The Commens Dictionary
Quote from ‘An Elementary Account of the Logic of Relatives’
Signs, or representations, are of three kinds: Icons, Indices, and Tokens. [—]
Indices are signs which stand for their objects in consequence of a real relation to them. An index is a sign which stands for its object in consequence of having a real relation to it. A pointing finger is its type. Of this sort are all natural signs and physical symptoms. The index has no generality in itself. It does not depend on a mental association, but upon a real reaction between the mind and the external world at the moment when the index acts. The index asserts nothing; it only says “There!” It takes hold of our eyes, as it were, and forcibly directs them to a particular object, and there it stops.