Logical positivism or logical empiricism are variants of neopositivism that embraced verificationism, a theory of knowledge combining strong empiricism—basing all knowledge on sensory experience—with mathematical logic and linguistics so that scientific statements could be conclusively proved false or true. Verificationism was inextricably tied with the covering law model of scientific explanation. As variants of analytic philosophy, verificationism dominated Anglo-American philosophy from the 1930s into the 1960s.
In the early 1920s, appalled both by the racism, bigotry, and nationalism flaring in Western society—as in the negative eugenics movement and World War I—and by the countermovement toward metaphysics, mysticism, and intuition, a group of scientists and philosophers were inspired by developments in mathematics, logic, linguistics, and physics, especially relativity theory, and sought to offer the world a transparent, meaningful, universal language. Hans Reichenbach led the Berlin Circle, while Moritz Schlick led the Vienna Circle, gathered around the University of Vienna and the Café Central. A 1929 pamphlet written by Otto Neurath, Hans Hahn, and Rudolf Carnap summarized the Vienna Circle's positions.
In the positivistic tradition promulgated in the 1830s by Auguste Comte, all epistemic—that is, all knowledge, which is justified belief—was scientific, and all sciences would be unified by a common content and method. Yet logical positivism was mostly influenced by Ernst Mach's phenomenalism—which accorded the mind virtually no power to attain knowledge beyond that delivered by direct sensory experience—and by the putative operationalism of Percy Bridgman, as well as by an interpretation of Ludwig Wittgenstein's philosophy of language. The verifiability principle demarcated the scientific as the verifiable and thus meaningful, whereas the unverifiable was unscientific and meaningless—metaphysic, emotive, or such—not candidate to further review by philosophers, tasked to organize knowledge, not develop new knowledge.
Logical positivism became famed for vigorous scientific antirealism—restricting science to observable aspects of nature—although that motivation and aspect has been exaggerated. Still, scientific theory's content would be direct observations, and its form would be mathematics modeling only patterns of sensory experience. Talk of unobservable aspects of nature, including causality, mechanism, and principles, was at best metaphorical—talk of observables in the abstract—or at worst emotional or metaphysical. By rational reconstruction, statements and concepts expressed in ordinary language would be replaced with more precise, standardized equivalents, and translated into a logical syntax reducible to symbolic logic. A scientific theory would be stated with its own method of verification, whereby a calculus in mathematical logic could be operated to verify the theory's falsity or truth.
In the late 1930s, members of the Vienna Circle fled Austria while A J Ayer imported verificationism to the English-speaking audience. With the close of World War II in 1945, logical positivism became milder, logical empiricism, led largely by Carl Hempel, a member of the Vienna Circle who had migrated to America. Ultimately, verificationism failed to resolve central problems, and after the Second World War, its doctrines increasingly came under attack by thinkers such as Nelson Goodman, Willard Van Orman Quine, Norwood Hanson, Karl Popper, Thomas Kuhn, J. L. Austin, Peter Strawson, Hilary Putnam, and Richard Rorty.
During the late 1920s, '30s, and '40s, Bertrand Russell and Ludwig Wittgenstein's formalism was developed by a group of philosophers in Vienna and Berlin, who formed the Vienna Circle and Berlin Circle, into a doctrine known as logical positivism (or logical empiricism). Logical positivism used formal logic to underpin an empiricist account of our knowledge of the world. Philosophers such as Rudolf Carnap and Hans Reichenbach, along with other members of the Vienna Circle, claimed that the truths of logic and mathematics were tautologies, and those of science were verifiable empirical claims. These two constituted the entire universe of meaningful judgements; anything else was nonsense. The claims of ethics and aesthetics were subjective preferences. Theology and other metaphysics were pseudo-statements, neither true nor false, simply meaningless nonsense.
Karl Popper's insistence upon the role of falsification in the philosophy of science was a reaction to the logical positivists. With the rise of Adolf Hitler and National Socialism in Germany and Austria, some members of the Vienna and Berlin Circles fled Germany, mainly to Britain and the USA, which helped to reinforce the dominance of logical positivism and analytic philosophy in the Anglophone world.
