A signal is usually defined as an instrument for the communication of messages, or as an intentional index. Traffic signals are signals in this sense. Signals are the foundation of communication semiotics.
In Jakobson’s (and that of many others) work (Sebeok, 1974, p. 212; Prieto, 1975, p. 125-141, etc.), semiotics is considered as being totally or partially “within the general science of communication” (Jakobson, 1974, p. 36), but “communication” remains a principal element without a definition.
Eric Buyssens is one of the post-Saussurrean linguists who understand semiotics to be the study of all forms of all systems of communication. Semiotics, in this perspective, studies the procedures of communication which are recognised as such, as a means of influencing another. The analysis of what is called semias (a term which includes a system of semes organised according to relations of opposition) is a semiotic act (communication occurs by means of a semiotic act) and constitutes one of the main contributions to linguistic semantics of the last few years. Luis J. Prieto continued Buyssen’s studies and is particularly interested in the problem of communication, that is, the transmission of a message or a semiotic act. For this writer, a signal is included in the broader category of indexes: its specificity comes from the fact that it was produced to serve as an index. Road or navigation signals are the most quoted examples. A signal is an instrument intended to transmit messages between a transmitter and receiver. In order for a message to be transmitted: 1) the receiver must understand what the intention of transmitting the message is; 2) the receiver must identify the message. These are the two functions that a signal must fulfil. A signal is defined as an instrument for the communication of messages. In fact, Bloomfield defines language as a system of signals (1933, p. 162).
Signal, index and symptom are three terms frequently used to represent a category of signs, as opposed to symbol or signal. There are those who define these terms as synonyms. In the typology of signs, when a sign is seen conventionally or automatically and sets off a reaction on the part of the receiver, it is said to function as a signal. Noth uses four groups of criteria and defines a signal as a significant, a semiotic stimulus, a kind of index or a certain type of basic sign.
According to Resnikow, a signal is (1) an artificially created sign with a conventional meaning (2) intended to produce a particular behaviour or change in behaviour; (3) they are created for use in the future; (4) they are usually simple in structure and easily remembered. The highway code is designed to be seen. There may be various signs on the same signal: a round shape corresponds to an “obligation”, the red border specifies that this obligation is negative (that means a prohibition), the black design inside designates those who are the “intended receivers of the prohibition”. Understanding the signal implies simultaneously understanding these three aspects of the information.
The word symptom suggests, above all, the field of medicine. For Hippocrates, symptoms were phenomena with meanings, understood as natural signs (Heidel, 1941, p. 62). In the Science of Medicine, Hippocrates states: “That which we cannot see should be visualised mentally and the doctor who is unable to see the nature of the disease, nor obtain information concerning it, should speculate, based on the symptoms”. If Hippocrates studied the disease as a naturalist, Galen “dared to modify nature as a scientist” (Majno, 1975, p. 396), considering the “anti-natural” which occurs in the body as a symptom and the sum of the symptoms as a syndrome. A syndrome is a configuration of signs with generally accepted indication. These two terms have strong medical connotations. Pain is a symptom which expresses a message forcing the nervous system to affect public and private behaviour. Thus, he sought signs of pain by means of phylogeny and ontogeny, hic et ubique (Sebeok, 1996, p. 63).
For Peirce a symptom is never distinguished from the idea of sign - something which represents something to someone - an index is a subspecies of this. “ The symptom itself is a legisign, a general type of a defined character. The occurrence in a particular case is a sinsign” (§ 8.335).The signals of a sickness do not “speak”, unlike signs of the weather. We have an index, said Peirce in 1885, when there is “a dual relationship between the sign and its object, independent of the mind that is using the sign... All natural signs and physical symptoms are of this nature” (3, p. 361). Symptoms, according to Peirce are unconscious indexes, which may be interpreted by their receivers, without the realism of an intentional transmitter. The footprint found by Robinson Crusoe in the sand was an index (or indication) “that there was another human being on the island”. St. Augustine’s class of signa naturalia, defined in contrast to signa data (In Doctrina Christiana, 2.1.2) beyond its orthodox sense (a rash as a symptom of badness), is illustrated by the footprints of an animal, as being seen as an omen or evidence. That is to say, symptoms function as traces - footprints, droppings, movements in the undergrowth, the remains of food, etc. - in the animal world, operating as metonyms (Sebeok, 1976, p. 133). In short, a symptom is a compulsive, automatic, non arbitrary signal. The apparent real, as Barthes said.
In information theory a signal is understood to be any unit which, following the rules of a code, is part of the composition of a message. L. Hjelsmlev uses “signals” to refer to the units of minimal manifestation of monoplane semiotics (scientific - algebra - non-scientific: games). Information theory defines sign as a technical term for a vehicle or significant signal. In Cherry’s definition, a signal is “the physical embodiment of a message (an utterance, a transmission, an exhibition of the happenings of signs” (1957, p. 308). In this definition, a signal is not a type of sign but only the occurrence of a sign type. Morris already defined a signal as being, in terms of semiotics, more primitive than a symbol. In Sebeok’s definition, a signal is a sign which mechanically (naturally) or conventionally (artificially) provokes some reaction in a receiver.
The receiver may be both a machine or an organism, and includes a personified supernatural being (Sebeok, 1972b, p. 514). The activity of signals is produced by an individual organism; represents information; is mediated by a physical carrier and is understood and replied to by one or more individuals. An example of a signal is the discharge of a pistol on discovering a footprint (a conventional trigger vs. a mechanical gun trigger). Benveniste defines a signal as a primitive semiotic stimulus, that is as “one physical fact linked to another physical fact by a natural or conventional relation”. The anthropo-semiotic nature of a symbol distinguishes it from a sign.
Lasar Ossipowitsch Resnikow, (1964) 1969, citado por W. Nöth, in Handbook of Semiotics, Indiana University Press, 1995, p. 112.