a group of philosophers and scientists who met periodically for discussions in Vienna from 1922 to 1938 and who proposed a self-consciously revolutionary conception of scientific knowledge. The Circle was initiated by the mathematician Hans Hahn to continue a prewar forum with the physicist Philip Frank and the social scientist Otto Neurath after the arrival in Vienna of Moritz Schlick, a philosopher who had studied with Max Planck. Carnap joined in 1926 (from 1931 in Prague); other members included Herbert Feigl (from 1930 in Iowa), Friedrich Waismann, Bergmann, Viktor Kraft, and Bela von Juhos. Viennese associates of the Circle included Kurt Gödel, Karl Menger, Felix Kaufmann, and Edgar Zilsel. (Popper was not a member or associate.) During its formative period the Circle’s activities were confined to discussion meetings (many on Wittgenstein’s Tractatus). In 1929 the Circle entered its public period with the formation of the Verein Ernst Mach, the publication of its manifesto Wissenschaftliche Weltauffassung: Der Wiener Kreis by Carnap, Hahn, and Neurath (translated in Neurath, Empiricism and Sociology, 1973), and the first of a series of philosophical monographs edited by Frank and Schlick. It also began collaboration with the independent but broadly like-minded Berlin ‘Society of Empirical Philosophy,’ including Reichenbach, Kurt Grelling, Kurt Lewin, Friedrich Kraus, Walter Dubislav, Hempel, and Richard von Mises: the groups together organized their first public conferences in Prague and Königsberg, acquired editorship of a philosophical journal renamed Erkenntnis, and later organized the international Unity of Science congresses. The death and dispersion of key members from 1934 onward (Hahn died in 1934, Neurath left for Holland in 1934, Carnap left for the United States in 1935, Schlick died in 1936) did not mean the extinction of Vienna Circle philosophy. Through the subsequent work of earlier visitors (Ayer, Ernest Nagel, Quine) and members and collaborators who emigrated to the United States (Carnap, Feigl, Frank, Hempel, and Reichenbach), the logical positivism of the Circle (Reichenbach and Neurath independently preferred ‘logical empiricism’) strongly influenced the development of analytic philosophy. The Circle’s discussions concerned the philosophy of formal and physical science, and even though their individual publications ranged much wider, it is the attitude toward science that defines the Circle within the philosophical movements of central Europe at the time. The Circle rejected the need for a specifically philosophical epistemology that bestowed justification on knowledge claims from beyond science itself. In this, the Circle may also have drawn on a distinct Austrian tradition (a thesis of its historian Neurath): in most of Germany, science and philosophy had parted ways during the nineteenth century. Starting with Helmholtz, of course, there also arose a movement that sought to distinguish the scientific respectability of the Kantian tradition from the speculations of German idealism, yet after 1880 neo-Kantians insisted on the autonomy of epistemology, disparaging earlier fellow travelers as ‘positivist.’ Yet the program of reducing the knowledge claim of science and providing legitimations to what’s left found wide favor with the more empirical-minded like Mach. Comprehensive description, not explanation, of natural phenomena became the task for theorists who no longer looked to philosophy for foundations, but found them in the utility of their preferred empirical procedures. Along with the positivists, the Vienna Circle thought uneconomical the Kantian answer to the question of the possibility of objectivity, the synthetic a priori. Moreover, the Vienna Circle and its conventionalist precursors Poincaré and Duhem saw them contradicted by the results of formal science. Riemann’s geometries showed that questions about the geometry of physical space were open to more than one answer: Was physical space Euclidean or non-Euclidean? It fell to Einstein and the pre-Circle Schlick (Space and Time in Contemporary Physics, 1917) to argue that relativity theory showed the untenability of Kant’s conception of space and time as forever fixed synthetic a priori forms of intuition. Yet Frege’s anti-psychologistic critique had also shown empiricism unable to account for knowledge of arithmetic and the conventionalists had ended the positivist dream of a theory of experiential elements that bridged the gap between descriptions of fact and general principles of science. How, then, could the Vienna Circle defend the claim – under attack as just one worldview among others – that science provides knowledge? The Circle confronted the problem of constitutive conventions. As befitted their self-image beyond Kant and Mach, they found their paradigmatic answer in the theory of relativity: they thought that irreducible conventions of measurement with wide-ranging implications were sharply separable from pure facts like point coincidences. Empirical theories were viewed as logical structures of statements freely created, yet accountable to experiential input via their predictive consequences identifiable by observation. The Vienna Circle defended empiricism by the reconceptualization of the relation between a priori and a posteriori inquiries. First, in a manner sympathetic to Frege’s and Russell’s doctrine of logicism and guided by Wittgenstein’s notion of tautology, arithmetic was considered a part of logic and treated as entirely analytical, without any empirical content; its truth was held to be exhausted by what is provable from the premises and rules of a formal symbolic system. (Carnap’s Logical Syntax of Language, 1934, assimilated Gödel’s incompleteness result by claiming that not every such proof could be demonstrated in those systems themselves which are powerful enough to represent classical arithmetic.) The synthetic a priori was not needed for formal science because all of its results were non-synthetic. Second, the Circle adopted verificationism: supposedly empirical concepts whose applicability was indiscernible were excluded from science. The terms for unobservables were to be reconstructed by logical operations from the observational terms. Only if such reconstructions were provided did the more theoretical parts of science retain their empirical character. (Just what kind of reduction was aimed for was not always clear and earlier radical positions were gradually weakened; Reichenbach instead considered the relation between observational and theoretical statements to be probabilistic.) Empirical science needed no synthetic a priori either; all of its statements were a posteriori.
