Photography is inescapably a memorial art. It selects, out of the flow of time, a moment to be preserved, with the moments before and after falling away like sheer cliffs.

At a dinner party earlier this year, I was in conversation with someone who asked me to define photography. I suggested that it is about retention: not only the ability to make an image directly out of the interaction between light and the tangible world, but also the possibility of saving that image. A shadow thrown onto a wall is not photography. But if the wall is photo­sensitive and the shadow remains after the body has moved on, that is photography. Human creativity, since the beginning of art, has found ways to double the visible world. What photography did was to give the world a way to double its own appearance.