Logical positivists typically considered philosophy as having a very limited function. For them, philosophy is concerned with the organization of thoughts, rather than having distinct topics of its own. The positivists adopted the principle of verificationism, according to which every meaningful statement is either analytic or is capable of being verified by experience. This caused the logical positivists to reject many traditional problems of philosophy, especially those of metaphysics or ontology, as meaningless.
Logical positivism was a movement without a fixed set of doctrines. The logical positivists held a wide range of views on many matters. Nonetheless, they were all interested in science and skeptical of theology and metaphysics. Early, most logical positivists proposed that all knowledge is based on logical inference from simple "protocol sentences" grounded in observable facts. Many logical positivists endorsed forms of materialism, metaphysical naturalism, and empiricism. (See James Ladyman, Understanding Philosophy of Science, p. 147)
Verifiability criterion of meaning
Perhaps the view for which the logical positivists are best known is the verifiability criterion of meaning, or verificationism. In one of its earlier and stronger formulations, this is the doctrine that a proposition is "cognitively meaningful" only if there is a finite procedure for conclusively determining its truth. An intended consequence of this opinion, for most logical positivists, is that metaphysical, theological, and ethical statements fail this criterion, and so are not cognitively meaningful. They distinguished cognitive from other varieties of meaningfulness (e.g. emotive, expressive, figurative), and most authors concede that the non-cognitive statements of the history of philosophy possess some other kind of meaningfulness. The positive characterization of cognitive meaningfulness varies from author to author. It has been described as the property of having a truth value, corresponding to a possible state of affairs, naming a proposition, or being intelligible or understandable in the sense in which scientific statements are intelligible or understandable.
Strong and weak verificationism
In response to criticism of verificationism, A. J. Ayer proposed a weak version. In Language, Truth and Logic he defines the distinction between "strong" and "weak" verification: "A proposition is said to be verifiable, in the strong sense of the term, if, and only if, its truth could be conclusively established by experience." (Ayer 1946:50) It is this sense of verifiable that causes the problem of verification with negative existential claims and positive universal claims. However, the weak sense of verification states that a proposition is "verifiable... if it is possible for experience to render it probable" (ibid.). After establishing this distinction, Ayer claims that "no proposition, other than a tautology, can possibly be anything more than a probable hypothesis" (Ayer 1946:51), and therefore can only be subject to weak verification. This defense was controversial among logical positivists, some of whom touted strong verification, and claimed that general propositions were indeed nonsense.
Analytic and synthetic knowledge
Logical positivists divided knowledge into analytic and synthetic categories. Analytic knowledge, such as mathematical theorems, is tautological (it is entirely deducable from its presuppositions) and thus can be validated a priori. Synthetic knowledge, such as assertions about the real world, must be verified a posteriori by observation. Logical positivists rejected the existence of any synthetic a priori knowledge. (For example, the scientific progress of general relativity demonstrates that philosophers are wrong to pronounce a priori that space should have a Euclidean nature.) The analytic-synthetic distinction was attacked by Quine's 1951 paper "Two Dogmas of Empiricism". Logical positivists also distinguished observational and theoretical terms. This distinction was criticised by Popper, who emphasised even basic observations as being "theory-laden".
Another characteristic feature of logical positivism is the commitment to "Unified Science"; that is, the development of a common language or, in Neurath's phrase, a "universal slang" in which all scientific propositions can be expressed. The adequacy of proposals or fragments of proposals for such a language was often asserted on the basis of various "reductions" or "explications" of the terms of one special science to the terms of another, putatively more fundamental one. Sometimes these reductions consisted of set-theoretic manipulations of a few logically primitive concepts (as in Carnap's (1928) Logical Structure of the World); sometimes these reductions consisted of allegedly analytic or a priori deductive relationships (as in Carnap's Testability and Meaning). A number of publications over a period of thirty years would attempt to elucidate this concept.
Mach's influence is most apparent in the logical positivists' persistent concern with metaphysics, the unity of science, and the interpretation of the theoretical terms of science, as well as the doctrines of reductionism and phenomenalism, later abandoned by many positivists.
