Verificationism was a movement in Western philosophy—in particular, analytic philosophy—that emerged in the 1920s by the efforts of a group of philosophers known as the logical positivists, who aimed to formulate criteria to ensure philosophical statements' meaningfulness and to objectively assess their falsity or truth. Initially, logical positivists sought a universal language whereby both ordinary language and physics—thereby all of the empirical sciences—could be represented formally via symbolic logic's axioms, whereupon the empirical sciences' basis in observation or experience could be clearly discerned and mimicked by philosophy.
Logical positivists' verifiability principle—that only statements about the world that are empirically verifiable or logically necessary are cognitively meaningful—cast theology, metaphysics, and evaluative judgements, such as ethics and aesthetics, as cognitively meaningless "pseudostatements" that were but emotively meaningful. The verificationist program's fundamental suppositions had varying formulations, which evolved from the 1920s to 1950s into the milder version logical empiricism. Yet all three of verificationism's shared basic suppositions—verifiability criterion, analytic/synthetic gap, and observation/theory gap—were by the 1960s found irreparably untenable, signaling the demise of verificationism and, with it, of the entire movement launched by logical positivism.
Although verificationist principles of a general sort—grounding scientific theory in some verifiable experience—are found retrospectively even with the American pragmatist C S Pierce and with the French conventionalist Pierre Duhem who fostered instrumentalism, the vigorous program termed verificationism was launched by the logical positivists who, emerging from Berlin Circle and Vienna Circle in the 1920s, sought epistemology whereby philosophy discourse would be meaningful, on par with empirical sciences.
Logical positivists garnered the verifiability criterion of cognitive meaningfulness from young Ludwig Wittgenstein's philosophy of language posed in his 1921 book Tractatus, and sought logicist reduction of mathematics to logic led by Bertrand Russell. Seeking grounding in such empiricism as of David Hume, Auguste Comte, and Ernst Mach—along with the positivism of the latter two—they borrowed some perspectives from Immanuel Kant, and found the exemplar of science to be Albert Einstein's general theory of relativity.
Early, some logical positivists within Vienna Circle recognized that the verifiability criterion was too stringent. Rudolf Carnap and Otto Neurath led a faction seeking "liberalization of empiricism", a faction that also switched from Mach's phenomenalism—which accorded the mind virtually no power to know objects, and restricted scientific talk to phenomena as experienced patterns of sensations—to Neurath's physicalism, which shifts talk to spatiotemporal, publicly observable objects and events.
Neurath also held that verification essentially compares statements to other statements, rather than statements directly to experience. And Carnap suggested that empiricism's basis is but pragmatic. In 1936, Carnap sought a switch from verification to confirmation, as an unrestricted generalization cannot be verified but, according to Carnap, can confirmed. Eventually, A J Ayer proposed two types of verification—strong and weak—while weak verification would be obtainable when a proposition is rendered probable. Meanwhile, Carnap sought to axiomatize a universal law's probability as a "degree of confirmation".
Despite employing abundant logical and mathematical tools to formalize a universal law's probability as "degree of confirmation", Rudolf Carnap never succeeded in this endeavor. In all of Carnap's formulations, a universal law's degree of confirmation is always zero. Carl Hempel, who had studied with Hans Reichenbach in Berlin Circle but was a protégé of Carnap in Vienna Circle, emigrated, as did Carnap and Reichenbach, to America. The three brought the movement to a milder stance, distinguishable as logical empiricism. Hempel's paradox of the ravens—aka, paradox of confirmation—found that an observation report confirming even one case of a universal law's prediction could not be logically formalized without an apparent absurdity.
The 1951 article "Two dogmas of empiricism", by Willard Van Orman Quine, attacked the analytic/synthetic division and apparently rendered the verificationist program untenable. Carl Hempel, one of verificationism's greatest internal critics, had recently concluded the same as to the verifiability criterion. In 1958, Norwood Hanson explained that even direct observations must be collected, sorted, and reported with guidance and constraint by theory, which sets a horizon of expectation and interpretation, how observational reports, never neutral, are laden with theory.
