Hans Hahn, the oldest of the three (1879–1934), was a mathematician. He received his degree in mathematics in 1902. Afterwards he studied under the direction of Ludwig Boltzmann in Vienna and David Hilbert, Felix Klein and Hermann Minkowski in Göttingen. In 1905 he received the Habilitation in mathematics. He taught at Innsbruck (1905–1906) and Vienna (from 1909).
Otto Neurath (1882–1945) studied sociology, economics and philosophy in Vienna and Berlin. From 1907 to 1914 he taught in Vienna at the Neuen Wiener Handelsakademie (Viennese Commercial Academy). Neurath married Olga, Hahn’s sister, in 1911.
Philipp Frank, the youngest of the group (1884–1966), studied physics at Göttingen and Vienna with Ludwig Boltzmann, David Hilbert and Felix Klein. From 1912, he held the chair of theoretical physics in the German University in Prague.
Their meetings were held in Viennese coffeehouses from 1907 onward. Frank remembered:
Presumably the meetings stopped in 1912, when Frank went to Prague, where he held the chair of theoretical physics left vacant by Albert Einstein. Hahn left Vienna during World War I and returned in 1921. The following year Hahn, with the collaboration of Frank, arranged to bring into the group Moritz Schlick, who held the chair of philosophy of the inductive sciences at the University of Vienna. Schlick had already published his two main works Raum und Zeit in die gegenwärtigen Physik (Space and Time in contemporary Physics) in 1917 and Allgemeine Erkenntnislehre (General Theory of Knowledge) in 1918. A central work for the newly founded discussion group was the Logisch-Philosophische Abhandlung (Tractatus Logico-Philosophicus), published by Ludwig Wittgenstein in 1921 (in 1922 in German-English version).
Under the direction of Schlick, a new regular series of meetings began. In 1926 Schlick and Hahn arranged to bring in Rudolf Carnap at the University of Vienna. In 1928 the Verein Ernst Mach (Ernst Mach Society) was founded, with Schlick as chairman. In 1929 the Vienna Circle manifesto Wissenschaftliche Weltauffassung. Der Wiener Kreis (The Scientific Conception of the World. The Vienna Circle) was published. The pamphlet is dedicated to Schlick, and its preface was signed by Hahn, Neurath and Carnap. In the appendix is a list of the members of the Vienna Circle.
The Vienna Circle was dispersed when the Nazi party came to power in Germany; many of its members emigrated to USA, where they taught in several universities. Schlick remained in Austria, but in 1936 he was killed by a Nazi sympathizer student in the University of Vienna.
Logical analysis is the method of clarification of philosophical problems; it makes an extensive use of symbolic logic and distinguishes the Vienna Circle empiricism from earlier versions. The task of philosophy lies in the clarification—through the method of logical analysis—of problems and assertions.
Logical analysis shows that there are two different kinds of statements; one kind includes statements reducible to simpler statements about the empirically given; the other kind includes statements which cannot be reduced to statements about experience and thus they are devoid of meaning. Metaphysical statements belong to this second kind and therefore they are meaningless. Hence many philosophical problems are rejected as pseudo-problems which arise from logical mistakes, while others are re-interpreted as empirical statements and thus becomes the subject of scientific inquiries.
One source of the logical mistakes that are at the origins of metaphysics is the ambiguity of natural language. “Ordinary language for instance uses the same part of speech, the substantive, for things (‘apple’) as well as for qualities (‘hardness’), relations (‘friendship’), and processes (‘sleep’); therefore it misleads one into a thing-like conception of functional concepts” (VC p. 329). Another source of mistakes is “the notion that thinking can either lead to knowledge out of its own resources without using any empirical material, or at least arrive at new contents by an inference from given states of affair” (VC p. 330). The latter notion is typical in Kantian philosophy, according to which there are synthetic statements a priori that expand knowledge without using the experience. Synthetic knowledge a priori is rejected by the Vienna Circle. Mathematics, which at a first sight seems an example of necessarily valid synthetic knowledge derived from pure reason alone, has instead a tautological character, that is its statements are analytical statements, thus very different from Kantian synthetic statements. The only two kinds of statements accepted by the Vienna Circle are synthetic statements a posteriori (i.e. scientific statements) and analytic statements a priori (i.e. logical and mathematical statements).
