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Philosophical doctrine
Verificationism, also known as the verification principle or the verifiability criterion of meaning, is the philosophical doctrine which asserts that a statement is meaningful only if it is either empirically verifiable (i. e. confirmed through the senses) or a truth of logic (e. g., tautologies). Verificationism rejects statements of metaphysics, theology, ethics, and aesthetics, as cognitively meaningless. Such statements may be meaningful in influencing emotions or behavior, but not in terms of conveying truth value, information, or factual content. Verificationism was a central theme of logical positivism, a movement in analytic philosophy that emerged in the 1920s by philosophers who sought to unify philosophy and science under a common naturalistic theory of knowledge.

Although earlier philosophical principles which aim to ground scientific theory in some verifiable experience are found within the work of American pragmatist C. S. Peirce and that of French conventionalist Pierre Duhem, the project of verificationism was launched by the logical positivists who, emerging from the Berlin Circle and the Vienna Circle in the 1920s, sought an epistemology whereby philosophical discourse would be, in their perception, as authoritative and meaningful as an empirical science. Logical positivists garnered the verifiability criterion of cognitive meaningfulness from Ludwig Wittgenstein's philosophy of language posed in his 1921 book Tractatus, and, led by Bertrand Russell, sought to reformulate the analytic–synthetic distinction in a way that would reduce mathematics and logic to semantical conventions. This would be pivotal to verificationism, in that logic and mathematics would otherwise be classified as synthetic a priori knowledge and defined as "meaningless" under verificationism. 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.

Logical positivists within the Vienna Circle recognized quickly that the verifiability criterion was too stringent. Notably, universal generalizations are empirically unverifiable, such that, under verificationism, vast domains of science and reason, such as scientific hypothesis, would be rendered meaningless. Rudolf Carnap, Otto Neurath, Hans Hahn and Philipp Frank led a faction seeking to make the verifiability criterion more inclusive, beginning a movement they referred to as the "liberalization of empiricism". Moritz Schlick and Friedrich Waismann led a "conservative wing" that maintained a strict verificationism. Whereas Schlick sought to reduce universal generalizations to frameworks of 'rules' from which verifiable statements can be derived, Hahn argued that the verifiability criterion should accommodate to less than conclusive verifiability. Among other ideas espoused by the liberalization movement were physicalism, over Mach's phenomenalism, coherentism over foundationalism, as well as pragmatism and fallibilism. In 1936, Carnap sought a switch from verification to confirmation. That same year saw the publication of A. J. Ayer's work, Language, Truth and Logic, in which he proposed two types of verification: strong and weak. This system espoused conclusive verification, yet accommodated for probabilistic inclusion where verifiability is inconclusive. Ayer also distinguished between practical and theoretical verifiability. Under the latter, propositions that cannot be verified in practice would still be meaningful if they can be verified in principle.

Verificationism has been criticized by some philosophers, such as Karl Popper, another graduate of the University of Vienna though not part of the Vienna Circle. Popper identified three core concerns with verificationism:

Verificationism rejects universal generalizations, like "all swans are white," as meaningless. Popper argues that while universal statements cannot be verified, they can be proven false. Further, Popper argues that universals can be scientifically useful. Verificationism allows for some obviously false existential statements, such as "werewolves exist," to be classified as scientifically meaningful because one could possibly find one somewhere that they have not yet looked. By its own logic, the verificationism is meaningless because it cannot be empirically verified. In The Logic of Scientific Discovery (1959), Popper proposed falsifiability as a preferable method for determining if a hypothesis is scientific, though it has been misunderstood as a revision of verificationism. Informed by the contrasting approaches of Albert Einstein and Sigmund Freud, Popper proposed falsifiability as a standard specific to the sciences, rather than as a theory of meaning. Popper regarded scientific hypotheses to be never completely verifiable, as well as not "confirmable" under Rudolf Carnap's thesis. It is pseudoscientists that come to a conclusion and seek out information to verify their claim. Scientists, on the other hand, use a hypothesis to indicate the most likely possible explanation based on current data, and they seek to prove the hypothesis wrong so that they can continue to learn. 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. 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, a theory commonly, if erroneously, attributed to verificationism. In 1967, John Passmore, a leading historian of 20th century philosophy, wrote, "Logical positivism is dead, or as dead as a philosophical movement ever becomes". Logical positivism's fall heralded postpositivism, where Popper's view of human knowledge as hypothetical, continually growing, and open to change ascended, was asked what he saw as its main defects, and answered that "nearly all of it was false".