It has been said that photography is obsessed with 'the thing that was.'  One of the unique characteristics inherent in the photographic medium is the indexical relationship between subject and object.  A photograph is proof that, even just for a second, that subject appeared in that way.  As time passes, we rely more and more on the photographic document to remind us of that moment; this is problematic because the photographic object, like memory, is transient.