‘If you look at walls soiled with a variety of stains or at stones with variegated patterns,’ Leonardo da Vinci advised fellow painters, ‘you will therein be able to see a resemblance to various landscapes graced with mountains, rivers, rocks, trees, plains, great valleys and hills in many combinations.’ By an irony of history, Leonardo (1452–1519) has come to resemble that stained wall: a Rorschach blot in which viewers discern phantoms of their own imagination.