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As historians are often painfully aware, the past, their field of study, doesn’t really exist. It is only a collection of memories and ideas held by people living in the present, and those impressions are conditioned by succeeding events. When we look at a photograph taken of the New York City skyline any time between the early 1970s and September 10, 2001, for example, we now do so with a sense of poignancy the photographer had no reason to feel. The past is over and therefore should be immutable, but, since it is merely a concept, history-or the way we look at it, which is the same thing-actually changes depending upon what happens in the future.