Rudolf Carnap

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Rudolf Carnap (May 18, 1891–September 14, 1970) was an influential philosopher who was active in central Europe before 1935 and in the United States thereafter. He was a leading member of the Vienna Circle and a prominent advocate of logical positivism.


Carnap was born in Ronsdorf, Germany, to a north German family that had been humble until his parents' generation. He began his formal education at the Barmen Gymnasium. From 1910 to 1914, he attended the University of Jena, intending to write a thesis in physics. But he also carefully studied Kant's Critique of Pure Reason in a course taught by Bruno Bauch, and was one of very few students to take Frege's courses in mathematical logic. After serving in the German army during WW I for three years, he was given permission to study physics at the University of Berlin, 1917-18, where Albert Einstein was a newly appointed professor. Carnap then attended the University of Freiburg, where he wrote a thesis setting out an axiomatic theory of space and time. The physics department said it was too philosophical, and Bruno Bauch of the philosophy department said it was pure physics. Carnap then wrote another thesis, under Bauch's supervision, on the theory of space from a more orthodox Kantian point of view, published as Carnap (1922).

In 1921, Carnap wrote a fateful letter to Bertrand Russell, who responded by copying out by hand long passages from his Principia Mathematica for Carnap's benefit, as neither Carnap nor Freiburg could afford a copy of this epochal work. In 1924 and 1925, he attended seminars led by Edmund Husserl, the founder of phenomenology, and continued to write on physics from a logical positivist perspective.

Carnap discovered a kindred spirit when he met Hans Reichenbach at a 1923 conference. Reichenbach introduced Carnap to Moritz Schlick, a professor at the University of Vienna who offered Carnap a position in his department, which Carnap took up in 1926. Carnap thereupon joined an informal group of Viennese intellectuals that came to be called the Vienna Circle, led by Moritz Schlick and including Hans Hahn, Friedrich Waismann, Otto Neurath, and Herbert Feigl, with occasional appearances by Hahn's student Kurt Gödel. When Wittgenstein visited Vienna, Carnap would meet with him. He (with Hahn and Neurath) wrote the 1929 manifesto of the Circle, and (with Hans Reichenbach) founded the philosophy journal Erkenntnis.

In 1928, Carnap published two important books:

  • The Logical Structure of the World, in which he developed a rigorous formal version of empiricism, defining all scientific terms in phenomenalistic terms. The formal system of the Aufbau, as this book is often called by virtue of the first word of its German title, was grounded in a single primitive dyadic predicate, which is satisfied if two individuals "resemble" each other. The Aufbau was greatly influenced by Principia Mathematica, and warrants comparison with the mereotopological metaphysics A. N. Whitehead developed over 1916-29. It appears, however, that Carnap soon became somewhat disenchanted with this book. In particular, he did not authorize an English translation until 1967.
  • Pseudoproblems in Philosophy asserted that many philosophical questions were meaningless, i.e., the way they were posed amounted to an abuse of language. An operational implication of this radical stance was taken to be the elimination of metaphysics from responsible human discourse. This is the notorious position for which Carnap was best known for many years.

In February 1930 Tarski lectured in Vienna, and in November 1930 Carnap visited Warsaw. On these occasions he learned much about Tarski's model theoretic approach to semantics. In 1931, Carnap was appointed Professor at the German language University of Prague. There he wrote the book that was to make him the most famous logical positivist and member of the Vienna Circle, his Logical Syntax of Language (Carnap 1934). In 1933, Willard Quine met Carnap in Prague and discussed the latter's work at some length. Thus began the lifelong mutual respect these two men shared, one that survived Quine's eventual forceful disagreements with a number of Carnap's philosophical conclusions.

