The reality of a photograph was insisted upon by Charles Peirce when he classified a photograph as an 'index' rather than an 'icon' or 'image.' Thus: "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 [indices], those by physical connection" - Charles Peirce