Wittgenstein's Tractatus Logico-Philosophicus was a text of great importance for the positivists. The Tractatus introduced many doctrines which later influenced logical positivism, including the concept of philosophy as a "critique of language," and the possibility of making a theoretically principled distinction between intelligible and nonsensical discourse. The Tractatus also adhered to a correspondence theory of truth which the positivists adopted, although some, like Otto Neurath, preferred a form of coherentism. Wittgenstein's influence is also evident in certain formulations of the verification principle. Compare, for example, Proposition 4.024 of the Tractatus, where Wittgenstein asserts that we understand a proposition when we know what happens if it is true, with Schlick's assertion that "To state the circumstances under which a proposition is true is the same as stating its meaning." The tractarian doctrine that the truths of logic are tautologies was widely believed among the logical positivists. Wittgenstein also influenced the logical positivists' interpretation of probability. According to Neurath, some logical positivists disliked the Tractatus, since they thought it included a great deal of metaphysics.
Contemporary developments in logic and the foundations of mathematics, especially Bertrand Russell and Alfred North Whitehead's monumental Principia Mathematica, impressed the more mathematically minded logical positivists such as Hans Hahn and Rudolf Carnap. "Language-planning" and syntactical techniques derived from these developments were used to defend logicism in the philosophy of mathematics and various reductionist theses. Russell's theory of types was employed to great effect in Carnap's early anti-metaphysical polemics.
Immanuel Kant also had an important influence on the positivists, both positive and negative. Negatively, Kant was often scorned by the positivists in their early debates, and Kant's doctrine of synthetic a priori truths was the doctrine they most wished to discredit. However, Kant's opinions about the nature of physical objects pervaded the protocol sentence debate, and Kantian opinions of the relationship between philosophy and science were shared by the positivists to some degree.
Logical positivism in Germany
Positivism in Germany is thought to have developed in response to Hegelian and neo-Hegelian metaphysics, which was a famous philosophy in Germany. Hegelian successors such as F.H. Bradley attempted to explain reality by postulating metaphysical entities that did not have any empirical basis. Logical positivists in response wanted to stop such metaphysical entities from being used as an explanation.
Another, less well-known factor that encouraged logical positivism was the urgency of solving new philosophical problems raised by new scientific developments. The Vienna Circle under the influence of Moritz Schlick and the Berlin Circle under the influence of Hans Reichenbach consisted of scientists, mathematicians, and scientists turned philosophers, who shared a common goal of solving new problems in the philosophy of science.
Logical positivism spread throughout almost the entire western world. It was disseminated throughout the European continent.[when?] It was spread to Britain by the influence of A. J. Ayer. And later, it was brought to American universities by members of the Vienna Circle after they fled Europe and settled in the United States during and after WWII. Logical positivism was essential to the development of early analytic philosophy. The term subsequently came to be almost interchangeable with "analytic philosophy" during the first half of the twentieth century. Logical positivism was immensely influential[clarification needed] in the philosophy of language. It represented the dominant philosophy of science between World War I and the Cold War.
Early critics of logical positivism said that its fundamental tenets could not themselves be formulated consistently. The verifiability criterion of meaning did not seem verifiable; but neither was it simply a logical tautology, since it had implications for the practice of science and the empirical truth of other statements. This presented severe problems for the logical consistency of the theory.
Another problem was that universal claims (e.g. "(all) philosophers are mortal") are problematic in terms of verification. The verifiability criterion was seen as being too strong. In its initial formulation, it made universal statements meaningless, and this was seen as a problem for science. This led to the weakening of the criterion.
