Logical positivism

A 20th century development of positivism which emphasizes questions of language and meaning and the role of logical relations like entailment.

It originated in the Vienna Circle and continued mainly in English-speaking countries (with Holland and Scandinavia) until World War II, after which it was replaced by linguistic philosophy in Britain and various movements in the USA and elsewhere.

Its central tenet is the verifiability principle, which in turn has its roots in David Hume’s (1711-1776) distinction – in the last paragraph of his An Enquiry Concerning Human Understanding (1748) – between ‘abstract reasoning concerning quantity or number’ and ‘experimental reasoning concerning matter of fact and existence’, all else being ‘sophistry and illusion’.

A J Ayer, ed., Logical Positivism (1959)


Logical positivists culled from Ludwig Wittgenstein’s early philosophy of language the verifiability principle or criterion of meaningfulness. As in Ernst Mach’s phenomenalism, whereby the mind knows only actual or potential sensory experience, verificationists took all sciences’ basic content to be only sensory experience. And some influence came from Percy Bridgman’s musings that others proclaimed as operationalism, whereby a physical theory is understood by what laboratory procedures scientists perform to test its predictions. In verificationism, only the verifiable was scientific, and thus meaningful (or cognitively meaningful), whereas the unverifiable, being unscientific, were meaningless “pseudostatements” (just emotively meaningful). Unscientific discourse, as in ethics and metaphysics, would be unfit for discourse by philosophers, newly tasked to organize knowledge, not develop new knowledge.


Logical positivism is sometimes stereotyped as forbidding talk of unobservables, such as microscopic entities or such notions as causality and general principles,[4] but that is an exaggeration. Rather, most neopositivists viewed talk of unobservables as metaphorical or elliptical: direct observations phrased abstractly or indirectly. So theoretical terms would garner meaning from observational terms via correspondence rules, and thereby theoretical laws would be reduced to empirical laws. Via Bertrand Russell’s logicism, reducing mathematics to logic, physics’ mathematical formulas would be converted to symbolic logic. Via Russell’s logical atomism, ordinary language would break into discrete units of meaning. Rational reconstruction, then, would convert ordinary statements into standardized equivalents, all networked and united by a logical syntax. A scientific theory would be stated with its method of verification, whereby a logical calculus or empirical operation could verify its falsity or truth.


In the late 1930s, logical positivists fled Germany and Austria for Britain and the United States. By then, many had replaced Mach’s phenomenalism with Otto Neurath’s physicalism, whereby science’s content is not actual or potential sensations, but instead is entities publicly observable. Rudolf Carnap, who had sparked logical positivism in the Vienna Circle, had sought to replace verification with simply confirmation. With World War II’s close in 1945, logical positivism became milder, logical empiricism, led largely by Carl Hempel, in America, who expounded the covering law model of scientific explanation. Logical positivism became a major underpinning of analytic philosophy,[5] and dominated philosophy in the English-speaking world, including philosophy of science, while influencing sciences, but especially social sciences, into the 1960s. Yet the movement failed to resolve its central problems,[6][7][8] and its doctrines were increasingly criticized, most trenchantly by Willard Van Orman Quine, Norwood Hanson, Karl Popper, Thomas Kuhn, and Carl Hempel.



Tractatus Logico-Philosophicus, by the young Ludwig Wittgenstein, introduced the view of philosophy as “critique of language”, offering the possibility of a theoretically principled distinction of intelligible versus nonsensical discourse. Tractatus adhered to a correspondence theory of truth (versus a coherence theory of truth). Wittgenstein’s influence also shows in some versions of the verifiability principle.[9][10] In tractarian doctrine, truths of logic are tautologies, a view widely accepted by logical positivists who were also influenced by Wittgenstein’s interpretation of probability although, according to Neurath, some logical positivists found Tractatus to contain too much metaphysics.[11]


Gottlob Frege began the program of reducing mathematics to logic, continued it with Bertrand Russell, but lost interest in this logicism, and Russell continued it with Alfred North Whitehead in their Principia Mathematica, inspiring some of the more mathematical logical posivists, such as Hans Hahn and Rudolf Carnap.[12] Carnap’s early anti-metaphysical works employed Russell’s theory of types.[13] Carnap envisioned a universal language that could reconstruct mathematics and thereby encode physics.[12] Yet Kurt Gödel’s incompleteness theorem showed this impossible except in trivial cases, and Alfred Tarski’s undefinability theorem shattered all hopes of reducing mathematics to logic.[12] Thus, a universal language failed to stem from Carnap’s 1934 work Logische Syntax der Sprache (Logical Syntax of Language).[12] Still, some logical positivists, including Carl Hempel, continued support of logicism.[12]


In Germany, Hegelian metaphysics was a dominant movement, and Hegelian successors such as F H Bradley explained reality by postulating metaphysical entities lacking empirical basis, drawing reaction in the form of positivism.[14] Starting in the late 19th century, there was a “back to Kant” movement. Ernst Mach’s positivism and phenomenalism were a major influence.



The Vienna Circle, gathering around University of Vienna and Café Central, was led principally by Moritz Schlick. Schlick had held a neo-Kantian position, but later converted, via Carnap’s 1928 book Der logische Aufbau der Welt, that is, The Logical Structure of the World. A 1929 pamphlet written by Otto Neurath, Hans Hahn, and Rudolf Carnap summarized the Vienna Circle’s positions. Another member of Vienna Circle to later prove very influential was Carl Hempel. A friendly but tenacious critic of the Circle was Karl Popper, whom Neurath nicknamed the “Official Opposition”.

