  Schlick claimed the method of definition by axioms was implicit because, unlike explicit definition, occurrences of the defined term cannot necessarily be replaced by a combination of the expressions which define it. And he praised the method for its specification of meanings independently of any intuitive content. Implicitly defined terms possess a clarity and precision of scope which cannot be achieved by concepts defined by abstraction from experience. Since, of course, axiomatic definitions stipulate the meanings of all their constituent concepts in terms of the remaining ones, the axioms effectively define concepts by their relations to one another. Thus implicit definitions are structural definitions, and their constituent terms are structurally defined (Schlick 2009, Sec. 7). The concepts thus defined are only related to the other elements of the axiom system and are not related to anything external to the axiom system until the definition is coordinated with extra-linguistic things. In Schlick’s earlier writings, he had stated that concepts themselves are functions which signify or designate the items with which they are coordinated or associated. Accordingly, even implicitly defined concepts must be coordinated with objects, elements of the class of things to which it applies (Schlick 1979a, 130; Schlick 2009, 23; Ryckman 1991, Sec. 3). Of course, these objects, like the concepts which designate them, are distinguished by possession of the properties in terms of which the designating concepts are defined. Such coordinations give empirical content to implicitly defined concepts, transforming them into full-blooded concepts rather than empty place-holders. Moreover, the concepts which dominate scientific thought at any given stage of its development must be modified, revised, and supplemented as science advances. Then a conceptual characterization of a given phenomena which was used at an earlier stage may not be at a later stage as, for instance, the judgment ‘A light-ray consists of a stream of moving particles’, drawn from Newtonian optics was later replaced by ‘A light-ray consists of electromagnetic waves’. 