A sign is a perceptible object – including a transient object such as a movement or sound – which stands for something else, its repraesentatum. In its function of serving as a sign, the physical object is dubbed ‘signal’. Leaving aside natural signs like dark clouds as a sign for imminent rain, a sign is transmitted from a sender to a receiver. Both interpret the signal as hinting at its repraesentatum, which it represents or evokes. In the most general sense, a sign renders inferences of something else possible (Keller 1995, ch. 10).
Classification by the physical nature of the signal
Signs may be classified by the physical nature of the signal, specifically by the sense that perceives it:
- A visual sign can be seen. Visual linguistic signs are gestures of a sign language or characters of some script.
- An auditory sign can be heard. Auditory linguistic signs are speech.
- A tactile sign can be felt. Tactile linguistic signs are commonly Braille.
Gestures are produced by moving body parts. Such movements are controlled. Therefore gestures are essentially signs sent intentionally. Auditory signs are produced by the voice. Vocal signs may be uttered voluntarily or spontaneously.
Classification by the link between signal and repraesentatum
The association between the sign and its repraesentatum has a basis which is known to sender and receiver. This basis may be an elementary relationship that imposes itself to the mind, viz. similarity or contiguity; or it may be specifically devised by the communication partners. Consequently signs are classified into three types by the kind of this basis (Peirce 1960):
- A sign which is similar to its repraesentatum is an icon.
Examples of icons include a topographic map and, in language, onomatopoetic1 items like buzz or bang. - A sign which is contiguous with its repraesentatum is an index.
Examples of indexes include sneezing as a symptom of a flue, a flare fired to draw attention to its source and, in language, an interjection like ouch as a sign of pain. - A sign which is associated with its repraesentatum by convention is a symbol.
Examples of symbols include the picture of a red heart symbolizing love and, in language, most words like dog and ache.
Examples of icons and indexes from natural languages suffer from the complication that they are not exhaustively motivated by similarity or contiguity, but always have a measure of conventionality. If this were not so, iconic and indexical words would be universal.
The most primordial signs are indexes (Keller 1995). Wernicke's region in the human brain, and its homologues in other vertebrate brains, are capable of recognizing a percept as indicating something else with which it has a relation of contiguity. Many gestures are indexical, above all the pointing at the repraesentatum. However, there are also symbolic gestures such as waving to bid farewell.
Since a symbol works on the basis of a convention specifically devoted to it, it needs no intrinsic motivation and can, in this respect, be completely arbitrary. For instance, there is nothing in the sound of the word dog which reminds one of a dog. This is true of the far majority of linguistic signs.
The repraesentatum
The repraesentatum of a sign may be a concrete object. Thus, the repraesentatum of a particular map may be a certain real town, the repraesentatum of a particular flare may be an individual boat in trouble, and the repraesentatum of a given red heart may be John's love to Linda. A linguistic sign used in a particular speech situation may likewise refer to a specific object. However, such a sign is always a token of a type. The type does not represent a particular object. Instead, its repraesentatum is a mental object, its meaning or, technically, its designatum; at any rate, something abstract.
1 Some native speakers and dictionaries prefer the form onomatopoeic. However, the Greek base for this adjective is the same as for poetic.