Logical positivism, also known aslogical empiricism orneo-positivism, was a philosophical movement, in theempiricist tradition, that sought to formulate ascientific philosophy in which philosophical discourse would be, in the perception of its proponents, as authoritative and meaningful asempirical science.[1]
Logical positivism's central thesis was theverification principle, also known as the "verifiability criterion of meaning", according to which a statement iscognitively meaningful only if it can be verified throughempirical observation or if it is atautology (true by virtue of its ownmeaning or its ownlogical form).[2] The verifiability criterion thus rejected statements ofmetaphysics,theology,ethics andaesthetics ascognitively meaningless in terms oftruth value orfactual content. Despite its ambition to overhaul philosophy by mimicking the structure and process of empirical science, logical positivism became erroneously stereotyped as an agenda to regulate the scientific process and to place strict standards on it.[1]
By the 1950s, problems identified within logical positivism's central tenets became seen as intractable, drawing escalating criticism among leading philosophers, notably fromWillard Van Orman Quine andKarl Popper, and even from within the movement, fromCarl Hempel. These problems would remain unresolved, precipitating the movement's eventual decline and abandonment by the 1960s. In 1967, philosopherJohn Passmore pronounced logical positivism "dead, or as dead as a philosophical movement ever becomes".[4]
TheVienna Circle was led principally byMoritz Schlick, congregating around theUniversity of Vienna and at theCafé Central. A manifesto written byOtto Neurath,Hans Hahn andRudolf Carnap in 1929 summarised the Vienna Circle's positions. Schlick had originally held aneo-Kantian position, but later converted, via Carnap's 1928 bookDer logische Aufbau der Welt (The Logical Structure of the World). The Viennese maintained closely cooperative ties with theBerlin Circle, among whomHans Reichenbach was pre-eminent.Carl Hempel, who studied under Reichenbach in Germany, was also to prove influential in the movement's later history.[13] A friendly but tenacious critic of the movement wasKarl Popper, whom Neurath nicknamed the "Official Opposition".[14]
Early in the movement, Carnap, Hahn, Neurath and others recognised that theverifiability criterion was too stringent in that it rejecteduniversal statements, which are vital toscientific hypothesis.[15] A radicalleft wing emerged from the Vienna Circle, led by Neurath and Carnap, who proposed revisions to weaken the criterion, a program they referred to as the "liberalisation of empiricism". A conservativeright wing, led by Schlick andWaismann, instead sought to classify universal statements as analytic truths, thereby to reconcile them with the existing criterion.[16] Within the liberal wing Carnap emphasisedfallibilism, as well aspragmatics, which he considered integral toempiricism. Neurath prescribed a move fromMach'sphenomenalism tophysicalism, though this would be opposed by Schlick. As Neurath and Carnap sought to pose science toward social reform, the split in the Vienna Circle also reflected political differences.[15]
Both Schlick and Carnap had been influenced by and sought to define logical positivism versus the neo-Kantianism ofErnst Cassirer, the contemporary leading figure of theMarburg school, and againstEdmund Husserl'sphenomenology. Logical positivists especially opposedMartin Heidegger's obscure metaphysics, the epitome of what they had rejected through their epistemological doctrines. In the early 1930s, Carnap debated Heidegger over "metaphysical pseudosentences".[17]
As the movement's first emissary to theNew World, Moritz Schlick visitedStanford University in 1929, yet otherwise remained in Vienna and was murdered in 1936 atthe University by a former student,Johann Nelböck, who was reportedly deranged.[17] That year,A. J. Ayer, a British attendee at various Vienna Circle meetings since 1933, publishedLanguage, Truth and Logic, which imported logical positivism to theEnglish-speaking world. In 1933, theNazi Party's rise to power in Germany had triggered flight of intellectuals, which accelerated upon Germany'sannexation of Austria in 1938.[17] The logical positivists, many of whom wereJewish, were targeted and continued flight throughout the pre-war period. Their philosophy thus became dominant in theEnglish-speaking world.[18]
By the late 1930s, many in the movement had replacedphenomenalism with Neurath'sphysicalism, wherebymaterial objects are not reducible tosensory stimuli but exist as publicly observable entities in thereal world. Neurath settled in England, where he died in 1945. Carnap, Reichenbach and Hempel settled permanently in America.[17]
According to theverifiability criterion of meaning, a statement iscognitively meaningful only if it is either verifiable byempirical observation or is ananalytic truth (i.e. true by virtue of its ownmeaning or its ownlogical form).[22]Cognitive meaningfulness was defined variably: possessingtruth value; or corresponding to a possible state of affairs; or intelligible or understandable as are scientific statements. Other types of meaning—for instance, emotive, expressive or figurative—were dismissed from further review.[23]
