Rudolf Carnap, a German-born philosopher and naturalized U.S. citizen, was a leading exponent of logical positivism and was one of the major philosophers of the twentieth century. He made significant contributions to philosophy of science, philosophy of language, the theory of probability, and classical, inductive and modal logic. He rejected metaphysics as meaningless because metaphysical statements cannot be proved or disproved by experience. He asserted that many philosophical problems are indeed pseudo-problems, the outcome of a misuse of language. Some of them can be resolved when we recognize that they are not expressing matters of fact, but rather concern the choice between different linguistic frameworks. Thus the logical analysis of language becomes the principal instrument in resolving philosophical problems. Since ordinary language is ambiguous, Carnap asserted the necessity of studying philosophical issues in artificial languages, which are governed by the rules of logic and mathematics. In such languages, he dealt with the problems of the meaning of a statement, the different interpretations of probability, the nature of explanation, and the distinctions between analytic and synthetic, a priori and a posteriori, and necessary and contingent statements.
Meaning and Verifiability
Perhaps the most famous tenet of logical empiricism is the verifiability principle, according to which a synthetic statement is meaningful only if it is verifiable. Carnap sought to give a logical formulation of this principle. In The Logical Structure of the World (1928) he asserted that a statement is meaningful only if every non-logical term is explicitly definable by means of a very restricted phenomenalistic language. A few years later, Carnap realized that this thesis was untenable because a phenomenalistic language is insufficient to define physical concepts. Thus he choose an objective language ("thing language") as the basic language, one in which every primitive term is a physical term. All other terms (biological, psychological, cultural) must be defined by means of basic terms. To overcome the problem that an explicit definition is often impossible, Carnap used dispositional concepts, which can be introduced by means of reduction sentences. For example, if A, B, C and D are observational terms and Q is a dispositional concept, then