The philosophical underpinning of the realism of documentary photography has been a positivism that assumes the unproblematic existence of an observable reality, a neutral, detached and unified observing subject, and a form of inductivism derived from sensory experience.
This is documentary photography's originary and formative way of thinking and it underpins the way that the focus of the camera apparatus assumes the sovereignty of geometrical perspective--Euclidean geometry as the cone of vision. This convention of perspective centres everything on the eye of the disembodied beholder standing outside the field of vision.
This is deemed to be a natural representation, the way things really looked, the way that we see,and the way things really are. The world is clear and distinct even at the margins, and the entire field of vision is measurable and visible.
The picture was objective and truthful. It was the emergence of digital technology that ruptured this mode of knowledge, representation and photographic realism not the philosophical critiques of positivism.