It is Secondness in the Second Correlate. The Index is the sign that represents its object by being materially connected to it. A pure Index can only make the existence of its object known to an interpreter, producing denotation (EP 2:460-461). In other words, the Index represents its object by blindly pointing to its existence. In fact, a finger pointing to something is a classic example of an Index. A pure Index does not bring any cognitive information about the Form of its object and, therefore, is not cognition (Short, 2004, p.12). In logic, it can only be a Selective (CP 2.296), or directions to find the Object of the proposition (CP 8.181).