The Commens Dictionary
Quote from ‘Letters to Lady Welby’
I define an Index as a sign determined by its dynamic object by virtue of being in a real relation to it. Such is a Proper Name (a legisign); such is the occurrence of a symptom of a disease (the symptom itself is a legisign, a general type of a definite character. The occurrence in a particular case is a sinsign).
‘Index’ (pub. 05.05.13-10:36). 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-letters-lady-welby-24.