 Lewis (1970) introduced a number of modifications concerning both the language of the Carnap sentence and its interpretation in order to attain proper definitions of theoretical terms. First, theoretical terms are considered to refer to individuals as opposed to relations and functions. This move is made coherent by allowing the basic language \(L(V_o)\) to contain relations like ‘x has property y’. The basic, i.e., non-theoretical language is thus no observation language in this account. Yet, it serves as the basis for introducing theoretical terms. The set \(V_o\) of ‘O-terms’ is best described as our antecedently understood vocabulary.