The Commens Dictionary
Quote from ‘On Existential Graphs’
An index represents its object by forcibly bringing it before the senses, or before the attention, appealing to “association by contiguity.” A pure index would present a pure sense-reaction. But again there is no such thing. Every index is considered as an individual sign; but this individuality will not bear cross-examination, but betrays more or less generality, because there is no pure index. Still we may call a proper name or demonstrative or personal pronoun an index. It appeals to individual recognition. Such words as yard which refer to individual prototypes have much of the index character. “Indefinite” pronouns, anything, something, etc. which tells us how to proceed in order to experience the object intended,—better called selective pronouns,—are almost indices.