"The familiarity of certain photographs builds our sense of the present and immediate past. Photographs lay down routes of reference, and serve as totems of causes: sentiment is more likely to crystallise around a photograph than around a verbal slogan. And photographs help construct, and revise, our sense of a more distant past, with the posthumous shocks engineered by the circulation of hitherto unknown photographs. Photographs that everyone recognises are now a constituent part of what a society chooses to think about, or declares that it has chosen to think about. It calls these ideas "memories" and, in the long run, that is a fiction. Strictly speaking, there is no such thing as collective memory - it is part of the same family of spurious notions as collective guilt. But there is collective instruction."

Susan Sontag, of course. there are more extracts from her new book, Regarding the Pain of Others here.