The Commens Dictionary
Quote from ‘Meaning Preface’
…the mode of representation may be by likeness or analogy, in which case, the sign may be called an Icon; or it may be by a real connexion, as a certain kind of rapid pulse is symptom of a fever, in which case the sign may be called an indication or Index; or finally the only connexion may lie in the fact that the Sign (a word, for example) is sure to be interpreted as standing for the Object, in which case the Sign may be called a Symbol…
MS [R] 637:33-34
‘Index’ (pub. 13.01.15-16:19). 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-meaning-preface-2.