The knowledge of science is given by its theories and laws, and is therefore expressed in statements. Science is a linguistic, or symbolic, representation of experience. For a scientific theory consists of sentences and equations, and these are, first of all, a series of signs. Signs, that is words as well as the symbols of mathematics, are arranged in a certain order, and we take account of experience by means of signs.
The notion of sign is very general. We say, for instance, that clouds are a sign of rain; or, we have natural signs which indicate a causal correlation between events. Sometimes, the sign is similar to the thing it stands for, in the manner in which a picture represents, and we have iconic signs. Finally, we have conventional signs, or symbols: the flag is the symbol for the country, or the red cross for medicine. The correlation is arbitrary, and it is a matter of convenience and of convention which sign is chosen. This is the case with language: words are conventional signs, or symbols, invented by men for the purpose of communication. They have arisen during the long history of mankind; but for this reason we are not always clear about the use a particular sign may have in a given situation.
The use of symbols not only presupposes a fixed shape of the physical marks by which they are represented. It requires equally that the symbols, or expressions (i.e. sequences of symbols), are combined in a definite manner if they are to function according to the intention of the user. The same word, for instance, is used repeatedly, and so we distinguish between the design, or type, of a sign and its individual occurrence, or token. But the symbols are also used in a certain way, that is, words (and other expressions) must be used according to rules if they are to possess meaning. Only then do we have a language.
This approach may appear unduly general. But it helps to remind us that the expressions of our language do not possess an inherent and fixed meaning, but that we give a meaning to them