Wittgenstein's principle of verifiability posed fairly obvious problems in any scientific context. No universal generalization can ever be verified. Perhaps independently, Karl Popper perceived the same problem... This led him to replace the requirement of verfiability with that of falsifiability, though only as a criterion to demarcate science from metaphysics, and not as one to be also used to demarcate meaningful from meaningless claims. It is also unclear what the status of the principle itself is, that is, whether it is meaningful by its own criterion of meaningfulness. Carnap, as well as other members of the Vienna Circle including Hahn and Neurath, realized that a weaker criterion of meaningfulness was necessary. Thus began the program of the "liberalization of empiricism." There was no unanimity within the Vienna Circle on this point. The differences between the members are sometimes described as those between a conservative "right" wing, led by Schlick and Waismann, which rejected both the liberalization of empiricism and the epistemological antifoundationalism of the move [from phenomenalism] to physicalism, and a radical "left" wing, led by Neurath and Carnap, which endorsed the opposite views. The "left" wing also emphasized fallibilism and pragmatics; Carnap went far enough along this line to suggest that empiricism itself was a proposal to be accepted on pragmatic grounds. This difference also reflected political attitudes insofar as Neurath and, to a lesser extent, Carnap viewed science as a tool for social reform.
The precise formulation of what came to be called the criterion of cognitive significance took three decades (see Hempel 1950; Carnap 1956 and 1961)... In an important pair of papers, "Testability and Meaning," Carnap (1936-1937) replaced the requirement of verification with that of confirmation; at this stage, he made no attempt to quantify the latter. Individual terms replace sentences as the unit of meaning. Universal generalizations are no longer problematic; though they cannot be conclusively verified, they can yet be confirmed. Moreover, in "Testability and Meaning," theoretical terms no longer require explicit definition from observational ones in order to acquire meaning; the connection between the two may be indirect through a system of implicit definitions. Carnap also provides an important pioneering discussion of disposition predicates.—Sahotra Sarkar, The Philosophy of Science: An Encyclopedia
Karl Popper's objection
A well-known critic of logical positivism was Karl Popper, who published the book Logik der Forschung in 1934 (translated by himself as The Logic of Scientific Discovery, published 1959). In it he argued that the positivists' criterion of verifiability was too strong a criterion for science, and should be replaced by a criterion of falsifiability. Popper thought that falsifiability was a better criterion because it did not invite the philosophical problems inherent in verifying an inductive inference, and it allowed statements from the physical sciences which seemed scientific but which did not satisfy the verification criterion.
Popper's concern was not with distinguishing meaningful from meaningless statements, but distinguishing scientific from metaphysical statements. Unlike the positivists, he did not claim that metaphysical statements must be meaningless; he also claimed that a statement which was "metaphysical" and unfalsifiable in one century (like the ancient Greek philosophy about atoms) could, in another century, be developed into falsifiable theories that have the metaphysical views as a consequence, and thus become scientific.
Hilary Putnam's objection
According to Hilary Putnam, a former student of Hans Reichenbach and Rudolf Carnap, making an observational/theoretical distinction is meaningless. The "received view" operates on the correspondence rule that states "The observational terms are taken as referring to specified phenomena or phenomenal properties, and the only interpretation given to the theoretical terms is their explicit definition provided by the correspondence rules." Putnam argues that introducing this dichotomy of observational terms and theoretical terms is the problem from which to start. Putnam demonstrates this with four objections:
- Something is referred to as "observational" if it is observable directly with our senses. Then an observation term cannot be applied to something unobservable. If this is the case, there are no observation terms.
- With Carnap's classification, some unobservable terms are not even theoretical and belong to neither observation terms nor theoretical terms. Some theoretical terms refer primarily to observation terms.
- Reports of observation terms frequently contain theoretical terms.
- A scientific theory may not contain any theoretical terms (an example of this is Darwin's original theory of evolution).
Subsequent objections from Quine and Kuhn
Subsequent philosophy of science tends to use certain aspects of both of these approaches. Willard Van Orman Quine criticized the distinction between analytic and synthetic statements and the reduction of meaningful statements to immediate experience. Work by Thomas Kuhn has claimed that it is not possible to provide truth conditions for science independent of its historical paradigm. But even this criticism was not unknown to the logical positivists: Otto Neurath compared science to a boat which we must rebuild on the open sea.
