The true content of a photograph is invisible, for it derives from a play, not with form, but with time. One might argue that photography is as close to music as to painting. . . a photograph bears witness to a human choice being exercised. This choice is not between photographing x and y: but between photographing at x moment or at y moment.
All photographs are there to remind us of what we forget. In this -as in other ways- they are the opposite of paintings. Paintings record what the painter remembers. Because each one of us forgets different things, a photo more than a painting may change its meaning according to who is looking at it.
Photographs bear witness to a human choice being exercised in a given situation. A photograph is a result of the photographer's decision that it is worth recording that this particular event or this particular object has been seen. If everything that existed were continually being photographed, every photograph would become meaningless.