For all practical purposes, then, indices cannot testify to the existence of their objects. As discussed above, indexicality emerges as a potential sign, or, better, as a particular kind of ground characterising indexical signs, but which may also be found outside signs. Perception would seem to be profused with indexicality. Indeed, proximity is a basic factor of perception according to Gestalt psychology, and is also one of the relationships included in topological space perception. The relation of part to whole is fundamental to Gestalt relations themselves.
All indexical relations involve either contiguity or factorality .
Any experience of two elements being related by proximity, conceived as a primordial perceptual fact, may be considered an actual perceptual context involving contiguity. The term abduction is employed here in Peirce's sense, to signify a general rule or regularity which is taken for granted and which links one singular fact with another. The archaeologist's art, from this point of view, would consist in transforming indexicalities of decayed cultures into proto-indices accessible to us. Dubois, Philippe, L'acte photographique. Greenlee, Douglas, Peirce's Concept of Sign.