Verificationism aims to formulate criteria of meaningfulness and to develop processes permitting unambiguous assessment of a theory's falsity or truth. Although verificatonist principles of a general sort—grounding scientific theory in some verifiable empiricism—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.
Initially, logical positivists sought a universal language whereby both ordinary language and physics could be axiomatized and reduced to symbolic logic while grounded on empiricism. Logical positivists' verifiability principle—that only statements logically or empirically verifiable are cognitively meaningful—rendered theology, metaphysics, and evaluative judgements, such as ethics and aesthetics, cognitively meaningless "pseudostatements", merely emotive or such. Although varying by exact descriptions proposed, the crucial components of this verificationism program, which extended into the 1960s, were the verifiability criterion, the analytic/synthetic gap, and the observation/theory gap.
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, August 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, widely regarded as the most important philosophy article of the 20th century, 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, that is, fundamental science—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". Nearing the 21st century, the vague concept of verification criteria—in varying forms varying from logical positivists' verificationism—was in some rehabilitation by Bas van Fraassen, Michael Dummett, Crispin Wright, Christopher Peacocke, David Wiggins, Richard Rorty, and others.