However, the persistence of metaphysics is connected not only with logical mistakes but also with “social and economical struggles” (VC p. 339). Metaphysics and theology are allied to traditional social forms, while the group of people who “faces modern times, rejects these views and takes its stand on the ground of empirical sciences” (VC p. 339). Thus the struggle between metaphysics and scientific world-conception is not only a struggle between different kinds of philosophies, but it is also—and perhaps primarily—a struggle between different political, social and economical attitudes. Of course, as the manifesto itself acknowledged, “not every adherent of the scientific world-conception will be a fighter” (VC p. 339). Many historians of the Vienna Circle see in the latter sentence an implicit reference to a contrast between the so called ‘left wing’ of the Vienna Circle, mainly represented by Neurath and Carnap, and Moritz Schlick. The aim of the left wing was to facilitate the penetration of the scientific world-conception in “the forms of personal and public life, in education, upbringing, architecture, and the shaping of economic and social life” (VC p. 339–340). In contrast, Schlick was primarily interested in the theoretical study of science and philosophy. Perhaps the sentence “Some, glad of solitude, will lead a withdrawn existence on the icy slopes of logic” (VC p. 339) is an ironic reference to Schlick.
The manifesto lists Walter Dubislav, Josef Frank, Kurt Grelling, Hasso Härlen, Eino Kaila, Heinrich Loewy, F. P. Ramsey, Hans Reichenbach, Kurt Reidemeister, and Edgar Zilsel as "Those sympathetic to the Vienna Circle" and Albert Einstein, Bertrand Russell and Ludwig Wittgenstein as "Leading representatives of the scientific world-conception".
The well known Thomas Kuhn’s work, The Structure of Scientific Revolutions, was published in this Encyclopedia in 1962, as the number two in the second volume.
A word W has a meaning if two conditions are satisfied. First, the mode of the occurrence of W in its elementary sentence form (i.e. the simplest sentence form in which W is capable of occurring) must be fixed. Secondly, if W occurs in an elementary sentence S, it is necessary to give an answer to the following questions (that are—according to Carnap—equivalent formulation of the same question):
(Carnap, 'The Elimination of Metaphysics Through Logical Analysis of Language' in Sarkar, Sahotra, cit., pp. 12)
An example offered by Carnap concerns the word 'arthropod'. The sentence form "the thing x is an arthropod" is an elementary sentence form that is derivable from "x is an animal", "x has a segmented body" and "x has jointed legs". Conversely, these sentences are derivable from "the thing x is an arthropod". Thus the meaning of the words 'arthropod' is determined.
According to Carnap, many words of metaphysics do not fulfill these requirements and thus they are meaningless. As an example, Carnap considers the word 'principle'. This word has a definite meaning, if the sentence "x is the principle of y" is supposed to be equivalent to the sentence "y exists by virtue of x" or "y arises out of x". The latter sentence is perfectly clear: y arises out of x when x is invariably followed by y, and the invariable association between x and y is empirically verifiable. But—says Carnap—metaphysicians are not satisfied with this interpretation of the meaning of 'principle'. They assert that no empirical relation between x and y can completely explain the meaning of "x is the principle of y", because there is something that cannot be grasped by means of the experience, something for which no empirical criterion can be specified. It is the lacking of any empirical criterion—says Carnap—that deprives of meaning the word 'principle' when it occurs in metaphysics. Therefore, metaphysical pseudo-statements such as "water is the principle of the world" or "the spirit is the principle of the world" are void of meaning because a meaningless word occurs in them.
