David Hockney is possibly the worlds most popular living painter, but he is also something else: an incisive and original thinker on art. Here are the fruits of his lifelong meditations on the problems and paradoxes of representing a three-dimensional world on a flat surface. How does drawing make one see things clearer, and clearer, and clearer still, as Hockney suggests? What significance do different media from a Lascaux cave wall to an iPad have for the way we see? What is the relationship between the images we make and the reality around us? How have changes in technology affected the way artists depict the world?