The idea of equivalence is an equation that is all at once. It takes in the entire mind. Everything works for a moment. The blessing of the photographic image is the precision involved. There is no chance, except the chance that there is an intelligent being who will press the shutter at the moment of the equation. Then the image stays as a cogent emblem of eternity. Photography allows you to do that. It insists that you be there. Walter Chappell
Another way of thinking about equivalency is to think of it as contemplative photography. You are basically working from within and see your self in the outside world via camera work. Something out there stimulates something inside of you that you may not be fully cognizant of until a later time and you make a picture using all your creative powers to have that picture emanate that intangible something you felt or recognized (or as Walter Chappell would put it, re-cognized). Walter Chappell:
Photograph to remember what you have not yet seen.
Force Of Seeing 