Symbols have one characteristic in common with signs; they point beyond themselves to something else. The red sign at the street corner points to the order to stop the movements of cars at certain intervals. A red light and the stopping of cars have essentially no relation to each other, but conventionally they are united as long as the convention lasts. The same is true of letters and numbers and partly even words. They point beyond themselves to sounds and meanings. They are given this special function by convention within a nation or by international conventions, as mathematical signs. Sometimes such signs are called symbols; but this is unfortunate because it makes the distinction between signs and symbols more difficult. Decisive is the fact that signs do not participate in the reality of that to which they point, while symbols do. Therefore, signs can be replaced for reasons of expediency or convention, while symbols cannot.