You can look and you can see. The two are not always simultaneous nor necessarily the same. Sometimes seeing involves a sensitivity to what isn't there. Sometimes seeing is more like imagining.
Artists squint to minimize detail and see the spread of shapes and patterns of light and dark. This play of light and dark in an image functions similarly to the bass or percussion in a piece of music. It is the structure on which everything else hangs and the container for the feeling tones of color.
Tension, comfort, melancholy or serenity, are all expressed in the interplay of light and dark. It provides us with definition, both visually and psychologically. One defines the other and together they help us define ourselves.