… the relation of a repraesentamen to its object (correlate) may be a real relation and, then, either an agreement or a difference, or it may be an ideal r[elati]on or one from which the reference to a correspondent (subject of representation) cannot be prescinded by position. [—] In the second case, there is a real difference of the repraesentamen from its object, that is to say not a mere difference in quality but also a bringing of them together in nature; in this case the representative character of the one will consist in constant accompaniment by the other, so that it indicates the existence of the latter without noting any characters of it. Such a representation may be termed an index.