The Commens Dictionary
Quote from ‘Reason's Rules’
An Index is a thing which having been forcibly affected by its object, forcibly affects its interpretant and causes that interpretant to be forcibly affected by the object, and to affect its interpretant in turn; and which, further, so far as it is a sign, becomes a sign in this way. So far as it is a sign in any other way or sense it belongs to one of the other types of sign and is not a pure Index.
[An] Index represents its object by virtue of a real relation with it and determines whatever interpretant may be in a real relation with it and the object.