Metaphysics,theology, as well as much ofethics andaesthetics failed this criterion, and so were found cognitively meaningless and onlyemotively meaningful (though, notably, Schlick considered ethical and aesthetic statements cognitively meaningful).[24][25] Ethics and aesthetics were considered subjective preferences, while theology and metaphysics contained "pseudostatements" that were neither true nor false. Thus, logical positivism indirectly assertedHume's law, the principle thatfactual statements cannot justifyevaluative statements, and that the two are separated by an unbridgeable gap.A. J. Ayer'sLanguage, Truth and Logic (1936) presented an extreme version of this principle—theboo/hooray doctrine—whereby all evaluative judgments are merely emotional reactions.[26]
Logical positivists in the Vienna Circle recognised quickly that the verifiability criterion was too restrictive.[13] Specifically,universal statements were noted to be empirically unverifiable, rendering vital domains of science andreason, such asscientific hypothesis,cognitively meaningless under verificationism. This would pose significant problems for the logical positivist program, absent revisions to its criterion of meaning.[27]
In his 1936 and 1937 papers,Testability and Meaning,Carnap proposedconfirmation in place of verification, determining that, though universal laws cannot be verified, they can be confirmed.[15] Carnap employed abundant logical and mathematical tools to research aninductive logic that would account for probability according todegrees of confirmation. However, he was never able to formulate a model. In Carnap's inductive logic, a universal law's degree of confirmation was always zero.[28] The formulation of what eventually came to be called the "criterion of cognitive significance", stemming from this research, took three decades (Hempel 1950, Carnap 1956, Carnap 1961).[15]Carl Hempel, who became a prominent critic of the logical positivist movement, elucidated theparadox of confirmation.[29]
In his 1936 book,Language, Truth and Logic,A. J. Ayer distinguishedstrong andweak verification. He stipulated that, "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", but is verifiable in the weak sense "if it is possible for experience to render it probable". He would add that, "no proposition, other than atautology, can possibly be anything more than a probablehypothesis". Thus, he would conclude that all are open to weak verification.[30]
Intheories of justification,a priori statements are those that can be known independently ofobservation, contrasting witha posteriori statements, which are dependent on observation. Statements may also be categorised intoanalytic andsynthetic: Analytic statements are true by virtue of their ownmeaning or their ownlogical form, therefore aretautologies that are true bynecessity but uninformative about the world. Synthetic statements, in comparison, arecontingent propositions that refer to a state of facts concerning the world.[31][32]
David Hume proposed an unambiguous distinction between analytic and synthetic, categorising knowledge exclusively as either "relations of ideas" (which area priori, analytic andabstract) or "matters of fact and real existence" (a posteriori, synthetic andconcrete), a classification referred to asHume's fork.[33][34]Immanuel Kant identified a further category of knowledge:Synthetica priori statements, which are informative about the world, but known without observation. This principle is encapsulated in Kant'stranscendental idealism, which attributes the mind a constructive role inphenomena wherebyintuitive truths—including synthetica priori conceptions ofspace andtime—function as an interpretative filter for an observer's experience of the world.[35] His thesis would serve to rescueNewton's law of universal gravitation from Hume'sproblem of induction by determininguniformity of nature to be in the category ofa priori knowledge.[36]
The Vienna Circle rejected Kant's conception of synthetica priori knowledge given its incompatibility with theverifiability criterion.[37] Yet, they adopted the Kantian position of defining mathematics and logic—ordinarily considered synthetic truths—asa priori.[38]Carnap's solution to this discrepancy would be to reinterpret logical truths as tautologies, redefining logic as analytic, building upon theoretical foundations established inWittgenstein'sTractatus. Mathematics, in turn, would be reduced to logic through thelogicist approach proposed byGottlob Frege. In effect, Carnap's reconstruction of analyticity expounded Hume's fork, affirming its analytic-synthetic distinction. This would be critically important in rendering the verification principle compatible with mathematics and logic.[39]
