An index is a sign that represents its object through a causal, spatial, or temporal connection. Smoke indexes fire. A weathervane indexes wind direction. A pointing finger indexes whatever it is directed toward. A fever indexes infection. Footprints index the passage of a body.
The defining feature of the index is that the connection between representamen and object is not arbitrary (as in a symbol) or based on resemblance (as in an icon) but existential — the sign is connected to the object by a real relation in the world (Peirce, 1931–1958).
In language, indexical expressions — pronouns like “I,” “this,” “here,” “now” — derive their meaning from the context of utterance rather than from convention alone. What “here” refers to depends on where the speaker is standing. This context-dependence is what makes them indexical (Chandler, 2007).
Related terms
- icon — a sign that resembles its object
- symbol — a sign connected to its object by convention
- sign — the triadic relation in which index is one type
Source: Peirce, Charles Sanders. Collected Papers. Harvard University Press, 1931–1958.