The Commens Dictionary
Quote from ‘Notes on Topical Geometry’
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.
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.
MS [R] 142:3-4
‘Index’ (pub. 12.01.15-11:05). Quote in M. Bergman & S. Paavola (Eds.), The Commens Dictionary: Peirce's Terms in His Own Words. New Edition. Retrieved from http://www.commens.org/dictionary/entry/quote-notes-topical-geometry.