ICON, INDEX and SYMBOL (Long Version)

Updated Sept 4, 2000 (R. Port), Linguistics L103, Fall, 2000

To the Short Version

People who study signs and communication differentiate three kinds of signs: an ICON from an INDEX from a SYMBOL. This distinction is very important and derives from philosopher C. S. Peirce in the late 19th century. This page is an attempt to sharpen the difference between these three which are described in your text (CELL, pp. 1-4). The critical issue is to appreciate what a symbol is. This is the key to understanding language and how it differs from any animal communication systems.

First, we must note that a sign is a stimulus pattern that has a meaning. The difference between the various kinds of sign has to do with how the meaning happens to be attached to (or associated with) the pattern.


The icon is the simplest since it is a pattern that physically resembles what it `stands for'.

  1. A picture of your face is an icon of you.
  2. The little square with a picture of a printer on your computer screen is an icon for the print function. (Whereas a little box that has the word `PRINT' is not an icon since it has no physical resemblance to printing or the printer.)
  3. The picture of a smoking cigarette with a diagonal bar across the picture is an icon that directly represents `Smoking? Don't do it' (at least it does with appropriate cultural experience).
  4. Your cat is preparing to jump up on your lap, so you put out the palm of your hand over the cat to prevent him from jumping. The first time, you may physically impede his jump (This is not a sign at all), but after a couple times, just putting your palm out briefly becomes an iconic sign for `You aren't welcome on my lap right now.' The gesture is an icon because it physically resembles an act of preventing him from jumping, even though it would not prevent him if he really wanted to do it.
  5. Words can be partly iconic too. Bow-wow, splash and hiccup resemble the sounds they represent- at least a little. And the bird called the whippoorwill produces a call resembling this English phrase, so whippoorwill is an iconic word. (These are also called onomotopoetic words.)
  6. Also words can be pronounced in an iconic way: His nose grew wa-a-a-ay out to here. Julia Childes grabbed that carrot and went CHOP CHOP CHOP CHOP. Aw, poor widdow ba-by!

Of course sometimes there could be a dispute about what `physical resemblance' means and how similar it must be. And just because we humans can recognize a little picture of something doesn't mean that any other animal could. (Do you think your cat could recognize a picture of a can of catfood and interpret it as ``Time to eat''? Not likely.) So physical resemblance is by no means a simple concept. But how and why an image or a sound has some particular semantic content for us humans is fairly easy to understand.


An `index' is defined by some sensory feature, A, (something directly visible, audible, smellable, etc) that correlates with and thus implies or `points to' B, something of interest to an animal. All animals exploit various kinds of indexical signs in dealing with the world. The more intelligent animals are good at learning and exploiting more sophisticated indices (thus a cat will use and learn many more indexical signs than a frog, a fish or an ant -- which tend to be restricted to ones acquired innately).


  1. dark clouds in the west are an index of impending rain (at least in Indiana),
  2. for a fish in the sea, the direction of greater light is the direction of warmer water,
  3. a limping gait is a sign that an animal is physically impaired,
  4. a scowling facial expression is an index of the person's displeasure or concern (to a human),
  5. sensing a pheremone in the air is an indexical sign (for some insects) that a sexually receptive member of its own species is located upwind,
  6. a particular alarm call in certain monkeys is a sign that the animal has either directly sensed (eg, seen, smelled, heard) a particular type of predator OR has heard another monkey give this predator alarm call.
  7. a particular pronunciation of a word is a index that someone comes from a particular geographic place or social group.

Note that all of these above depend on a certain statistical regularity of part A (the signal pattern) with part B (the behaviorally relevant state). The exploitation of this regularity requires first, detecting property A (which is not necessarily simple) and either learning (or innately knowing) its correlation with the B. In that case the animal will use A as an index for B.

Note that for humans, some indices can be artificial and manmade (rather than environmentally natural or innate to particular species):

  1. a beep from your oven can signal that the cookies are ready to be removed,
  2. a red stoplight is a sign that you should stop your car if you don't want to risk an accident,
  3. in an animal behavior experiment, a flashing light could be a sign that food will be available in a certain place or that a shock will soon follow.
  4. a person can wave their hand as a sign of recognition and greeting (though this may be partly iconic too).

Notice that the correlation need not be perfect. It isn't always warmer closer to the sea surface, dark clouds in the west don't always mean the rain is coming this way, and even a stoplight can be broken sometimes. This doesn't detract from the usefulness of these signs as a way for an animal to guide its life in a confusing and only partly predictable world.

Words are said to be indexical when they directly point to their meaning - without depending on any relationship to other words. Thus, words like here, there, I, me, you, this, etc. For all of these there is an implied pointing gesture. (Remember in Latin, index really meant the index finger.)


Words as Symbols.

