The main aim of the so-called structuralist approach in the philosophy of science is to explicate the conceptual structure and empirical content of scientific theories and to account for the different ways (historical or systematical) they are related to each other. Structuralism tries to meet this aim by providing a (logical and set-theoretical) framework that allows a description of scientific theories which is more refined, more precise, and more complete than traditional ones, for example, the received view of logical empiricism. This new approach also tries to make perspicuous what is common to the logical structure of scientific theories in general and to take pragmatic and conventional aspects into account. (See Balzer, Moulines, & Sneed, 1987, for the latest version of structuralism.) One of the most original results of structuralism is concerned with the role of theoretical terms in science. The main idea is roughly this: A term is theoretical not because it is unobservable (as logical positivism claimed) but because it cannot be applied without presupposing a successful application of the theory in which it appears.