This is more of a scientific question perhaps -- not sure -- but how is it possible that we can close our eyes and still see light i.e. how is it possible we close our eyes and think of images or memories in which the sun is shining and objects can be seen as if the real sun were shining? To see a postcard or a picture with our eyes open requires an outside light source, the sun or a lamp. But then I can close my eyes and see the same postcard and picture without any outside light source. How is that possible?