Contemporary status within philosophy
Key tenets of logical positivism, including its atomistic philosophy of science, the verifiability principle, and the fact–value distinction, came under attack after the Second World War by philosophers such as Nelson Goodman, Quine, J. L. Austin, and Peter Strawson. Nicholas G. Fotion comments that "By the late 1960s it became obvious that the movement had pretty much run its course." Most philosophers consider logical positivism to be, as John Passmore expressed it, "dead, or as dead as a philosophical movement ever becomes". By the late 1970s, its ideas were so generally recognized to be seriously defective that one of its own main proponents, A. J. Ayer, could say in an interview: "I suppose the most important [defect]...was that nearly all of it was false." It retains an important place in the history of analytic philosophy as the antecedent of contemporary philosophies, such as constructive empiricism, positivism, and postpositivism.
- See, e.g., : "Vienna Circle" in Stanford Encyclopedia of Philosophy
- Smith, L.D. (1986). Behaviorism and Logical Positivism: A Reassessment of the Alliance. Stanford University Press. p. 314. ISBN 9780804713016. LCCN 85030366. "The secondary and historical literature on logical positivism affords substantial grounds for concluding that logical positivism failed to solve many of the central problems it generated for itself. Prominent among the unsolved problems was the failure to find an acceptable statement of the verifiability (later confirmability) criterion of meaningfulness. Until a competing tradition emerged (about the late 1950's), the problems of logical positivism continued to be attacked from within that tradition. But as the new tradition in the philosophy of science began to demonstrate its effectiveness—by dissolving and rephrasing old problems as well as by generating new ones—philosophers began to shift allegiances to the new tradition, even though that tradition has yet to receive a canonical formulation."
- Bunge, M.A. (1996). Finding Philosophy in Social Science. Yale University Press. p. 317. ISBN 9780300066067. LCCN lc96004399. "To conclude, logical positivism was progressive compared with the classical positivism of Ptolemy, Hume, d'Alembert, Compte, Mill, and Mach. It was even more so by comparison with its contemporary rivals—neo-Thomisism, neo-Kantianism, intuitionism, dialectical materialism, phenomenology, and existentialism. However, neo-positivism failed dismally to give a faithful account of science, whether natural or social. It failed because it remained anchored to sense-data and to a phenomenalist metaphysics, overrated the power of induction and underrated that of hypothesis, and denounced realism and materialism as metaphysical nonsense. Although it has never been practiced consistently in the advanced natural sciences and has been criticized by many philosophers, notably Popper (1959 , 1963), logical positivism remains the tacit philosophy of many scientists. Regrettably, the anti-positivism fashionable in the metatheory of social science is often nothing but an excuse for sloppiness and wild speculation."
- "Popper, Falsifiability, and the Failure of Positivism". 7 August 2000. Retrieved 30 June 2012. "The upshot is that the positivists seem caught between insisting on the V.C. [Verifiability Criterion]—but for no defensible reason—or admitting that the V.C. requires a background language, etc., which opens the door to relativism, etc. In light of this dilemma, many folk—especially following Popper's "last-ditch" effort to "save" empiricism/positivism/realism with the falsifiability criterion—have agreed that positivism is a dead-end."
- Carnap, R. (1928). The Logical Structure of the World. ?.
- Popper, Karl R. (2002). The Logic of Scientific Discovery. Routledge. ISBN 0-415-27844-9.
- Prominent amongst these were Ludwig Wittgenstein and Rudolf Carnap. Karl Popper might also be included, since despite his rejection of the term his method has much in common with the analytic tradition.
- Logical Empiricism, Stanford Encyclopedia of Philosophy
- For a classic survey of other versions of verificationism, see Hempel, Carl. "Problems and Changes in the Empiricist Criterion of Meaning". Revue Internationale de Philosophie 41 (1950), pp 41-63.
- For the classic expression of this view, see Carnap, op. cit. Moritz Schlick, a major logical positivist, did not consider ethical (or aesthetic) sentences to be cognitively meaningless. See Schlick, Moritz. "The Future Of Philosophy". The Linguistic Turn. Ed. Richard Rorty. Chicago: University of Chicago Press, 1992. 43-53.