However, there are pseudo-statements in which occur only meaningful words; these pseudo-statements are formed in a counter-syntactical way. An example is the word sequence "Caesar is a prime number"; every word has a definite meaning, but the sequence has no meaning. The problem is that "prime number" is a predicate of numbers, not a predicate of human beings. In the example the nonsense is evident; however, in natural language the rules of grammar do not prohibit the formation of analogous meaningless word sequences that are not so easily detectable. In the grammar of natural languages, every sequence of the kind "x is y", where x is a noun and y is a predicate, is acceptable. In fact, in the grammar there is no distinction between predicate which can be affirmed of human beings and predicate which can be affirmed of numbers. So "Caesar is a general" and "Caesar is a prime number" are both well-formed, in contrast for example with "Caesar is and", which is ill-formed. In a logically constructed language—says Carnap—a distinction between the various kinds of predicate is specified, and pseudo-statements as "Caesar is a prime number" are ill-formed. Now, and this is the main point of Carnap's argument, metaphysical statements in which do not occur meaningless words, are indeed meaningless because they are formed in a way which is admissible in natural languages, but not in logically constructed languages. Carnap attempts to indicate the most frequent sources of errors from which metaphysical pseudo-statements can arise. One source of mistakes is the ambiguity of the verb 'to be', which is sometimes used as a copula ("I am hungry") and sometimes to designate existence ("I am"). The latter statement incorrectly suggests a predicative form, and thus it suggests that existence is a predicate. Only modern logic, with the introduction of an explicit sign to designate existence (the sign ), which occurs only in statements such as , never as a predicate, has showed that existence is not a predicate, and thus has revealed the logical error from which pseudo-statements such as "cogito, ergo sum" has aroused.
Another source of mistakes is type confusions, in which a predicate of a kind is used as a predicate of another kind. For example the pseudo-statements "we know the Nothing" is analogous to "we know the rain", but while the latter is well-formed, the former is ill-formed, at least in a logically constructed language, because 'Nothing' is incorrectly used as a noun. In a formal language, 'Nothing' only means , such as "there is nothing which is outside", i.e. , and thus 'Nothing' never occurs as a noun or as a predicate.
What is the role of metaphysics? According to Carnap, although metaphysics has no theoretical content, it has content indeed: metaphysical pseudo-statements express the attitude of a person towards life. Metaphysics is an art like lyrical poetry. The metaphysician, instead of using the medium of art, works with the medium of the theoretical; he confuses art with science, attitude towards life with knowledge, and thus produces an unsatisfactory and inadequate work. "Metaphysicians are musicians without musical ability" (Carnap, 'The Elimination of Metaphysics', in Sarkar, Sahotra, cit.,p. 30).
Between 1928 and 1937, the Vienna Circle published ten books in a collection named Schriften zur wissenschaftlichen Weltauffassung (Monographs on the Scientific World-Conception), edited by Schlick and Frank. Karl Raimund Popper’s book Logik der Forschung was published in this collection. Seven works were published in another collection, called Einheitswissenschaft (Unified Science). In 1930 Rudolf Carnap and Hans Reichenbach undertook the editorship of the journal Erkenntnis, which was published between 1930 and 1940 (from 1939 the editors were Otto Neurath, Rudolf Carnap and Charles Morris).
The following is the list of works published in the two collections edit by the Vienna Circle.
Schriften zur wissenschaftlichen Weltauffassung (Monographs on the Scientific World-Conception), edit by Schlick and Frank:
Einheitswissenschaft (Unified Science), edit by Carnap, Frank, Hahn, Neurath, Joergensen (after Hahn's death), Morris (from 1938):
These works are translated in Unified Science: The Vienna Circle Monograph Series Originally Edited by Otto Neurath, Kluwer, 1987.
Monographs, arranged in chronological order, published in the International Encyclopedia of Unified Science:
Another link to the U.S. is Willard Van Orman Quine who traveled in 1932–1933 as Sheldon Traveling Fellow to Vienna, Prag, and Warsaw.
Alfred Jules Ayer acquainted the British acedeme with the work of the Vienna Circle. Also Karl Popper was important for the reception and critique of their work, even though he never participated in the meetings of the Vienna Circle.