Karl Popper's 1959 book proposing falsificationism, originally published in German in 1934, reached the Anglosphere and was soon mistaken for a new type of verificationism, yet refuted verificationism. Falsificationism's demarcation falsifiable grants a theory the status scientific—simply, empirically testable—not the status meaningful, a status that Popper did not aim to arbiter. Popper found no scientific theory either verifiable or, as in Carnap's "liberalization of empiricism", confirmable, and found unscientific, metaphysical, ethical, and aesthetic statements often rich in meaning while also underpinning or fueling science as the origin of scientific theories.
Thomas Kuhn's landmark book of 1962, The Structure of Scientific Revolutions—which identified paradigms of science overturned by revolutionary science within fundamental physics—critically destabilized confidence in scientific foundationalism, commonly if erroneously attributed to verificationism. Popper, who had long claimed to have killed verificationism but recognized that some would confuse his falsificationism for more of it, was knighted in 1965. At 1967, John Passmore, a leading historian of 20th-century philosophy, wrote, "Logical positivism is dead, or as dead as a philosophical movement ever becomes"—a general view among philosophers. Logical positivism's fall heralded postpositivism, where Popper's view of human knowledge as hypothetical, continually growing, and open to change ascended, and verificationism became mostly maligned.
Although Karl Popper's falsificationism became soon widely criticized by philosophers, Popper has been the only philosopher of science often praised by scientists, whereas verificationists have been likened to economists of the 19th century who took circuitous, protracted measures to refuse falsification, that is, refutation, of their preconceived principles. Still, logical positivists practiced Popper's principles—conjecturing and refuting—until they ran their course, catapulting Popper, initially a contentious misfit, to carry the richest philosophy out of interwar Vienna. And his falsificationism, as did verificationism, poses a criterion, falsifiability, to ensure that empiricism anchors scientific theory.
In a 1979 interview, A J Ayer, who had introduced logical positivism to the Anglosphere in the 1930s, was asked what he saw as its main defects, and answered that "nearly all of it was false". Still, he soon admitted still holding "the same general approach". The "general approach" of empiricism and reductionism—whereby mental phenomena resolve to the material or physical, and philosophical questions largely resolve to ones of language and meaning—has run through Western philosophy since the 17th century and lived beyond logical positivism's fall.
In 1977, Ayer had noted, "The verification principle is seldom mentioned and when it is mentioned it is usually scorned; it continues, however, to be put to work. The attitude of many philosophers reminds me of the relationship between Pip and Magwitch in Dickens's Great Expectations. They have lived on the money, but are ashamed to acknowledge its source". In the late 20th and early 21st centuries, the general concept of verification criteria—in forms that differed from those of the logical positivists—was defended by Bas van Fraassen, Michael Dummett, Crispin Wright, Christopher Peacocke, David Wiggins, Richard Rorty, and others.
- Editors of Encyclopædia Britannica, "Verifiability principle", Encyclopædia Britannica, Website accessed 12 Mar 2014.
- Whereas logical positivism was launched seeking a theory of verification, logical empiricism sought merely a theory of confirmation, yet logical positivism is often an umbrella term both for itself in the strict sense and for logical empiricism, too, which both can also be termed neopositivism.
- Wesley C Salmon, Four Decades of Scientific Explanation (Minneapolis: University of Minnesota Press, 1990 / Pittsburgh: University of Pittsburgh Press, 2006), p 123: "It is imperative, I believe, to separate the question of the existence of entities not directly observable by means of the unaided human senses from the issue of the meaningfulness of a theoretical vocabulary. Logical empiricists like Carnap and Hempel suggested that the terms of our scientific language can be subdivided into two parts: an observational vocabulary, containing such terms as 'table', dog', 'red', 'larger than', etc, and a theoretical vocabulary containing such terms as 'electron', 'atom', 'molecule', 'gene', 'excited state (of an atom)', etc. The viability of a sharp observational-theoretical distinction was frequently called into question, but that particular problem need not detain us now".
- James Fetzer, "Carl Hempel", in Edward N Zalta, ed, The Stanford Encyclopedia of Philosophy (Spring 2013): "However surprising it may initially seem, contemporary developments in the philosophy of science can only be properly appreciated in relation to the historical background of logical positivism. Hempel himself attained a certain degree of prominence as a critic of this movement. Language, Truth and Logic (1936; 2nd edition, 1946), authored by A J Ayer, offers a lucid exposition of the movement, which was—with certain variations—based upon the analytic/synthetic distinction, the observational/theoretical distinction, and the verifiability criterion of meaningfulness.