Carnap devoted much of his career to the cornerstonedoctrine ofrational reconstruction, whereby scientific theories can be formalised intopredicate logic and the components of a theory categorised intoobservation terms andtheoretical terms.[40] Observation terms are specified by direct observation and thus assumed to have fixed empirical definitions, whereas theoretical terms refer to theunobservables of a theory, includingabstract conceptions such asmathematical formulas. The two categories ofprimitive terms would be interconnected in meaning via adeductive interpretative framework, referred to ascorrespondence rules.[41]
Early in his research, Carnap postulated that correspondence rules could be used to define theoretical terms from observation terms, contending that scientific knowledge could be unified byreducing theoretical laws to "protocol sentences" grounded in observable facts. He would soon abandon this model of reconstruction, suggesting instead that theoretical terms could be defined implicitly by theaxioms of a theory. Furthermore, that observation terms could, in some cases, garner meaning from theoretical terms via correspondence rules.[42] Here, definition is said to be 'implicit' in that the axioms serve to exclude those interpretations that falsify the theory. Thus, axioms define theoretical terms indirectly by restricting the set of possible interpretations to those that are true interpretations.[41]
Carnap's early anti-metaphysical works employed Russell'stheory of types.[45] Like Russell, Carnap envisioned a universal language that could reconstruct mathematics and thereby encode physics. YetKurt Gödel'sincompleteness theorem showed this to be impossible, except in trivial cases, andAlfred Tarski'sundefinability theorem finally undermined all hopes of reducing mathematics to logic. Thus, a universal language failed to stem from Carnap's 1934 workLogische Syntax der Sprache (Logical Syntax of Language). Still, some logical positivists, includingCarl Hempel, continued support of logicism.[44]
Carl Hempel was prominent in the development of thedeductive-nomological (DN) model, then the foremost model ofscientific explanation defended even among critics of neo-positivism such asPopper.[47] According to the DN model, a scientific explanation is valid only if it takes the form of adeductive inference from a set of explanatorypremises (explanans) to the observation or theory to be explained (explanandum).[48] The model stipulates that the premises must refer to at least onelaw, which it defines as anunrestricted generalization of theconditional form: "IfA, thenB".[49] Laws therefore differ from mereregularities ("George always carries only $1 bills in his wallet") which do not necessarily supportcounterfactual claims.[50] Furthermore, laws must be empirically verifiable in compliance with the verification principle.[48]
The DN model ignores causal mechanisms beyond the principle ofconstant conjunction ("first eventA and then always eventB") in accordance with theHumeanempiricist postulate that, though sequences of events are observable, the underpinningcausal principles are not.[47] Hempel stated that well-formulated natural laws (empirically confirmed regularities) are satisfactory in approximating causal explanation.[48]
Hempel later proposed a probabilistic model of scientific explanation: The inductive-statistical (IS) model. Derivation of statistical laws from other statistical laws would further be designated as the deductive-statistical (DS) model. The DN and IS models are collectively referred to as the "covering law model" or "subsumption theory", the latter referring to the movement's stated goals of "theory reduction".[48][51]
Logical positivists were committed to the vision of aunified science encompassing all scientific fields (including thespecial sciences, such asbiology,anthropology,sociology andeconomics, andthe fundamental science, orfundamental physics) which would be synthesised into a singularepistemic entity.[52][48] Key to this concept was the doctrine oftheory reduction, according to which the covering law model would be used to interconnect the special sciences and, thereupon, toreduce all laws in the special sciences to fundamental physics.[53]
The movement envisioned a universal scientific language that could express statements with commonmeaning intelligible to all scientific fields.Carnap sought to realise this goal through the systematic reduction of the linguistic terms of more specialised fields to those of more fundamental fields. Various methods of reduction were proposed, referring to the use ofset theory to manipulate logicallyprimitive concepts (as in Carnap'sLogical Structure of the World, 1928) or viaanalytic anda priori deductive operations (as described inTestability and Meaning, 1936, 1937). A number of publications over a period of thirty years would attempt to elucidate this concept.[54][55]