Carnap, under no illusions about what the Third Reich was about to unleash on Europe, and whose socialist and pacifist convictions made him a marked man, emigrated to the United States in 1935 and became a naturalized citizen in 1941. Meanwhile back in Vienna, Moritz Schlick was assassinated in 1936. From 1936 to 1952, Carnap was a professor of philosophy at the University of Chicago. Thanks in part to Quine's good offices, Carnap spent the years 1939-41 at Harvard, where he was reunited with Tarski. Carnap (1963) later expressed some irritation about his time at Chicago, where he and Charles W. Morris were the only members of the department committed to the primacy of science and logic. (Their Chicago colleagues included Richard McKeon, Mortimer Adler, Charles Hartshorne, and Manley Thompson.) Carnap's years at Chicago were nonetheless highly productive ones. He wrote books on semantics (Carnap 1942, 1943, 1956), modal logic, coming very close in Carnap (1956) to the now-standard possible worlds semantics for that logic Saul Kripke proposed starting in 1959, and on the philosophical foundations of probability and induction (Carnap 1950, 1952).

After a stint at the Institute for Advanced Study at Princeton, he joined the philosophy department at UCLA in 1954, Hans Reichenbach having died the previous year. He had earlier declined an offer of a similar position at the University of California, because taking up that position required that he sign a McCarthy-era loyalty oath, a practice to which he was opposed on principle. While at UCLA, he wrote on scientific knowledge, the analytic - synthetic dichotomy, and the verification principle. His writings on thermodynamics and on the foundations of probability and induction, were published posthumously as Carnap (1971, 1977, 1980).

Carnap taught himself Esperanto when he was a mere fourteen years of age, and remained very sympathetic to it (Carnap 1963). He later attended a World Congress of Esperanto and employed the language while traveling.

Carnap had four children by his first marriage, which ended in divorce in 1929. His second wife committed suicide in 1964. He died in Santa Monica, California.



Rejecting metaphysics

For more information, see: The elimination of metaphysics.

Carnap hold this view until the end of his life. But the reason or vindication of rejection varied through the times. The main motivation was that Carnap experienced from the history of philosophy that the metaphysical debates were endless and not fruitful, no progress was noticeable in such areas form the antics until the present. According to Carnap's view metaphysics is "bad poetry", i.e. it is the expression of feelings in an inappropriate, theoretical style.

The first vindication of rejection was verificationism. Later he described metaphysics as a failure of mode of speech. He distinguished formal and physical mode of speech, and pointed out that some times a question put into physical mode of speech is inappropriate. However translating it back to formal mode makes the question answerable and devoid of metaphysics. In "Empiricism Semantics and Ontology" Carnap gave a new analysis for the nature of metaphysical statements. See below.

According to the principle of rejecting metaphysics, Carnap rejected to articulate opinion about the truth of realism, solipsism, idealism dualism, and such.


Carnap hold this thesis for a very short time. It is also questionable what "verification" meant for Carnap. Does this mean proof or not? This is unclear. As a result of Popper's critique Carnap soon revised his concept and used later only confirmation or corroboration as criteria.

Linguistical atomism

In the early stage of logical positivism it is common that Carnap writesd about sentences as the basis of linguistical analysis. Linguistical atomism was never stated by Carnap, however there is no sign that he or his fellows would be aware of linguistical holism. Holism starts to appear in his philosophy later, and one can say that this development is earlier than the external critique. When Quine criticises logical positivism about atomism, Carnap was already well aware of holism. According to this, he usually made his statements relative to a fixed lingustical system.

Scientific philosophy

In the philosophy of the Vienna Circle the scientific world view (Wissenschaftliche Welauffassung) played a main role. The development of physics and especially the development of mathematics and logic made this perspective possible. Most of the philosophers in the Vienna Circle and also Carnap came from physics and were fascinated by the relativity-theory. This may be the origin of the scientific view.

According to the scientific view, all the meaningful questions regarding reality is scientific. This connects to the rejection of methaphysics and synthetic apriori.

