I've done a lot of reading about photography over the last two years or so--beginner "how-to"s, intermediate and advanced technical texts about chemistry and optics, histories of the medium, piles of monographs of the masters, collections of contemporary practitioners and living masters, and lots of philosophy (about representation, epistemics and ontologics of aesthetic, etc)--and without a doubt, the absolute most singularly important book I've read is Henri Van Leir's "Philosophy of Photography." His work has transformed how I think about myself as a practitioner of conventional optical-chemical photography, how I understand the digital "revolution" in image making, and has made obvious to me how I must proceed to explore my work. I would like to present his ideas here that perhaps someone would like to talk about them with me, because so far in my life I'm up to (prospectively) one other person.

Van Lier's philosophical musings on the epistemic nature of photography use the word "indice" as something that is a physical mark in the universe that is connotative (like the latent image on film captured through the lens), void of intention that is required to make it denotative. He uses the word "index" as the intended denotative content of a photograph, which comes from the thematic or contextual aspect of the "spectacle" that the photographer intended to capture.
*the bolding is my emphasis, italics as original.

"After having been scrutinized all of its characteristics, it might be said that photography is best understood in light of the opposition often made today between the real and reality.Reality designates the real inso far as it is already seized and organized in sign systems, thus assuming the forms of intentionally, contentionally and systemically defined signs accordingly distributed in objects and actions, which are the designates that denominate or represent the signs in question. By contrast, the real is that which escapes this conception of reality. It is all that is before, after and underneath reality, it is all that is not yet domesticated by our technical, scientific, and social relations, and which Sartre, for instance, dubbed the quasi-relations of the in-itself.
Indices hover between the real and reality. They are the chaotic, unnamable and unrepresentable quasi-relations -- mostly suddenly -- constitute relations: schemas, words, drawings, or digits. From there, they enter into relaity, but often only hypothetically, partially and fragilely, in overlap with other possible relations, and consumed by other quasi-relations. In their emergence, indices are not only aided by the internal decision of their more or less analogical or namable texture and structure, but also by the index which, by designating indices, increasse the latter's likelihood to be viewed in a particular context, and thus to be seen as either this or that. Therefore, to start with,indices belong to the real, and only appertain to reality in the final stage, which is furthermore rarely decisive. Moreover, photographic imprints are indices of indices with respect to possible spectacles. They are (very direct) indices of the imbuing photons, and, through their multiple abstractive meditaion, they are (very indirect) indices of external objects and actions. As such, a photograph is not merely a blend of reality and the real. It is a phenomenon where what is represented of reality comes to us across the frame of the real. Moreover, this is a double frame involving the chemistry of the film and the physicality of the lens. However, the termacross is still inexact. One has to use the term within, since the photograph is infintely slender and lacks a before or after, back or front. In a figurative sense, photographs are therefore fragments of reality within the (double) frame of the real.

It is true that, in the case of advertising, pornographic, industrial, and family photographs, extremely imperious indexes and remarkable analogies may ensure that we forget this frame and can only percieve stimuli-signs. However, even in this case, the quasi-relations of the real do not border on therelations of reality; the former can be seen as the mould in which these relations are in continuous and precarious germination. This confirms the priority of perceptual, motive, semiotic, and indicial field effects. Indeed, why is ti that between the quasi-relations of this matrix and the created fleeting relations there is no soliditifaction at any time, as their place of reciprocal conversions, field effects, curvatures and fluctuations? In a figurative sense, a photograph is reality emerging from the real. Conversely, it is reality gnawed at by the real.

One can rephrase this by introducing a different set of categories. The Greeks opposed Chaos -- non-information and noise -- to Cosmos -- (cosmetic) order, which was translated alsmot literally in Latinas Mundus, the cleanly (the non-filthy). In this frame, Chaos pertains to the real, while the Cosmos-Mundus belongs to the realm of reality, of which man could indeed be the ruler and the semiotic epitome, the Microcosm. According to Cicero, Latin had the virtue of introducing a more comprehensive notion, namely that of Universe, the turn-towards-one, capable of embracing Cosmos and Chaos, order and disorder, information and noise, negentropy and entropy, improbabilty and probability, refinement and obscenity, scene and non-scene. One can now clearly see the place of photography. Through its indexes and certain more or less indexed indices, the photograph offers fragments of the Cosmos-Mundus. However, the chemistry of its latent image and the abstractive configuration of its lenses belong to the Universe World of which they are states. The tips of the Cosmos-Mundus therefore appear as states of the Universe.

