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. 1) The "icon" acts chiefly by a factual similarity between its signans and signatum, e.g., between the picture of an animal and the animal pictured; the former stands for the latter "merely because it resembles it". 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 [...]. 3) The "symbol" acts chiefly by imputed, learned contiguity between signans and signatum. This connection "consists in its being a rule" and does not depend on the presence or absence of any similarity or physical contiguity.
- Jakobson (2004), a pag.15-16