Karl Popper, a graduate of theUniversity of Vienna, was an outspoken critic of the logical positivist movement from its inception. InLogik der Forschung (1934, published in English in 1959 asThe Logic of Scientific Discovery) he attackedverificationism directly, contending that theproblem of induction renders it impossible forscientific hypotheses and otheruniversal statements to be verified conclusively. Any attempt to do so, he argued, would commit the fallacy ofaffirming the consequent, given that verification cannot—in itself—exclude alternative valid explanations for a specific phenomenon or instance of observation.[58] He would later affirm that the content of the verifiability criterion cannot be empirically verified, thus is meaningless by its own proposition and ultimatelyself-defeating as a principle.[59]
In the same book, Popper proposedfalsifiability, which he presented, not as a criterion ofcognitive meaning like verificationism (as commonly misunderstood),[60] but as a criterion to distinguish scientific from non-scientific statements, thereby todemarcate the boundaries of science. Popper observed that, though universal statements cannot be verified, they can be falsified, and that the most productive scientific theories were apparently those that carried the greatest 'predictive risks' of being falsified by observation.[61] He would conclude that thescientific method should be ahypothetico-deductive model, wherein scientific hypotheses must be falsifiable (per his criterion), held as provisionally true until proven false by observation, and arecorroborated by supporting evidence rather than verified or confirmed.[62]
In rejecting neo-positivist views of cognitive meaningfulness, Popper consideredmetaphysics to be rich in meaning and important in the origination of scientific theories andvalue systems to be integral to science's quest for truth. At the same time, he disparagedpseudoscience, referring to theconfirmation biases that embolden support for unfalsifiable conjectures (notably those inpsychology andpsychoanalysis) andad hoc arguments used to entrench predictive theories that have been proven conclusively false.[61]
In his influential 1951 paperTwo Dogmas of Empiricism, American philosopher andlogicistWillard Van Orman Quine challenged theanalytic-synthetic distinction. Specifically, Quine examined the concept ofanalyticity, determining that all attempts to explain the idea reduce ultimately tocircular reasoning. He would conclude that, if analyticity is untenable, so too is the neo-positivist proposition to redefine its boundaries.[63] YetCarnap's reconstruction of analyticity was necessary for logic and mathematics to be deemed meaningful under verificationism. Quine's arguments encompassed numerous criticisms on this topic he had articulated to Carnap since 1933.[64] His work effectively pronounced the verifiability criterion untenable, threatening to uproot the broader logical positivist project.[65]
In 1958,Norwood Hanson'sPatterns of Discovery characterised the concept oftheory-ladenness. Hanson andThomas Kuhn held that even direct observations are never truly neutral in that they areladen with theory, i.e. influenced by a system of theoreticalpresuppositions that function as an interpretative framework for thesenses.[66] Accordingly, individuals subscribed to different theories might report radically different observations even as they investigate the same phenomena. Hanson's thesis attacked theobservation-theory distinction, which draws a dividing line between observational and non-observational (theoretical) language. More broadly, its findings challenged the central-most tenets ofempiricism in questioning the infallibility and objectivity of empirical observation.[67]
Though foundationalism was often considered a constituent doctrine of logical positivism (and Kuhn's thesis anepistemological criticism of the movement) such views were simplistic:[70] In the 1930s,Neurath had argued for the adoption ofcoherentism, famously comparing the progress of science toreconstruction of a boat at sea.[71]Carnap had entertained foundationalism from 1929 to 1930, but he,Hans Hahn and others would later join Neurath in converting to a coherentist philosophy. Theconservative wing of theVienna Circle underMoritz Schlick subscribed to a form of foundationalism, but its principles were defined unconventionally or ambiguously.[72]
In some sense, Kuhn's book unified science, but through historical and social assessment rather than bynetworking the scientific specialties using epistemological orlinguistic models.[73] His ideas were adopted quickly by scholars in non-scientific disciplines, such as the social sciences in which neo-positivists were dominant,[46] ushering academia intopostpositivism or postempiricism.[73]
In his critique of thereceived view in 1962,Hilary Putnam attacked theobservation-theory distinction.[74] Putnam proposed that the division between "observation terms" and "theoretical terms" was untenable, determining that both categories have the potential to betheory-laden. Accordingly, he remarked that observational reports frequently refer to theoretical terms in practice.[75] He illustrated cases in which observation terms can be applied to entities thatCarnap would classify asunobservables. For example, inNewton'scorpuscular theory of light, observation concepts can be applied to the consideration of bothsub-microscopic and macroscopic objects.[76]
