An informant index is a proposition. For one cannot better define a proposition (as distinguished from the assertion whereby one assumes responsibility for its truth) than as a representation of which one part serves, directly or indirectly, as an index of its object, while the other part excites in the mind an image of the same object.
http://www.commens.org/dictionary/entry/quote-logical-tracts-no-1-existential-graphs-5