Signs (or "representamina" in Peirce nomenclature) offer three basic varieties of semiosis, three distinct "representative qualities" based on different relationships between the signans and signatum. This difference enables him to discern three cardinal types of signs. [...] 2) The "index" acts chiefly by a factual, existential contiguity between its signans and signatum, and "psychologically, the action of indices depends upon association by contiguity"; e.g., smoke is an index of fire.
- Jakobson (2004), a pag.15-16