Signs are of three kinds,
1st, the icon, which represents its object by virtue of a character which it would equally possess did the object and the interpreting mind not exist;
2nd, the index, which represents its object by virtue of a character which it could not possess did the object not exist, but which it would equally possess did the interpreting mind not operate;
3rd, the symbol, which represents its object by virtue of a character which is conferred upon it by an operation of the mind.
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An index represents an object by virtue of its connection with it. It makes no difference whether the connection is natural, or artificial, or merely mental.
http://www.commens.org/dictionary/entry/quote-notes-topical-geometry