Before I dive on into to my most recent research and thinking, I thought it might be useful to outline some of the key concepts and research that underpin my current project. First and foremost amongst them: the index.
Put simply, the index is a type of sign; a sign that, in Rosalind Krauss’s words, “establishes its meaning along the axis of [a] physical connection to [its] referent.”  The identification of the photograph as a type of index goes back to the American philosopher C.S. Peirce, who identified three principal types of signs in his writing.
Likenesses (or icons), “which serve to convey ideas of the things they represent simply by imitating them.”  Examples of likenesses include: paintings, drawings and sculptures.
Indications (or indices), “which show something about things on account of their being physically connected with them.”  Examples of indications include: guideposts, relative pronouns and photographs.
Symbols, which have become associated with their meanings by usage.  Examples of symbols include: words, phrases, speeches, books and libraries.
In my writing, I will use the terms ‘icon’ and ‘index’, as opposed to Peirce’s ‘likeness’ and ‘indication’.
In one oft-quoted paragraph, Peirce makes the following pronouncement on the status of photographs.
Photographs, especially instantaneous photographs, are very instructive, because we know that they are in certain respects, exactly like the objects they represent. But this resemblance is due to the photographs having been produced under such circumstances that they were physically forced to correspond point by point to nature. In that aspect, then, they belong to the second class of signs, those by physical connection.  [my emphasis]
So, according to Peirce, the photograph belongs to the category of the index, not the icon, because their method of production physically forces them to “correspond point by point to nature”. His language is not dissimilar to that of Henry Fox Talbot, one of the pioneers of photography, who described photographs as images “impressed by nature’s hand”.  There are many other examples I could cite, but the point that I want to draw out is the idea that the photograph – by virtue of its automaticity – has some intrinsic connection to the real is a very old one. While there have always been those that argue against this point of view (whom I will address later), I think it is fair to say that this has been the dominant point of view for over one hundred and fifty years.
Some theorists have gone further to stress that the physical connection to the referent – the index – is the essential quality of camera work. As Roland Barthes states in Camera Lucida:
I call “photographic referent” not optionally the real thing to which an image or sign refers but the necessarily real thing which has been placed before the lens, without which there would be no photograph. […] since this constraint exists only for photography, we must consider it, by reduction as the very essence, the noeme of photography.  [emphasis in original]
Similarly, Rosalind Krauss identifies the index as the “sign essential to camerawork”.  Generally speaking, in photographic theory the index is regarded as a trace or footprint left on the film by the chemical interaction of light. Krauss makes this explicit when she singles out the photogram as forcing, or making explicit what is true of all photography, that “[e]very photograph is the result of a physical imprint transferred by light reflection onto a sensitive surface.”  Statements like this (unintentionally perhaps) tend to perpetuate the idea that the index can only be a trace. However, to return to Peirce, the trace is only one category of index.
Into the category of the index Peirce places not just traces but pointers. A relative pronoun, such as ‘that’, is a pointer, a word which is meaningless – essentially an empty sign – and only becomes relevant when it is related to another word, ‘that house’ for instance.  From this, it might be inferred that a photograph too is a pointer. It is created by a processes that forces it to “correspond point by point to nature”, but at the same time it points to something in the world: ‘that!’ I think this is an important point to keep in mind when discussing how the index operates on the viewer, which I will elaborate on at a later date.
The index is a type of sign that bears a physical connection to its referent; in photography, this is generally conceived as a physical imprint of light on chemical film (a trace). It is a non-negotiable connection to the real, identified by Roland Barthes and Rosalind Krauss as the quality essential to camerawork.
 Rosalind Krauss, “Notes on the Index,” October (3) Spring 1977: 70.
 C.S. Peirce, The Essential Peirce: Selected Philosophical Writings, eds. Nathan Houser and Christian Kloesel (Bloomington: Indiana University Press, 1992), 5.
 Ibid., 6.
 William Henry Fox Talbot, The Pencil of Nature [Online] (Project Gutenberg), https://www.gutenberg.org/ebooks/33447 (accessed November 3 2014).
 Roland Barthes, Camera Lucida (London: Vintage, 2000), 84.
 Rosalind Krauss, Perpetual Inventory (Cambridge, MT: The MIT Press, 2010).
 Rosalind Krauss, Notes on the Index, 75.
 C.S. Peirce, The Essential Peirce: Selected Philosophical Writings.