Logical positivism (later referred to as logical empiricism, rational empiricism, or neo-positivism) is a school of philosophy that combines positivism—which states that the only authentic knowledge is scientific knowledge—with some king of logical analysis, which is similar, but not the same as logicism.
Logical positivism denied the soundness of metaphysics and large swathes of traditional philosophy, and affirmed that statements about metaphysics, religion and some claims of ethics are devoid of cognitive meaning and thus nothing but expression of feelings or desires; only statements about mathematics, logic and natural sciences have a definite meaning.
Logical positivism originated in the Vienna Circle in the 1920s, where Moritz Schlick, Rudolf Carnap, Otto Neurath, and others (see Philosophers associated with logical positivism in this article) divided meaningful statements into those which are analytic (true a priori), and those which are synthetic (verified by sensory experience, a posteriori) - this was perhaps presaged by Hume's fork. Logical positivism refuted synthetic a priori knowledge: an evident criticism to Kantian philosophy.
Origins of logical positivism
The logical positivists were very much influenced by and were great admirers of the early work of Wittgenstein (from the period of the Tractatus Logico-Philosophicus). Wittgenstein himself was not a logical positivist, although he was on friendly terms with many members of the Vienna Circle while in Vienna, especially fellow aristocrat Moritz Schlick. Wittgenstein's influence is evident in the formulation of the verifiability principle. See for example Proposition 4.024 of the Tractatus, where Wittgenstein asserts that we understand a proposition when we know what happens if it is true, and compare it with Schlick's assertion that "The definition of the circumstances under which a statement is true is perfectly 'equivalent' to the definition of its meaning". Wittgenstein also influenced the logical positivists' interpretation of probability.
However, Wittgenstein's relations were not entirely amicable after he left Vienna. Wittgenstein worked mostly in cooperation for nearly a decade with Circle member Friedrich Waismann, to impose form and structure on his often oracular utterances, using him as a secretary and speaking of cooperating on a book with him. When Waismann came to Cambridge in 1937, Wittgenstein barely acknowledged him.
Not all logical positivists' reactions to the Tractatus were positive: according to Neurath, it was full of metaphysics. Carnap (in his autobiography in The Philosophy of Rudolf Carnap) said that Wittgenstein's influence on the Vienna Circle was overestimated. Moreover, Wittgenstein did not take part in the Vienna Circle's discussions; there were separate meetings between him, Schlick, Carnap and Waismann, but soon Carnap was not admitted to those meetings.
Carnap was highly influenced by Bertrand Russell, with whom he had correspondence. The Aufbau is inspired by the Principia Mathematica.
Ernst Mach was another main influence. The name of the Vienna Circle "Verein Ernst Mach" indicates this.
Gotlob Frege was a teacher of Rudolf Carnap and made a big contribution in developing his views.
The assertions of logical positivism
Although the logical positivists held a wide range of beliefs on many matters, they were all interested in science and skeptical of theology and metaphysics. Following Ludwig Wittgenstein, many held the correspondence theory of truth, although some, like Neurath, held the coherence theory of truth. One part of the Circle believed that all knowledge should be based on simple "protocol sentences" grounded in observable facts, however this was debated in the protocol sentence debate. Hence, many logical positivists supported forms of empiricism and some of them supported some kind of reductionism.
Logical positivism is perhaps best known for the verifiability criterion of meaning, which asserts that a statement is meaningful if and only if it is empirically verifiable. One intended consequence of the verification criterion is that all non-empirical forms of discourse, including a large part of classical ethics and aesthetics, are not "literally" or "cognitively" meaningful, and thus belong to "metaphysics". Rejection of metaphysics was also held after the liberalization of the verifiability criterion.
Most of the logical positivists were in favour of methodology of science. One exception may be Neurath, who had more "anarchistic" views. Some members, like Philip Frank developed a methodology, which was close to Karl Popper's. The main theorist Carnap however had a methodology based on degree of confirmation, and had a long battle with Popper over the so called question of induction.
Logical positivists rejected the possibility of synthetic apriori, which also means that they hold the distinction of synthetic and analytic. Especially Carnap had a long debate with Quine about this.
The "received view" about logical positivism is that they were atomists in regard of the role of sentences in analyzing the languages. It is true that early positivism took sentences as the basis of linguistical analysis, but atomism was never stated explicitly. Before any major criticism logical positivists themselves came to the idea of linguistic holism, i.e. that sentences can only be analyzed in context. From this point Carnap for example made his claims relative to a fixed language.
