 The replacement of theoretical relation and function symbols with individual terms was judged counterintuitive by Papineau (1996). A reformulation, however, of Lewis’s definitions using second- or higher-order variables is not difficult to accomplish, as Schurz (2005) has shown. In this reformulation the problem arises that theoretical terms are usually not uniquely interpreted since our observational evidence is most of the time insufficient to determine the extension of theoretical relation and function symbols completely. Theoretical functions, such as temperature, pressure, electromagnetic force etc., are determined only for objects that have been subjected to appropriate measurements, however indirect. In view of this problem, Schurz (2005) suggests letting the higher-order quantifiers range only over those extensions that correspond to natural kind properties. This restriction renders the requirement of unique interpretation of theoretical terms plausible once again. Such a reading was also suggested by Psillos (1999: ch. 3) with reference to Lewis’s (1984) discussion of Putnam’s (1980) model-theoretic argument. In that paper, Lewis himself suggests the restriction of the interpretation of descriptive symbols to extensions corresponding to natural kind properties.