As a field of academic enquiry, semiology has its origin in linguistics as developed by the Swiss academic Ferdinand de Saussure. In the late nineteenth and early twentieth centuries, Saussure gave an influential series of lectures on linguistics in which he proposed semiology as a model for the investigation of language and language systems. Saussure's work was unusual in several respects, not least because, counter to the dominant approach advocated by linguists at the time, he was not concerned with uncovering the etymology of language but with the ways in which language was used in the here and now, an approach that is now usually referred to as "'synchronic' rather than 'diachronic."' Saussure did not publish his work, but following his death in 1912, his students collected his lecture notes and published them as Course in General Linguistics .
Saussure's major concern was to develop a science of signs. A sign can be understood as anything that carries meaning, although Saussure himself was interested exclusively in linguistic signs—that is, words. He argued that a sign consists of two indivisible components: the signifier (the way the sign is communicated) and the signified (the mental concept the sign communicates). We know that something is a sign because its two parts are indivisible—that is, we see something and we can make sense of it by giving a name to it. Saussure called this process of reading and making sense of a sign "signification."
By way of an example, the three letters C- A- T, in this specific order, mean something in our language system and culture. They stand in for a cat. So in this order, these three letters are a sign. The signifier here is the three letters in THIS specific order, and the signified is OUR mental concept of a cat. Crucially, Saussure notes, the relationship between the signifier and the signified is an arbitrary one. For example, the word "cat" does not look like a cat, nor does it have any essential "catness" about it. Through convention, people have agreed that those three letters stand for the concept of cat in our language and culture. The evidence of this is that in Switzerland and France, for example, the four letters C- H- A- T are a sign meaning the same thing in French.
In the United States during this same period, the pragmatist and philosopher Charles Peirce was investigating signs and sign systems, and he developed a theoretical model that he called semiotics. Peirce's semiotics was not confined to linguistic theory in the same way as Saussure's; it was more fully integrated into his philosophical interests, and it is this broader application of a theory of meaning systems that distinguishes his work.
Peirce argued that signs can be categorized as belonging to three distinct categories; iconic, indexical, and symbolic. An iconic sign looks like the thing it represents. For Peirce, this was the most effective of all forms of sign system. An indexical sign possesses some kind of physical link between the sign and the thing it represents, providing evidence that the thing represented was there. Smoke, for example, is an indexical sign of fire. A symbolic sign is arbitrarily linked to what it represents; it neither looks like the thing represented nor possesses a physical link to the thing represented. It is a sign that stands in the place of the thing represented. The written word is the best example of a symbolic sign.
Signs in Peirce's model can belong to more than one category simultaneously. This is important in film, where cinematic images are both iconic—that is, they look like the thing represented—and indexical—that is, they are evidence that someone/thing was present to be photographed. Animated and computer-generated images can be iconic but not indexical. Similarly, sound can be iconic (a voice can sound like the filmed person's voice), indexical (noises in another room can suggest that someone is there), or symbolic (a musical theme can suggest a character in a film).