An index is a representamen whose special representative character depends upon its factual connection with its object and is independent of its being interpreted as a sign.

An index may be nearly or quite free from all iconic character; as Bunker Hill Monument, which was intended, as its designer said, merely to say “Here!” Or it may be predominantly iconic; as a photograph which resembles its object closely by virtue having been in physical connection with it. The iconic element may, as in this case, […] be combined with the indexical element in the whole representamen, or these characters may belong to separate parts of the representamen; as one of those hygroscopes where a little woman comes out of the house when the air is dry and goes in when it is moist, as a real woman would. This latter kind of index which conveys definite information is a proposition.