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".
- C J Misak, Verificationism: Its History and Prospects (New York: Routledge, 1995), p viii.
- Miran Epstein, ch 2 "Introduction to philosophy of science", in Clive Seale, ed, Researching Society and Culture, 3rd edn (London: Sage Publications, 2012), pp 18–19.
- Karl Popper, ch 4, subch "Science: Conjectures and refutations", in Andrew Bailey, ed, First Philosophy: Fundamental Problems and Readings in Philosophy, 2nd edn (Peterborough Ontario: Broadview Press, 2011), pp 338–42.
- Despite Hume's radical empiricism, set forth near 1740, Hume was also committed to common sense, and apparently did not take his own skepticism, such as the problem of induction, as drastically as others later did [Antony G Flew, A Dictionary of Philosophy, rev 2nd edn (New York: St Martin's Press, 1984), "Hume", p 156].
- Sahotra Sarkar & Jessica Pfeifer, eds, The Philosophy of Science: An Encyclopedia, Volume 1: A–M (New York: Routledge, 2006), "Rudolf Carnap", p 83.
- Antony G Flew, A Dictionary of Philosophy, rev 2nd edn (New York: St Martin's Press, 1984), "Neurath", p 245.
- Mauro Murzi "Rudolf Carnap (1891–1970)", Internet Encyclopedia of Philosophy, 12 Apr 2001.
- Bruce Caldwell, Beyond Positivism: Economic Methodology in the 20th Century, rev edn (London: Routledge, 1994), p 47–48.
- Malachi Haim Hacohen, Karl Popper: The Formative Years, 1902–1945: Politics and Philosophy in Interwar Vienna (Cambridge: Cambridge University Press, 2000), pp 212–13.
- Peter Godfrey-Smith, Theory and Reality: An Introduction to the Philosophy of Science (Chicago: University of Chicago Press, 2005), p 57–59.
- Falsificationism is simply Popper's scientific epistemology, whereas critical rationalism is Popper's general epistemology.
- Samir Okasha, Philosophy of Science: A Very Short Introduction (New York: Oxford University Press, 2002) ch 5.
- "But for a brief lapse around 1929/30, then, the post-Aufbau Carnap fully represents the position of Vienna Circle anti-foundationalism. In this he joined Neurath whose long-standing anti-foundationalism is evident from his famous simile likening scientists to sailors who have to repair their boat without ever being able to pull into dry dock (1932b). Their positions contrasted at least prima facie with that of Schlick (1934) who explicitly defended the idea of foundations in the Circle's protocol-sentence debate. Even Schlick conceded, however, that all scientific statements were fallible ones, so his position on foundationalism was by no means the traditional one. The point of his 'foundations' remained less than wholly clear and different interpretation of it have been put forward. ... While all in the Circle thus recognized as futile the attempt to restore certainty to scientific knowledge claims, not all members embraced positions that rejected foundationalism tout court. Clearly, however, attributing foundationalist ambitions to the Circle as a whole constitutes a total misunderstanding of its internal dynamics and historical development, if it does not bespeak wilfull ignorance. At most, a foundationalist faction around Schlick can be distinguished from the so-called left wing whose members pioneered anti-foundationalism with regard to both the empirical and formal sciences" [Thomas Uebel, "Vienna Circle", sec "3.3 Reductionism and foundationalism: Two criticisms partly rebutted", in Edward N Zalta, ed, The Stanford Encyclopedia of Philosophy (Summer 2012 edn)].
- Oswald Hanfling, ch 5 "Logical positivism", in Stuart G Shanker, Philosophy of Science, Logic and Mathematics in the Twentieth Century (London: Routledge, 1996), pp 193–94.
- Mark Blaug The Methodology of Economics: Or, How Economists Explain, 2nd edn (Cambridge UK: Cambridge University Press, 1992), ch 3 "The verificationists, a largely nineteenth-century story", p 51.