Rudolf Carnap

German-American philosopher
born May 18, 1891, Ronsdorf, Ger.
died Sept. 14, 1970, Santa Monica, Calif., U.S.
Main
German-born U.S. philosopher of Logical Positivism. He made
important contributions to logic, the analysis of language,
the theory of probability, and the philosophy of science.
Education.
From 1910 to 1914 Carnap studied mathematics, physics, and
philosophy at the universities of Jena and Freiburg im
Breisgau. At Jena he attended the lectures of Gottlob Frege,
now widely acknowledged as the greatest logician of the 19th
century, whose ideas exerted a deep influence on Carnap.
After serving in World War I, Carnap earned his doctorate
in 1921 at Jena with a dissertation on the concept of space.
He argued that the conflicts among the various theories of
space then held by scholars resulted from the fact that
those theories actually dealt with quite different subjects;
he called them, respectively, formal space, physical space,
and intuitive space and exhibited their principal
characteristics and fundamental differences.
For several years afterward Carnap was engaged in private
research in logic and the foundations of physics and wrote a
number of essays on problems of space, time, and causality,
as well as a textbook in symbolic, or mathematical, logic
(Abriss der Logistik, 1929; a considerably different later
German version appeared in English translation: Introduction
to Symbolic Logic and Its Applications, 1958).
Career in Vienna and Prague.
In 1926 Moritz Schlick, the founder of the Vienna Circle—a
small group of philosophers, mathematicians, and other
scholars who met regularly to discuss philosophical
issues—invited Carnap to join the faculty of the University
of Vienna, where he soon became an influential member of the
Circle. Out of their discussions developed the initial ideas
of Logical Positivism, or Logical Empiricism. This school of
thought shared its basic Empiricist orientation with David
Hume, a Scottish Empiricist, and Ernst Mach, an Austrian
physicist and philosopher. Its leading members, informed and
inspired by the methods and theories of contemporary
mathematics and science, sought to develop a “scientific
world view” by bringing to philosophical inquiry the
precision and rigour of the exact sciences. As one means to
this end, Carnap made extensive use of the concepts and
techniques of symbolic logic in preference to the often
inadequate analytic devices of traditional logic.
Carnap and his associates established close connections
with like-minded scholars in other countries, among them a
group of Empiricists that had formed in Berlin under the
leadership of Hans Reichenbach, an eminent philosopher of
science. With Reichenbach, Carnap founded a periodical,
Erkenntnis (1930–40), as a forum for the new “scientific
philosophy.”
The basic thesis of Empiricism, in a familiar but quite
vague formulation, is that all of man’s concepts and beliefs
concerning the world ultimately derive from his immediate
experience. In some of his most important writings, Carnap
sought, in effect, to give this idea a clear and precise
interpretation. Setting aside, as a psychological rather
than a philosophical problem, the question of how human
beings arrive at their ideas about the world, he proceeded
to construe Empiricism as a systematic-logical thesis about
the evidential grounding of empirical knowledge. To this
end, he gave the issue a characteristically linguistic turn
by asking how the terms and sentences that, in scientific or
in everyday language, serve to express assertions about the
world are related to those terms and sentences by which the
data of immediate experience can be described. The
Empiricist thesis, as construed and defended by Carnap, then
asserts that the terms and sentences of the first kind are
“reducible” to those of the second kind in a clearly
specifiable sense. Carnap’s conception of the relevant sense
of reducibility, which he always stated in precise logical
terms, was initially rather narrow but gradually became more
liberal.
In his first great work, Der logische Aufbau der Welt
(1928; Eng. trans.—with a smaller work—The Logical Structure
of the World: Pseudoproblems in Philosophy), Carnap
developed, with unprecedented rigour, a version of the
Empiricist reducibility thesis according to which all terms
suited to describe actual or possible empirical facts are
fully definable by terms referring exclusively to aspects of
immediate experience, so that all empirical statements are
fully translatable into statements about immediate
experiences.
Prompted by discussions with his associates in Vienna,
Carnap soon began to develop a more liberal version of
Empiricism, which he elaborated while he was professor of
natural philosophy at the German University in Prague
(1931–35); he eventually presented it in full detail in his
essay “Testability and Meaning” (Philosophy of Science, vol.
3 [1936] and 4 [1937]). Carnap argued that the terms of
empirical science are not fully definable in purely
experiential terms but can at least be partly defined by
means of “reduction sentences,” which are logically
much-refined versions of operational definitions, and
“observation sentences,” whose truth can be checked by
direct observation. Carnap stressed that usually such tests
cannot provide strict proof or disproof but only more or
less strong “confirmation” for an empirical statement.