Putnam advocatedscientific realism, whereby scientific theory describes areal world existing independently of the senses. He rejected positivism, which he dismissed as a form ofmetaphysical idealism, in that it precluded any possibility to acquire knowledge of the unobservable aspects of nature. He also spurnedinstrumentalism, according to which a scientific theory is judged, not by whether it corresponds to reality, but by the extent to which it allows empirical predictions or resolves conceptual problems.[17]
In 1967,John Passmore wrote, "Logical positivism is dead, or as dead as a philosophical movement ever becomes".[4] His opinions concurred with widespread sentiment in academic circles that the movement had run its course by the late 1960s.[77] Logical positivism's fall heraldedpostpositivism, distinguished byPopper'scritical rationalism—which characterised human knowledge as continuously evolving via conjectures and refutations—andKuhn's historical and social perspectives on the saltatory course of scientific progress.[78]
In a 1976 interview,A. J. Ayer, who had introduced logical positivism to theEnglish-speaking world in the 1930s,[79] was asked what he saw as its main defects and answered that, "nearly all of it was false". Yet, he maintained that it was "true in spirit", referring to the principles ofempiricism andreductionism wherebymental phenomena resolve to the material or physical and philosophical questions largely resolve to ones of language and meaning.[4][80] Despite its problems, logical positivism helped to anchoranalytic philosophy in the English-speaking world and its influence extended beyond philosophy in shaping the course ofpsychology and thesocial sciences. In the post-war period,Carl Hempel's contributions were vitally important in establishing the subdiscipline of thephilosophy of science.[17]
Logical positivism's fall reopened the debate over themetaphysical merit of scientific theory, whether it can offer knowledge of the world beyond human experience (scientific realism) or whether it is simply an instrument to predict human experience (instrumentalism).[81][82] Philosophers increasingly critiqued the movement's doctrine and history, often misrepresenting it without thorough examination,[83] sometimes reducing it to oversimplifications and stereotypes, such as its association withfoundationalism.[84]
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^Tractatus Proposition 4.024 bears resemblance to Schlick's statement, "To state the circumstances under which a proposition is true is the same as stating its meaning".
^Schlick, Moritz (1932). "Positivismus und realismus".Erkenntnis.3 (1):1–31.doi:10.1007/BF01886406. English translation in:Sarkar, Sahotra, ed. (1996).Logical Empiricism at its Peak: Schlick, Carnap, and Neurath. New York: Garland Publishing. p. 38.
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^abBunge, M.A. (1996).Finding Philosophy in Social Science. Yale University Press. p. 317.ISBN978-0300066067.LCCNlc96004399.However, neo-positivism failed dismally to give a faithful account of science, whether natural or social. It failed because it remained anchored tosense-data and to aphenomenalist metaphysics, overrated the power ofinduction and underrated that ofhypothesis, and denouncedrealism andmaterialism 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.
^Hempel, Carl G (1950). "Problems and changes in the empiricist criterion of meaning".Revue Internationale de Philosophie.41:41–63.
^Various different views are discussed in Ayer'sLanguage, Truth, and Logic, Schlick's "Positivism and realism" (reprinted inSarkar 1996 andAyer 1959) and Carnap'sPhilosophy and Logical Syntax.
^Allen, Barry (2007). "Turning back the linguistic turn in the theory of knowledge".Thesis Eleven.89 (1):6–22.doi:10.1177/0725513607076129.S2CID145778455.In his famous novelNineteen Eighty-FourGeorge Orwell gave a nice (if for us ironical) explanation of the boon Carnap expects from the logical reform of grammar. Right-thinkingIngsoc party members are as offended as Carnap by the unruliness of language. It's a scandal that grammar allows such pseudo-statements as 'It is the right of the people to alter or abolish Government' (Jefferson), or 'Das Nichts nichtet' (Heidegger). Language as it is makes no objection to such statements, and to Carnap, as to the Party, that's a sore defect.Newspeak, a reformed grammar under development at theMinistry of Truth, will do what Carnap wants philosophical grammar to do.
^Schlick, Moritz (1992). "The future Of philosophy". In Richard Rorty (ed.).The Linguistic Turn. Chicago: University of Chicago Press. pp. 43–53.
^Ayer, A.J (1936).Language, Truth, and Meaning. pp. 2,63–77.
^John Vicker (2011)."The problem of induction". In Edward N. Zalta (ed.).The Stanford Encyclopedia of Philosophy (Fall 2011 ed.). Retrieved24 August 2012.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, anyuniversally 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.
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