 A few notational conventions and preliminary considerations are necessary to explain the formal accounts of theoretical terms. Essential to all of these accounts is the division of the set of descriptive symbols into a set \(V_o\) of observational and another set \(V_t\) of theoretical terms. (The descriptive symbols of a formal language are simply the non-logical ones.) A scientific theory thus be formulated in a language \(L(V_o,V_t)\). The division of the descriptive vocabulary gives rise to a related distinction between T- and C-axioms among the axioms of a scientific theory. The T-axioms contain only \(V_t\) symbols as descriptive ones, while the C-axioms contain both \(V_o\) and \(V_t\) symbols. The latter axioms establish a connection between the theoretical and the observational terms. TC designates the conjunction of T- and C-axioms and \(A(\TC)\) the set of these axioms. Let \(n_1 ,\ldots ,n_k\) be the elements of \(V_o\) and \(t_1 ,\ldots ,t_n\) the elements of \(V_t\). Then, TC is a proposition of the following type: