• Section I (from ch. 23 of Carnap’s book). Carnap frames his introduction to the distinction between theories and other parts of science in ch. 23 as a distinction between theoretical laws and empirical laws. The various issues surrounding this distinction between the theoretical and the observable will probably form the heart of our discussion of this chapter.

However, not everyone would accept Carnap’s assumption that a theory consists of laws. The most common alternative view sees a theory as given by a model or collection of models. Such models are often very abstract, so they must be presented by descriptions that could be seen as laws; but, on this alternative to Carnap’s view, the theory is to be found in the model, not the description of it.

Another respect in which this view differs from Carnap is that while, for him, laws provide the only grasp we have of the theory, someone who holds that theories are models may hold that one route to the model is by way of analogy. The classical example is the kinetic theory of gases, which Carnap describes in the second paragraph of §II (p. 321) and again at the beginning of §III (pp. 326-327). The analogy here is with ordinary physical objects, and it is only an analogy because certain features of these objects (for example, the material composing them) may be irrelevant for the model.

This disagreement about the nature of scientific theories is related to the difference between positivists and realists. A positivist thinks the first question we should always ask is how an aspect of science is connected to our sense experience, and the connection between nonobservable entities and sense experience comes by way of laws that enable us to predict some sense experiences on the basis of others (or enable us to derive further laws that do this). On the other hand, a realist thinks that the first question we should ask about science is the sort of underlying reality that science is uncovering, and a model provides a way of pointing to that reality by way of analogy.

So, in addition to thinking through the difference between the theoretical and observable as Carnap discusses it, ask yourself which of these two ways of understanding the theoretical seems preferable. Should we think of laws as primary, models serving merely as a way of suggesting or illustrating laws, or should we think of models as primary, with laws serving merely as a way of describing models?

• Section II (from ch. 24). Although Carnap agrees with someone who thinks of theories as models in holding that theories have a substantial degree of autonomy in relation to the laws that may be derived from them, he also insists that there must be some connection between the nonobservable vocabulary of the theory and the observable vocabulary used to state empirical laws. He describes this views on this connection in this section. He refers early on to P. W. Bridgman (1882-1961), an American physicist. Bridgman originated the idea of “operational definitions,” and one of Carnap’s chief aims in this section is to distinguish his views from Bridgman’s. (Carnap also mentions N. R. Campbell, 1880-1949, a British physicist who wrote on the philosophy of science; Campbell is a classical source for the view of theories that centers on the idea of analogy.)

• Section III (from ch. 25). Carnap addresses two topics in this section: (i) reduction and the unity of science and (ii) the demarcation of science and non-science. He notes that it is possible for one branch of a science to be derived from another that began as the study of entirely different phenomena. When this happens, the branch whose laws have been explained is sometimes said to be reduced to the branch that explains these laws. As Carnap notes in passing on p. 329, this sort of reduction can draw together branches of science. Such unification was seen as an important goal by the logical positivists. Indeed, they called one of their chief publications the International Encyclopedia of Unified Science. (In a way, you will read part of this work since it is one of central ironies of the philosophy of science in the 20th century that the work that did the most to move the philosophy of science away from logical positivism—Kuhn’s The Structure of Scientific Revolutions—was first published as a component of the Encyclopedia of Unified Science.) Carnap discusses the second topic, demarcation, more briefly; he views are an example of the position Thagard labeled “verificationism” (see pp. 66, 68f).