- Examples of these different views can be found in Scheffler's Anatomy of Inquiry, Ayer's Language, Truth, and Logic, Schlick's "Positivism and Realism" (rpt. in Sarkar (1996) and Ayer (1959)), and Carnap's Philosophy and Logical Syntax.
- For a thorough consideration of what the thesis of the unity of science amounts to, see Frost-Arnold, Gregory, "The Large-Scale Structure of Logical Empiricism: Unity of Science and the Rejection of Metaphysics" at 
- "Positivismus und Realismus", Erkenntnis 3:1-31, English trans. in Sarkar, Sahotra (ed.) Logical Empiricism at its Peak: Schlick, Carnap, and Neurath. New York: Garland Pub., 1996, p. 38
- For a very informative summary of the effect the Tractatus had on the main logical positivists, see the Entwicklung der Thesen des "Wiener Kreises"
- See Carnap, Rudolf. "The Elimination Of Metaphysics Through Logical Analysis of Language". Erkenntnis 2 (1932). Rpt. in Logical Positivism. Ed. Alfred Jules Ayer. New York: Free Press, 1959. 60-81.
- See the essays by Schlick, Carnap, and Neurath in Ayer's Logical Positivism.
- Friedman, Michael, Reconsidering Logical Positivism. Cambridge, UK: Cambridge University Press, 1999.
- Suppe, Frederick, The Positivist Model of Scientific Theories, in: Scientific Inquiry, Robert Klee editor, New York, USA: Oxford University Press, 1999, pp. 16-24.
- Putnam, H. (1985). Philosophical Papers: Volume 3, Realism and Reason. Philosophical Papers. Cambridge University Press. p. 184. ISBN 9780521313940. LCCN lc82012903.
- Fetzer, James (2012). "Carl Hempel". In Edward N. Zalta. The Stanford Encyclopedia of Philosophy (Summer 2012 ed.). "It would fall to Hempel to become perhaps the most astute critic of that movement and to contribute to its refinement as logical empiricism... Hempel himself attained a certain degree of prominence as a critic of this movement... The analytic/synthetic distinction and the observational/theoretical distinction were tied together by the verifiability criterion of meaningfulness... By this standard, sentences that are non-analytic but also non-verifiable, including various theological or metaphysical assertions concerning God or The Absolute, qualify as cognitively meaningless. This was viewed as a desirable result. But, as Hempel would demonstrate, its scope was far too sweeping, since it also rendered meaningless the distinctively scientific assertions made by laws and theories... The analytic/synthetic distinction took a decided hit when the noted logician, Willard van Orman Quine, published “Two Dogmas of Empiricism” (1953), challenging its adequacy... While the analytic/synthetic distinction appears to be justifiable in modeling important properties of languages, the observational/theoretical distinction does not fare equally well. Within logical positivism, observation language was assumed to consist of names and predicates whose applicability or not can be ascertained, under suitable conditions, by means of direct observation... Karl Popper (1965, 1968), however, would carry the argument in a different direction by looking at the ontic nature of properties... Hempel (1950, 1951), meanwhile, demonstrated that the verifiability criterion could not be sustained. Since it restricts empirical knowledge to observation sentences and their deductive consequences, scientific theories are reduced to logical constructions from observables. In a series of studies about cognitive significance and empirical testability, he demonstrated that the verifiability criterion implies that existential generalizations are meaningful, but that universal generalizations are not, even though they include general laws, the principal objects of scientific discovery. Hypotheses about relative frequencies in finite sequences are meaningful, but hypotheses concerning limits in infinite sequences are not. The verifiability criterion thus imposed a standard that was too strong to accommodate the characteristic claims of science and was not justifiable... Both theoretical and dispositional predicates, which refer to non-observables, posed serious problems for the positivist position, since the verifiability criterion implies they must be reducible to observables or are empirically meaningless... The need to dismantle the verifiability criterion of meaningfulness together with the demise of the observational/theoretical distinction meant that logical positivism no longer represented a rationally defensible position. At least two of its defining tenets had been shown to be without merit. Since most philosophers believed that Quine had shown the analytic/synthetic distinction was also untenable, moreover, many concluded that the enterprise had been a total failure. Among the important benefits of Hempel's critique, however, was the production of more general and flexible criteria of cognitive significance... Hempel suggested multiple criteria for assessing the cognitive significance of different theoretical systems, where significance is not categorical but rather a matter of degree... The elegance of Hempel's study laid to rest any lingering aspirations for simple criteria of cognitive significance and signaled the demise of logical positivism as a philosophical movement. Precisely what remained, however, was in doubt. Presumably, anyone who rejected one or more of the three principles defining positivism—the analytic/synthetic distinction, the observational/theoretical distinction, and the verifiability criterion of significance—was not a logical positivist. The precise outlines of its philosophical successor, which would be known as “logical empiricism”, were not entirely evident. Perhaps this study came the closest to defining its intellectual core. Those who accepted Hempel's four criteria and viewed cognitive significance as a matter of degree were members, at least in spirit. But some new problems were beginning to surface with respect to Hempel's covering-law explication of explanation and old problems remained from his studies of induction, the most remarkable of which was known as “the paradox of confirmation”."
