The term "symbol' is a relative term: for anything to be a symbol, it must have a reference. It is a symbol only by virtue of standing for something, or being a surrogate, often for a stimulus not present. The words of natural language are symbols the reference of most of which goes back in the last analysis to factors in conscious experience. Such reference is not precise, and since it is desirable that logical symbolism be precise, logic has come to be elaborated in artificial languages the symbols of which do not depend on reference to experience. The symbols of a strict logical system should not depend on any reference they have outside the systematic structure.