Bill often makes this comment during his videos because it is so important and so basic to great art. In order to create the illusion of light in your painting you often have to surround it with dark. This was called chiaroscuro by the “Old Masters” and Bill understood not only the term but the technique. If you look at some of his candle paintings, you can see his mastery at work. He loved to paint on a black canvas for the same reason. It allowed the light he put into his paintings to stand out.