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Charles Peirce also divided signs (or, as Saussure would have it, signifiers) into three categories. These are:

  • Iconic: These signs have some resemblance to that which they signify. A picture of a man on the door of the men's bathroom is an iconic sign, as is a photograph of a person: the image bears a resemblance to its referent.

  • Indexical: These signs have a causal link to their referents. Often, these signs can be considered to be parts of their referents: smoke, for example, is an indexical sign of a fire. A footprint, too, is an indexical sign: the print is caused by the foot.

  • Symbolic: These signs have no intrinsic relationship to their referents: they are completely arbitrary. An octagonal red sign, meaning "Stop," is an example of a symbolic sign, as there is no real reason that red octagons should be associated with stopping a car. Almost all human language (onomatopoeiac words are debatable) is symbolic in nature.

These categories are not exclusive: it's possible to find signs which combine elements of different categories. For example, a red traffic light is both symbolic and indexical, as the red light arbitrarily signifies "stop," while the illumination of the light is caused by the flow of electricity. However, signs are usually classified according to their most meaningful properties: therefore, the light would be considered a symbolic sign, as its indexical properties are incidental to its primary significance. In other words, it doesn't matter whether it's a candle or a light bulb making the red light: red lights mean "stop."

I guess a photograph of a stop sign that someone's stepped on would combine all three categories...