The Commens Dictionary
Quote from ‘Logical Tracts. No. 1. On Existential Graphs’
An index is a representamen whose representative force depends upon its being factually connected with its object, and does not depend upon its being interpreted as a sign.
For example, the symptoms of disease are indices. For though they cannot serve as signs without being interpreted as such, yet that which renders them fit to be the signs they are is their factual connexion with the diseases, which would exist though nobody had remarked it.
An index must essentially be an individual existent fact or thing. Strictissime, therefore, it cannot function as an icon too, since an icon is only an appearance in consciousness. But an index must have some appearance connected with it; and according as that does or does not […] contribute to its representative force, we have an important division of indices into those which give information and those which merely serve to identify individuals. Of identifying indices, the letters which are attached to the singular points of a geometrical diagram are examples. Remembering that an icon, – or, indeed, any appearance, – has its being only in consciousness, we can readily convince ourselves that any informing index has an icon connected psychologically with it. A symptom calls up in the iatrical mind certain memories of disease. A weather-cock calls up an image of a quarter of the horizon.