Technically photography is at the intersection of two distinct procedures; one of a chemical order: the action of light on certain substances; the other of a physical order: the formation of the image through an optical device.
(Roland Barthes 1982: 10)
The painter’s camera obscura is only one of the causes of Photography; the essential one, perhaps, was the chemical discovery.
For most of its history the photographic image itself relied upon two essential ingredients: the camera’s projection of an image and the subsequent chemical development of the image. In this context it would be wise to heed the words of the Gestalt psychologists and remember that the whole is more than the sum of the parts, ...