Now, what about a noun word in a human language? Let's say English `KITTY'? Isn't this just a kind of arbitrary index? Isn't KITTY just an index for the presence of a cat (just for English speakers of course)? In support, one might note that a small child and its mother would be likely to say KITTY in the presence of a cat (so there should be some correlation between the cat and the word KITTY). The sounds [kIDi] correlate partially with the presence of cat (so A predicts B). Doesn't that show that this is just an indexical sign like those above? Unfortunately no - even if its true that most early words for children are learned indexically (that is, by pointing to what they refer to). In general, however, it is very rare for the utterance of a word to correlate with the thing it refers to. Sometimes such a correlation exists, of course, buta word in any language is vastly more complex and sophisticated even for language-learning infants. Notice that:

  1. You and your baby will also freely use the word KITTY when a cat is NOT around (so the correlation between KITTY and the cat is a very weak). [If your dog knows the `word' TAKE-A-WALK, try just discussing taking a walk in earshot of the dog and see what happens! Dogs have no grasp of `Talking about taking-a-walk'. That's because take-a-walk is only an indexical sign for your dog, not a symbol as it is for you and your baby.]
  2. Many words in every language describe objects that noone has ever seen, like MONSTER, UNICORN, GHOST, DEVIL, etc. (so the possibility of a ny correlation is ruled out completely)! What percent of the time that you utter the word ROCKET or TRAIN, do you suppose there is a physical rocket or train present? My guess is 0% for ROCKET and about 1-3% train. If there is no correlation or an extremely weak one, then these words cannot be indices.
  3. On the other hand, any word has strong associations with other words that are `activated' whenever a word is heard or read. Thus KITTY activates words like CAT, FUR, BABY, PURR, PUPPY, PLAY, SAUCER, MILK, YARNBALL, CATFOOD, etc.
    By `activate', I mean that you are more likely to think of or utter these other words after hearing or saying KITTY. (There are many kinds of experimental evidence for this, plus intuition.) This suggests that KITTY may be somehow physically linked to these other words in the brain. It suggests that KITTY gets some of its meaning from the selective activation of just these particular words (and their associated emotional content) when the word KITTY is spoken.

These word-word relationships (sometimes called word-associates) are critical for anchoring the meaning of a word without requiring any correlation in space and time between the signal (the sound of the word) and its meaning. Indices do not require any such set of relationships to work as signs. In summary, symbols like most words in a human language are (a) easily removable from their context, and (b) are closely associated with large sets of other words.

Notice that humans easily learn words for things we have never experienced. Children who grow up in the tropics learn to correctly use words like SNOW and ICE without ever seeing snow or ice. It is not a big problem for them because they have heard descriptions of them in terms of words they do know, like COLD, WHITE, CLEAR, HARD, SOFT, FLUFFY, WATER, MELT, FALLING, SLIPPERY, etc. From these descriptions, they get a pretty good idea what snow and ice are like - enough to read and produce the words appropriately.

This is the enormous power of human symbols: When you have learned a basic vocabulary (based in part on indexical relationships), you can use it to bootstrap to many other new concepts and words. And given the possibility of cultural transmission from generation to generation, human knowledge and understanding become cumulative and have grown at a very rapid rate (relative to the creation and transmission of innate knowledge).


Apparently no living nonhuman animals are able to use word-like symbols.

There are, however, some (disputed) claims that a few individual animals (mostly higher primates like monkeys, chimpanzees and gorillas) have been trained by humans to use a small (< 50) inventory of symbol-like units using hand signs or small physical tokens. If this claim is true, it implies a huge divide between humans and nonhuman animals. It means that no animal communication systems can be understood as just `simple versions of human languages'. This claim is daring and provocative, but probably true. [Of course, if one believes that humans are derived from nonhuman animals, then somehow our ancestors must have passed through stages that were intermediate between index-based communication systems (like dogs, monkeys, bees, whales, etc) and modern-human symbolic language even though we have very little direct evidence about how this evolution took place.]

Nonword Symbols.

Words (especially nouns, verbs and adjectives) are the architype for symbols. But the most common use of the term symbol in everyday, nontechnical language is for signs that are not words: eg, a flag or totem animal as the symbol of a country (bald eagle for USA, bear for Russia, etc), a cross for Christianity, star of David for Judaism, swastika for Nazism, a particular type font for a specific product (eg, Coca-Cola, Indiana University, etc).

It seems that a similar set of associations to other words exist for such symbols. Thus, the US FLAG (that is, the graphic pattern in red, white and blue, not the English word FLAG) gets its meaning partly from its association to words and concepts like: HOMELAND, WASHINGTON, BALD EAGLE, PATRIOTISM, MOM, DAD, APPLE PIE, PRIDE, HEROISM, DEMOCRACY, `OH SAY CAN YOU SEE...', `I PLEDGE ALLEGIANCE...', SACRIFICE, etc etc.

Mathematical and logical symbols also get their meaning from their relation to other symbols. Thus pi is defined as the ratio of the circumference of a circle to its diameter: pi = c/d.

So, nonword symbols are much like words but often lack a phonetic form.


The term sign is often used for all three of these: icons, indices and symbols. All have a signal aspect, some physical pattern (eg, a sound or visible shape) and a meaning (some semantic content that is implied or `brought to mind') by the signal. But they differ in that icons have a physical resemblance between the signal and the meaning and an index has a correlation in space and time with its meaning. But a symbol is an arbitrary pattern (usually a sound pattern in a language) that gets its meaning primarily from its mental association with other symbols and only secondarily from its correlation with environmentally relevant properties.