Carnap and other Vienna Circle members, including Hahn and Neurath, saw need for a weaker criterion of meaningfulness than verifiability.[15] A radical “left” wing—led by Neurath and Carnap—began the program of “liberalization of empiricism”, and they also emphasized fallibilism and pragmatics, which latter Carnap even suggested as empiricism’s basis.[15] A conservative “right” wing—led by Schlick and Waismann—rejected both the liberalization of empiricism and the epistemological nonfoundationalism of a move from phenomenalism to physicalism.[15] As Neurath and somewhat Carnap posed science toward social reform, the split in Vienna Circle also reflected political views.[15]


The Berlin Circle was led principally by Hans Reichenbach.


Both Moritz Schlick and Rudolf Carnap had been influenced by and sought to define logical positivism versus the neo-Kantianism of Ernst Cassirer—the then leading figure of Marburg school, so called—and against Edmund Husserl’s phenomenology. Logical positivists especially opposed Martin Heidegger’s obscure metaphysics, the epitome of what logical positivism rejected. In the early 1930s, Carnap debated Heidegger over “metaphysical pseudosentences”.[16] Despite its revolutionary aims, logical positivism was but one view among many vying within Europe, and logical positivists initially spoke their language.[16]


As the movement’s first emissary to the New World, Moritz Schlick visited Stanford University in 1929, yet otherwise remained in Vienna and was murdered at the University, reportedly by a deranged student, in 1936.[16] That year, a British attendee at some Vienna Circle meetings since 1933, A. J. Ayer saw his Language, Truth and Logic, written in English, import logical positivism to the English-speaking world. By then, the Nazi Party’s 1933 rise to power in Germany had triggered flight of intellectuals.[16] In exile in England, Otto Neurath died in 1945.[16] Rudolf Carnap, Hans Reichenbach, and Carl Hempel—Carnap’s protégé who had studied in Berlin with Reichenbach—settled permanently in America.[16] Upon Germany’s annexation of Austria in 1938, remaining logical positivists, many of whom were also Jewish, were targeted and continued flight. Logical positivism thus became dominant in the English-speaking world.


Analytic/synthetic gap

Concerning reality, the necessary is a state true in all possible worlds—mere logical validity—whereas the contingent hinges on the way the particular world is. Concerning knowledge, the a priori is knowable before or without, whereas the a posteriori is knowable only after or through, relevant experience. Concerning statements, the analytic is true via terms’ arrangement and meanings, thus a tautology—true by logical necessity but uninformative about the world—whereas the synthetic adds reference to a state of facts, a contingency.

In 1739, David Hume cast a fork aggressively dividing “relations of ideas” from “matters of fact and real existence”, such that all truths are of one type or the other.[17][18] By Hume’s fork, truths by relations among ideas (abstract) all align on one side (analytic, necessary, a priori), whereas truths by states of actualities (concrete) always align on the other side (synthetic, contingent, a posteriori).[17] Of any treatises containing neither, Hume orders, “Commit it then to the flames, for it can contain nothing but sophistry and illusion”.[17]

Thus awakened from “dogmatic slumber”, Immanuel Kant quested to answer Hume’s challenge—but by explaining how metaphysics is possible. Eventually, in his 1781 work, Kant crossed the tines of Hume’s fork to identify another range of truths by necessity—synthetic a priori, statements claiming states of facts but known true before experience—by arriving at transcendental idealism, attributing the mind a constructive role in phenomena by arranging sense data into the very experience spacetime, and substance. Thus, Kant saved Newton’s law of universal gravitation from Hume’s problem of induction by finding uniformity of nature to be a priori knowledge. Logical positivists rejected Kant’s synthethic a priori, and adopted Hume’s fork, whereby a statement is either analytic and a priori (thus necessary and verifiable logically) or synthetic and a posteriori (thus contingent and verifiable empirically).[17]

Observation/theory gap

Early, most logical positivists proposed that all knowledge is based on logical inference from simple “protocol sentences” grounded in observable facts. In the 1936 and 1937 papers “Testability and meaning”, individual terms replace sentences as the units of meaning.[15] Further, theoretical terms no longer need to acquire meaning by explicit definition from observational terms: the connection may be indirect, through a system of implicit definitions.[15] Carnap also provided an important, pioneering discussion of disposition predicates.[15]

Cognitive meaningfulness


The logical positivists’ initial stance was that a statement is “cognitively meaningful” only if some finite procedure conclusively determines its truth.[19] By this verifiability principle, only statements verifiable either by their analyticity or by empiricism were cognitively meaningful. Metaphysics, ontology, as well as much of ethics failed this criterion, and so were found cognitively meaningless. Moritz Schlick, however, did not view ethical or aesthetic statements as cognitively meaningless.[20] Cognitive meaningfulness was variously defined: having a truth value; corresponding to a possible state of affairs; intelligible or understandable as are scientific statements.[21]

Ethics and aesthetics were subjective preferences, while theology and other metaphysics contained “pseudostatements”, neither true nor false. This meaningfulness was cognitive, although other types of meaningfulness—for instance, emotive, expressive, or figurative—occurred in metaphysical discourse, dismissed from further review. Thus, logical positivism indirectly asserted Hume’s law, the principle that is statements cannot justify ought statements, but are separated by an unbridgeable gap. A. J. Ayer’s 1936 book asserted an extreme variant—the boo/hooray doctrine—whereby all evaluative judgments are but emotional reactions.

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