- Uebel, Thomas (2008). "Vienna Circle". In Edward N. Zalta. The Stanford Encyclopedia of Philosophy (Fall 2008 ed.). "What Carnap later called the “liberalization of empiricism” was underway and different camps became discernible within the Circle... In the first place, this liberalization meant the accomodation of universally quantified statements and the return, as it were, to salient aspects of Carnap's 1928 conception. Everybody had noted that the Wittgensteinian verificationist criterion rendered universally quantified statements meaningless. Schlick (1931) thus followed Wittgenstein's own suggestion to treat them instead as representing rules for the formation of verifiable singular statements. (His abandonment of conclusive verifiability is indicated only in Schlick 1936a.) A second element that began to do so soon was the recognition of the problem of the irreducibility of disposition terms to observation terms... A third element was that disagreement arose as to whether the in-principle verifiability or support turned on what was merely logically possible or on what was nomologically possible, as a matter of physical law etc. A fourth element, finally, was that differences emerged as to whether the criterion of significance was to apply to all languages or whether it was to apply primarily to constructed, formal languages. Schlick retained the focus on logical possibility and natural languages throughout, but Carnap had firmly settled his focus on nomological possibility and constructed languages by the mid-thirties. Concerned with natural language, Schlick (1932, 1936a) deemed all statements meaningful for which it was logically possible to conceive of a procedure of verification; concerned with constructed languages only, Carnap (1936-37) deemed meaningful only statements for whom it was nomologically possible to conceive of a procedure of confirmation of disconfirmation.
Many of these issues were openly discussed at the Paris congress in 1935. Already in 1932 Carnap had sought to sharpen his previous criterion by stipulating that those statements were meaningful that were syntactically well-formed and whose non-logical terms were reducible to terms occurring in the basic observational evidence statements of science. While Carnap's focus on the reduction of descriptive terms allows for the conclusive verification of some statements, his criterion also allowed universally quantified statements to be meaningful, provided they were syntactically and terminologically correct (1932a, §2). It was not until one of his Paris addresses, however, that Carnap officially declared the meaning criterion to be mere confirmability. Carnap's new criterion required neither verification nor falsification but only partial testability so as now to include not only universal statements but also the disposition statements of science... Though plausible initially, the device of introducing non-observational terms in this way gave rise to a number of difficulties which impugned the supposedly clear distinctions between logical and empirical matters and analytic and synthetic statements (Hempel 1951). Independently, Carnap himself (1939) soon gave up the hope that all theoretical terms of science could be related to an observational base by such reduction chains. This admission raised a serious problem for the formulation of a meaning criterion: how was one to rule out unwanted metaphysical claims while admitting as significant highly abstract scientific claims?"
