There may be a mere relation of reason between the sign and the thing signified; in that case the sign is an icon. Or there may be a direct physical connection; in that case, the sign is an index. Or there may be a relation which consists in the fact that the mind associates the sign with its object; in that case the sign is a name.
http://www.commens.org/dictionary/entry/quote-one-two-three-fundamental-categories-thought-and-nature-0