There is a third way of formulating this. In ancient times, what mattered most was the event, to the extent that, since the times of the pharaohs and of the Romans, many lived for their tomb or posthumous glory, that is to say, for the final consecrations of the event that they had been. Thepossible, uncertain event was distrusted. For the most part, the photograph belongs to the latter. The contingency of the photographic shot and its development. The possibility of indicial imprints, and the possibility of indexed indices. The contingency of re-cuts and ulterior layouts. An event implies a certain emphasis, or else value judgment, and willy-nilly seems to refer to some banality. The possible, as situated between imprint and the indicial, between indices and indexes, between reality and the real, between Cosmos and Mundus, is more readily accessible, and, as is the case with a process, is situated in a course (things have to run their course, as Beckett would put it) that does not necessarily have an end or goal. Moreover, the latter are even discouraged. Reality is comprised of events and objects while the real is characterized by process and relay.

Therefore, the photograph is in every sense a matter of black. What is most important for photography -- as with interstellar space -- is the night. In film rolls and blank paper, the camera, darkrooms and printing laboratiries, it is the night, the darkness and non-light out of which luminous eventualities manifest themselves punctually and incidentally, emerging out of the dark only to return to it. The photographic photon traverses the night of the device only to take hold again of shadows, in the form of negatives and latent images. And this darkness is contained in a room with its secret and genital workings. Here, only solely speaks of spools, paper impregnators, baths, and developing. The photograph is more uterine than phallic. The architect, the dancer, the painter, the sculptor, the artisan, and the writer all work in a lighted room; even their nights are filled with light. By contrast, the photographer inhabits the camera obscura, and he ultimately and always draws in the future viewers with him.

The photograph is even the most vivacious experience of what physicists call the black box, where one can clearly percieve the entrance (input) and the exit (output), without ever knowing quite well what takes place between the two. The function of reality and the cosmos is to dissimulate black boxes, to make us believe that everything can be reduced to signs, referents, objects and events, and therefore to links that clarify and reveal causality. The apprehension of the real and the universe is to dare to confront black boxes wherever they might be, which is to say, almost everywhere when keeping in mind that there are fewer clear-cut cases of causality than what Heisenberg called series of probabilities. These series of probabilityes are statistically claculatble and predictable; however, this does not entail that they are uninterruptedly describable. No matter where it is taken, a photograph renders place and duration, which are peculair to reality, in the form of space-time, non-duration and non-space, which are characteristic of the real. Invented and used by earthlings, the photograph is the stuff of extraterrestrials."
-- Henri Van Lier, "Philosophy of Photography" (2007), 36-38.

"In its lenses, the photographic process gathers and makes use of the main messenger of the universe, electromagnetic waves. These have remarkable characteristsics. Their movement is linear, apart from an enormous gravitational effect. Their refractivity, when passing from one environment to another, is goverened by fixed laws. Their interference fringes are continuous and calculable. They are isotropic: in a vacuum, their speed is constant in every direction. According to the theory of relativity, this speed is insurmoutnable and gives rise to the cosmic constant c. Because of this, simultaneities are created and thus also a coordinated space and time, a space-time. Through the electromagnetic waves between the space-time islets prodigiously moving away or drawing closer, a kind of unity is instituted which ensures that any event belongs to the Universe, to the turn-towards-One. Working on this set of features revolving around c is, for the lens engineer as well as for the photographer, in itself a remarkable way of connecting with the nature of things.

Furthermore, the solar system privileges particular electromagnetic waves. As the sun's surface temperature is 5800 degrees Kelvin, its most intense electromagnetic radiation has a wavelength of about 2.9mm (the length of a privileged wave of a black body at 1 degree Kelvin) divided by 5800, or in other words, 500 nano-meters. Thus, Evolution selected the human eye for its adaptation to waves between 400 and 700 nano-meters; 500 for green at the centre, 400 for blue, 700 for red. As a consequence, and in return for other optic capabilities, man captures light in a most balanced and integrating manner. This remarkable ability is one of the elements - together with the standing posture, the hand, the larynx, the neocortex, and the omnivorous diet - contributing to his transformation into the signifying animal, this mammal where analogical and digital signs blossomed, or, in short, humankind as the place where signs originated. The integrating gaze is the fundamental practical, scientific, and aesthetic experience bespeaking the concord between humankind and the solar system, and beyond. In other words, these abilities render man the cosmic and universal animal."
-- Henri Van Lier, "Philosophy of Photography" (2007), 59-60.

the rest of the book is here, online, for free forever: