“The Thing Itself” makes an interesting point when differentiating between the subject of the photograph and the actual photograph. According to this short text from The Photographer’s Eye by John Szarkowski, “the subject and the picture were not the same thing… It was the photographer’s problem to see not simply the reality before him but the still invisible picture, and to make his choices in terms of the latter.”
Essentially this suggests that the main subject of the photo, whatever the main focus is right in front of the lens, isn’t the entire truth of the photograph and what lies in the background of the subject is probably the source of truth. The photographer should make his artistic choices based on what is going on behind the subject as well.
The photograph is more important than the thing itself because it’s the entire photograph that tells a story. A subject can easily contradict the message the photograph is trying to convey.
I am not quite sure what you mean when you write in the second paragraph that “what lies in the background of the subject is probably the source of truth.” I think John Szarkowski is saying that photographers try to make a great image-meaning a visually engaging image- that conveys a point of view. They do not in their photographs objectively show what was there in front of the camera.