Full Idea
For Peirce there are three different kinds of sign, which are different kinds of representation, built on different relationships: an 'icon' represents what it resembles, an 'index' is a natural sign, and a 'symbol' is a conventional sign.
Gist of Idea
Icons resemble their subject, an index is a natural sign, and symbols are conventional
Source
report of Charles Sanders Peirce (Logic as Semiotic: Theory of Signs [1897]) by Barry Maund - Perception Ch.4
Book Reference
Maund,Barry: 'Perception' [Acumen 2003], p.77
A Reaction
Maund makes use of natural signs (like footprints) to explain representative perception. Peirce's distinctions seem useful in philosophy of mind generally, if the brain somehow represents what it experiences. How subjective are signs?