Rejection of the synthetic apriori

Carnap hold Kant's distinction of analytic and synthetic despite of the fierce attack of Quine until the end of his life. Despite of Kant Carnap regarded synthetic apriory statements impossible. A good example is geometry, where Carnap distinguished physical geometry as synthetic, and pure geometry as pure analytic. In the debate with Quine Carnap tried to define analytic statements. He made various attempts, which were not good enough for Quine. It is still an open question in philosophy, if this distinction can be made.

Moderate conventionalism

Carnap's conventionalism is usually exemplified with his Principle of Tolerance. Some interpret this almost as Feyerabend's "anything goes", making Carnap a relativist. However, Carnap's conventionalism is limited. We are only choose freely in the analytic part of frameworks, the synthetic component is not free of choice. This means that Carnap is not relativistic at all regarding "reality".

Methodology of science

Carnap was interested in the methodology of science, but most of his works do not deal with this explicitly. Some methodology can be taken from writings about linguistical analysis. There is however two big part of Carnap's philosophy, which are partially methodology. The first one is the distinction of theoretical and observational language, the second is the method of induction, and the calculation of degree of confirmation. However Carnap never stated that degree of confirmation decides the acceptance of a theory. He stated this to be a practical question, i.e. there is no methodology for this. This is also connected to the framework-theory of Carnap.


Aufbau is usually described as a very reductionistic approach. Later Carnap accepted implicit and extensive definitions, which is a liberalisation of reductionism. Even later Carnap introduces observational and theoretical languages, and the theoretical terms are only partially defined by correspondence rules. The liberalisation of reductionism is connected to the liberalisation of verificationism. However Carnap also repeatedly rejected metaphysics, which makes these liberalisations problematic.

Protocoll-sentence debate

The protocoll-sentence debate was an internal "war" in the Vienna Circle. Some members were more foundationalists and others more conventionalists. Carnap tried to mediate and synthesize various views. Later, with the construction of an observational and theoretical language he also gave a kind of answer to this question.


In Aufbau Carnap preferred phenomenalism to physicalism, although he stated the different mode of speech as equivalent. It was Neurath' influence that shifted Carnap to physicalism. He wrote many articles and a booklet on physicalism, where the intertranslability and reduction to a physical language is investigated. A main issue is the reduction of psychology to physics. Physicalism is also connected to the UNity of Science, since physicalist language was intended to be the universal language.

Unity of Science

Unity of Science can be of various kind. In Aufbau Carnap tried to create a constitutional system, where unity was based on reduction. Later a unified language was conceived, which should be the physical language. The unity is based on translatability here.


This theory was introduced in "Empiricism, Semantics and Ontology" [[1]]. Here he develops his theory of frameworks and distinguishes internal and external questions. An internal question is a simple case, like "Does a even prime number greater than 2?" He points out that external questions are only meaningful if they are considered as practical questions, like "Is the acceptance of a system of numbers fruitful for science?". If they are not put in practical form, but in existential, or theoretical form, then we get metaphysical questions, like "Are the numbers really existing?".

Re-evaluation of Carnap's philosophy

According to Reisch 1991 the views of Kuhn and Carnap are much closer than believed by the received view, which was described for example by Suppe. Reisch states that Kuhn's Structure is based on logical positivism. He also states that some of the most important theses of Kuhn, like incommensurability, the theory-ladenness of observations are already present at Carnap. In the opinion of Reisch Carnap’s theory about internal-external distinction and linguistical framework corresponds to the paradigm-theory of Kuhn. This idea was continued by Earman, Irzik and Friedman, all of whom published similar ideas, with similar conclusions.

Oliveira (Oliveira 2002) criticizes these authors and calls them revisionists. He mentions the fact that Carnap did not refer to Kuhn in his writings. He explains the letters published by Reisch with Carnap's view of not regarding history of science as philosophy, and that he accepted Kuhn's book because he did not regard it as a competing view. According to Oliveira, Carnap may have regarded Kuhn's book simply as a good history of science in the context of a sharp discovery-justification distinction, as sustained by logical positivists.

See also