Semiotic Index

        A sign operates as an index when there is a concrete, tangible relationship between the signifier and the signified, as in:

Smoke→Fire

Fingerprint→Identity

Analogue groove in phonograph record→Voice/Other Sound Source

Some naturally occurring semiotic indices are natural binary oppositions, as well:

Reduced pupil opening→fear/anger/dislike  vs. Dilated pupil opening→attraction/pleasure