David Hockney is a painter and a writer. In his book. Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters, Hockney introduces a theory that has shaken up art historians and directors of museums and galleries. Hockney closely examined the canvases of some of the most respected European masters. Then he tried to prove that painters such as Johannes Vermeer, Jan van Eyck, and Caravaggio used optical tools to aid them in their work.