In his 1916, Cours de linguistique générale, the French linguist, Ferdinand de Saussure, defined language as a human fact elaborated from a system of linguistic signs. According to him, the linguistic sign was the combination of an acoustic image and a concept. Each word we use is a linguistic sign. For instance, the term 'ox' is made of the acoustic image [o-k-s], to which the concept of a big animal with a couple of horns, and so on, is associated. The concept is the mental representation we make of the word described by the acoustic image.
From now on, we will use the term 'word' for 'linguistic sign'. Each concrete word has a concept which is associated to it. The latter has common features in the minds of people sharing a common culture. For instance the word 'table' evokes a top with four legs for almost everybody in Western Civilization. With time, and individuals, the concept may change a little. The concept of a table in the twelfth century was probably slightly different from today's concept. For one, it was probably more rustic. However, basically, the very concept did not change. Consequently, hypothetically, a twelfth-century man could recognize a contemporary table as we can recognize a twelfth-century table. This makes us say that the concepts associated to concrete words do not evolve very much. This quasi-unchanging nature of the concept makes them easy to define.
Each abstract word is also the association of an acoustic image with a concept. However, contrary to what we had with concrete words, the concept cannot be materialized. This is the reason why it is more difficult to give a definition of abstract words than of concrete words. For abstract words, we will talk about 'notions', to indicate that the concepts which are associated are more or less confused. The meaning of notions is more likely to change with time because of their abstract feature. Indeed, with time, the abstract concept related to a notion can change easily. For instance, the notion of music as we hear it today is very far from what it was in the twelfth century. We consider music a work of the twelfth century, whereas the twelfth-century man we talked about earlier may not consider today's electronic music a form of music. Hence, with time and the evolution of society, the meaning of a notion is likely to be modified, and even changed.
Like each notion, its meaning can evolve with time and society. Indeed, the evolution of the society which uses the notion has repercussions on the definition given to the notion by the latter. Each time, the improvement of knowledge concerning the notion will tend to specify the concept. People will try to define it, yet the concept in itself cannot be truly defined since it remains abstract. Let us focus on the example of Art. The notion of Art has evolved with time. Little by little people came to define different forms of Art, thus enlarging the borders of the definition. The graffiti was first seen as a form of urban delinquency before some people came to expose those paintings as a form of modern Art. Today, the graffiti has become one more concept for the notion of Art, thus enlarging the notion and consequently modifying it. What I mean is that notions are in perpetual mutations, they evolve because of the abstract feature of their concepts with the evolution of society through time.
When we use the term 'drug', we use a notional word. Actually, and contrary to what people think, the term 'drug' is an abstract word. The number of different definitions in the various books dealing with the subject confirm this. When people try to think about the concept of drug, they think about examples of substances considered drugs rather than about the concept itself. Heroin, cocaine or marijuana are, in fact, different concepts related to the concept of drug. People, tend to confuse the notion of drug with those concepts. To such an extent that some people, or even books, tend to see the notion of drug as a concrete word. In the common language, drug has even almost become a concrete word. However, it is a genuine notion, perpetually evolving with time and the society in which the term is used. Hence, in the same manner as the graffiti has become a form of Art by entering in the list of the various concepts associated with the notion of Art, we could imagine that, with time, goods such as sugar, or Coca-Cola-which we would never have thought of as drugs-could be added on the list of concepts related to the notion of drug and enlarge the list of drugs.
From this point of view, Coca-Cola is particularly interesting. First, because the story of the soft drink since its creation in the late nineteenth century gives an account of the evolution of the concept of drug since it used to be considered a drug and it no longer is. Secondly, because through its story we can witness the evolution of America as regards drugs. And third, because from the example of today's number one American soft drink, and the evolution of the concepts we could be tempted to consider Coca-Cola a drug.