- Vicker, John (2011). "The Problem of Induction". In Edward N. Zalta. The Stanford Encyclopedia of Philosophy (Fall 2011 ed.). "This initial formulation of the criterion was soon seen to be too strong; it counted as meaningless not only metaphysical statements but also statements that are clearly empirically meaningful, such as that all copper conducts electricity and, indeed, any universally quantified statement of infinite scope, as well as statements that were at the time beyond the reach of experience for technical, and not conceptual, reasons, such as that there are mountains on the back side of the moon. These difficulties led to modification of the criterion: The latter to allow empirical verification if not in fact then at least in principle, the former to soften verification to empirical confirmation."
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- Richardson, Alan and Thomas Uebel (eds.) The Cambridge Companion to Logical Positivism. New York: Cambridge University Press, 2007.
- Salmon, Wesley and Wolters, Gereon (ed.) Logic, Language, and the Structure of Scientific Theories: Proceedings of the Carnap-Reichenbach Centennial, University of Konstanz, 21–24 May 1991, Pittsburgh: University of Pittsburgh Press, 1994.
- Sarkar, Sahotra (ed.) The Emergence of Logical Empiricism: From 1900 to the Vienna Circle. New York: Garland Publishing, 1996.
- Sarkar, Sahotra (ed.) Logical Empiricism at its Peak: Schlick, Carnap, and Neurath. New York: Garland Pub., 1996.
- Sarkar, Sahotra (ed.) Logical Empiricism and the Special Sciences: Reichenbach, Feigl, and Nagel. New York: Garland Pub., 1996.
- Sarkar, Sahotra (ed.) Decline and Obsolescence of Logical Empiricism: Carnap vs. Quine and the Critics. New York: Garland Pub., 1996.
- Sarkar, Sahotra (ed.) The Legacy of the Vienna Circle: Modern Reappraisals. New York: Garland Pub., 1996.
- Spohn, Wolfgang (ed.) Erkenntnis Orientated: A Centennial Volume for Rudolf Carnap and Hans Reichenbach, Boston: Kluwer Academic Publishers, 1991.
- Werkmeister, William (May 1937). "Seven Theses of Logical Positivism Critically Examined". The Philosophical Review (Cornell University) 46 (3): 276–297. doi:10.2307/2181086. JSTOR 2181086.
- Articles by logical positivists
- Carnap, Rudolf. 'Empiricism, Semantics, and Ontology.'
- Excerpt from Carnap, Rudolf. Philosophy and Logical Syntax.
- Feigl, Herbert. 'Positivism in the Twentieth Century (Logical Empiricism)', Dictionary of the History of Ideas, 1974, Gale Group (Electronic Edition)
- Hempel, Carl. 'Problems and Changes in the Empiricist Criterion of Meaning.'
- Articles on logical positivism
- Logical Empiricism entry by Richard Creath in the Stanford Encyclopedia of Philosophy
- Kemerling, Garth. 'Logical Positivism', Philosophy Pages
- Murzi, Mauro. 'Logical Positivism', The New Encyclopedia of Unbelief, Tom Flynn (ed.). Prometheus Books, 2007 (PDF version)
- Murzi, Mauro. 'The Philosophy of Logical Positivism.'
- Passmore, John. 'Logical Positivism', The Encyclopedia of Philosophy, Paul Edwards (ed.). New York: Macmillan, 1967, first edition
- Articles on related philosophical topics
- Hájek, Alan. 'Interpretations of Probability', The Stanford Encyclopedia of Philosophy (Summer 2003 Edition), Edward N. Zalta (ed.)
- Rey, Georges. 'The Analytic/Synthetic Distinction', The Stanford Encyclopedia of Philosophy (Fall 2003 Edition), Edward N. Zalta (ed.)
- Ryckman, Thomas A., 'Early Philosophical Interpretations of General Relativity', The Stanford Encyclopedia of Philosophy (Winter 2001 Edition), Edward N. Zalta (ed.)
- Woleński, Jan. 'Lvov-Warsaw School', The Stanford Encyclopedia of Philosophy (Summer 2003 Edition), Edward N. Zalta (ed.)
- Woodward, James. 'Scientific Explanation', The Stanford Encyclopedia of Philosophy (Summer 2003 Edition), Edward N. Zalta (ed.)