The Beholder's Share

One of my favorite painting teachers expressed an idea to me that has always stuck.  He taught us never to include too much detail, and told us that the viewer's brain would fill it in.  (This was complete with optical illusion demonstrations, and lots of science.  Pretty fun, for an intro level art class!)  But the real lesson here is that a work of art needs a viewer, and that the viewer will better appreciate art that includes them.  "Great art is great, because it is ambiguous."