Combined with the view that the analysis of the logical form of expressions allowed for the exact determination of their combinatorial value, verificationism was to exhibit the knowledge claims of science and eliminate metaphysics. Whatever meaning did not survive identification with the scientific was deemed irrelevant to knowledge claims (Reichenbach did not share this view either). Since the Circle also observed the then long-discussed ban on issuing unconditional value statements in science, its metaethical positions may be broadly characterized as endorsing noncognitivism. Its members were not simply emotivists, however, holding that value judgments were mere expressions of feeling, but sought to distinguish the factual and evaluative contents of value judgments. Those who, like Schlick (Questions of Ethics, 1930), engaged in metaethics, distinguished the expressive component (x desires y) of value judgments from their implied descriptive component (doing z furthers aim y) and held that the demand inherent in moral principles possessed validity if the implied description was true and the expressed desire was endorsed. This analysis of normative concepts did not render them meaningless but allowed for psychological and sociological studies of ethical systems; Menger’s formal variant (Morality, Decision and Social Organization, 1934) proved influential for decision theory. The semiotic view that knowledge required structured representations was developed in close contact with foundational research in mathematics and depended on the ‘new’ logic of Frege, Russell, and Wittgenstein, out of which quantification theory was emerging. Major new results were quickly integrated (albeit controversially) and Carnap’s works reflect the development of the conception of logic itself. In his Logical Syntax he adopted the ‘Principle of Tolerance’ vis-à-vis the question of the foundation of the formal sciences: the choice of logics (and languages) was conventional and constrained, apart from the demand for consistency, only by pragmatic considerations. The proposed language form and its difference from alternatives simply had to be stated as exactly as possible: whether a logico-linguistic framework as a whole correctly represented reality was a cognitively meaningless question. Yet what was the status of the verifiability principle? Carnap’s suggestion that it represents not a discovery but a proposal for future scientific language use deserves to be taken seriously, for it not only characterizes his own conventionalism, but also amplifies the Circle’s linguistic turn, according to which all philosophy concerned ways of representing, rather than the nature of the represented. What the Vienna Circle ‘discovered’ was how much of science was conventional: its verificationism was a proposal for accommodating the creativity of scientific theorizing without accommodating idealism. Whether an empirical claim in order to be meaningful needed to be actually verified or only potentially verifiable, or fallible or only potentially testable, and whether so by current or only by future means, became matters of discussion during the 1930s. Equally important for the question whether the Circle’s conventionalism avoided idealism and metaphysics were the issues of the status of theoretical discourse about unobservables and the nature of science’s empirical foundation. The view suggested in Schlick’s early General Theory of Knowledge (1918, 2d. ed. 1925) and Frank’s The Causal Law and its Limitations (1932) and elaborated in Carnap’s ‘Logical Foundations of the Unity of Science’ (in Foundations of the Unity of Science I.1, 1938) characterized the theoretical language as an uninterpreted calculus that is related to the fully interpreted observational language only by partial definitions. Did such an instrumentalism require for its empirical anchor the sharp separation of observational from theoretical terms? Could such a separation even be maintained?
Consider the unity of science thesis. According to the methodological version, endorsed by all members, all of science abides by the same criteria: no basic methodological differences separate the natural from the social or cultural sciences (Geisteswissenschaften) as claimed by those who distinguish between