if a cloud passes in front of the sun), the amount of light reflected by the objects in the scene changes too, but your perception of the objects remains the same - you see them as having the same shape, colour and texture as before. Similarly, you do not perceive an object as increasing in size as you approach it, even though the image it casts upon your retina does get larger. Form, movement and space are the essential elements of perception.
The brain receives fragments of information from approximately 1 million axons in each of the optic nerves. It combines and organises these fragments into the perception of a scene - objects having different forms, colours and textures, residing at different locations in three-dimensional space. Even when our bodies or our eyes move, exposing the photoreceptors to entirely new patterns of visual information, our perception of the scene before us does not change. We see a stable world, not a moving one, because the brain keeps track of our own movements and those of our eyes and compensates for the constantly changing patterns of neural firing that these movements cause.