A Photograph as an icon and an index

"In relation to photography, similarity generally means visual resemblance: a photographed portrait, like a painted one, is an icon. An index, however, represents its object through contact: it points at its object, or it is itself a trace of, or mark made by, that object. A thumbprint is an index. Because the item had to be there for an indexical representation of it to exist, it is often thought that an index is inherently more persuasive that in icon. A photograph is both an icon and an index; it is like an icon with a seal of approval, or, as Barthes calls it, a "certificate of presence" ".

Photography Degree Zero, Reflections on Roland Barthes's Camera Lucida, page 76.