Sentences that do not thus yield observational
implications and therefore cannot possibly be tested and
confirmed by observational findings were said to be
empirically meaningless. By reference to this testability
criterion of empirical significance, Carnap and other
Logical Empiricists rejected various doctrines of
speculative metaphysics and of theology, not as being false
but as making no significant assertions at all.
Carnap argued that the observational statements by
reference to which empirical statements can be tested may be
construed as sentences describing directly and publicly
observable aspects of physical objects, such as the needle
of a measuring instrument turning to a particular point on
the scale or a subject in a psychological test showing a
change in pulse rate. All such sentences, he noted, can be
formulated in terms that are part of the vocabulary of
physics. This was the basic idea of his “physicalism,”
according to which all terms and statements of empirical
science—from the physical to the social and historical
disciplines—can be reduced to terms and statements in the
language of physics.
In later writings, Carnap liberalized his conception of
reducibility and of empirical significance even further so
as to give a more adequate account of the relation between
scientific theories and scientific evidence.
Career in the United States.
By the time “Testability and Meaning” appeared in print,
Carnap had moved to the United States, mainly because of the
growing threat of German National Socialism. From 1936 to
1952 he served on the faculty of the University of Chicago.
During the 1940–41 school year, Carnap was a visiting
professor at Harvard University and was an active
participant in a discussion group that included Bertrand
Russell, Alfred Tarski, and W.V.O. Quine.
Soon after going to Chicago, Carnap joined with the
sociologist Otto Neurath, a former fellow member of the
Vienna Circle, and with an academic colleague, the
Pragmatist philosopher Charles W. Morris, in founding the
International Encyclopedia of Unified Science, which was
published, beginning in 1938, as a series of monographs on
general problems in the philosophy of science and on
philosophical issues concerning mathematics or particular
branches of empirical science.
Since his Vienna years, Carnap had been much concerned
also with problems in logic and in the philosophy of
language. He held that philosophical perplexities often
arise from a misunderstanding or misuse of language and that
the way to resolve them is by “logical analysis of
language.” On this point, he agreed with the “ordinary
language” school of Analytic Philosophy, which had its
origins in England. He differed from it, however, in
insisting that more technical issues—e.g., those in the
philosophy of science or of mathematics—cannot be adequately
dealt with by considerations of ordinary linguistic usage
but require clarification by reference to artificially
constructed languages that are formulated in logical
symbolism and that have their structure and interpretation
precisely specified by so-called syntactic and semantic
rules. Carnap developed these ideas and the theoretical
apparatus for their implementation in a series of works,
including Logische Syntax der Sprache (1934; The Logical
Syntax of Language) and Meaning and Necessity (1947; 2nd
enlarged ed., 1956).
Carnap’s interest in artificial languages included
advocacy of international auxiliary languages such as
Esperanto and Interlingua to facilitate scholarly
communication and to further international understanding.
One idea in logic and the theory of knowledge that
occupied much of Carnap’s attention was that of analyticity.
In contrast to the 19th-century radical Empiricism of John
Stuart Mill, Carnap and other Logical Empiricists held that
the statements of logic and mathematics, unlike those of
empirical science, are analytic—i.e., true solely by virtue
of the meanings of their constituent terms—and that they can
therefore be established a priori (without any empirical
test). Carnap repeatedly returned to the task of formulating
a precise characterization and theory of analyticity. His
ideas were met with skepticism by some, however—among them
Quine, who argued that the notion of analytic truth is
inherently obscure and the attempt to delimit a class of
statements that are true a priori should be abandoned as
misguided.
From about 1945 onward, Carnap turned his efforts
increasingly to problems of inductive reasoning and of
rational belief and decision. His principal aim was to
construct a formal system of inductive logic; its central
concept, corresponding to that of deductive implication,
would be that of probabilistic implication—or, more
precisely, a concept representing the degree of rational
credibility or of probability that a given body of evidence
may be said to confer upon a proposed hypothesis. Carnap
presented a rigorous theory of this kind in his Logical
Foundations of Probability (1950).
Carnap spent the years from 1952 to 1954 at the Institute
for Advanced Study in Princeton, where he continued his work
in probability theory. Subsequently, he accepted a
professorship at the University of California at Los
Angeles. During those years and indeed until his death,
Carnap was occupied principally with modifications and
considerable extensions of his inductive logic.
Carl G. Hempel