Logical positivism was essential to the development of early analytic philosophy. It was disseminated throughout the European continent and, later, in American universities by the members of the Vienna Circle. A.J. Ayer is considered responsible for the spread of logical positivism to Britain. The term subsequently came to be almost interchangeable with "analytic philosophy" in the first half of the twentieth century. Logical positivism was immensely influential in the philosophy of language and represented the dominant philosophy of science between World War I and the Cold War. Many subsequent commentators on "logical positivism" have attributed to its proponents a greater unity of purpose and creed than they actually shared, overlooking the complex disagreements among the logical positivists themselves.
Unity of Science can be of various kind. In Aufbau Carnap tried to create a constitutional system, where unity was based on reduction. Later a unified language was conceived, which should be the physical language. The unity is based on translatability here. Neurath's concept about the unity of science was quite different. It was a practical unity without any system, a mosaic of fields.
Einheitswissenschaft (Unified Science)
The Vienna Circle published a collection called Einheitswissenschaft (Unified Science)—edited by Rudolf Carnap, Philipp Frank, Hans Hahn, Otto Neurath, Jorgen Jorgensen (after Hahn's death) and Charles Morris—the aim of which was to present a unified vision of science. The collection was dismissed, after the publication of several monographies, because of the problems arising from World War II. The list of philosophers and scientists who contributed to these works is impressive. The complete list of contributors is given here for the historical record.
Einheitswissenschaft (Unified Science), edited by Carnap, Frank, Hahn, Neurath, Jorgensen (after Hahn's death), and Morris (from 1938):
- Hans Hahn, Logik, Mathematik und Naturerkennen, 1933
- Otto Neurath, Einheitswissenschaft und Psychologie, 1933
- Rudolf Carnap, Die Aufgabe der Wissenschaftlogik, 1934
- Philipp Frank, Das Ende der mechanistischen Physik, 1935
- Otto Neurath, Was bedeutet rationale Wirtschaftsbetrachtung, 1935
- Otto Neurath, E. Brunswik, C. Hull, G. Mannoury, J. Woodger, Zur Enzyklopädie der Einheitswissenschaft. Vorträge, 1938
- Richard von Mises, Ernst Mach und die empiristische Wissenschaftauffassung, 1939
These works are translated in Unified Science: The Vienna Circle Monograph Series, originally edited by Otto Neurath:, Kluwer, 1987.
International Encyclopedia of Unified Science
In 1938 the publication of the International Encyclopedia of Unified Science started under the auspice of logical positivists. It was an ambitious project, and never completed. Only the first section, Foundations of the Unity of Sciences, was published; it contained two volumes, for a total of twenty monographs published from 1938 to 1969:
- Otto Neurath, Niels Bohr, John Dewey, Bertrand Russell, Rudolf Carnap, Charles Morris, Encyclopedia and unified science, 1938, vol.1 n.1
- Charles Morris, Foundations of the theory of signs, 1938, vol.1 n.2
- Victor Lenzen, Procedures of empirical sciences, 1938, vol.1 n.5
- Rudolf Carnap, Foundations of logic and mathematics, 1939, vol.1 n.3
- Leonard Bloomfield, Linguistic aspects of science, 1939, vol.1 n.4
- Ernest Nagel, Principles of the theory of probability, 1939, vol.1 n.6
- John Dewey, Theory of valuation, 1939, vol.2 n.4
- Giorgio De Santillana and Egdard Zilsel, The development of rationalism and empiricism, 1941, vol.2 n.8
- Otto Neurath, Foundations of social sciences, 1944, vol.2 n.1
- Joseph Henri Woodger, The technique of theory construction, 1949, vol.2 n.5
- Philipp Frank, Foundations of physics, 1946, vol.1 n.7
- Erwin Frinlay-Freundlich, Cosmology, 1951, vol.1 n.8
- Jorgen Jorgensen, The development of logical empiricism, 1951, vol.2 n.9
- Egon Brunswik, The conceptual framework of psychology, 1952, vol.1 n.10
- Carl Hempel, Fundamentals of concept formation in empirical science, 1952, vol.2 n.7
- Felix Mainx, Foundations of biology, 1955, vol.1 n.9
- Abraham Edel, Science and the structure of ethics, 1961, vol.2 n.3
- Thomas Kuhn, The structure of scientific revolutions, 1962, vol.2 n.2
- Gherard Tintner, Methodology of mathematical economics and econometrics, 1968, vol.2 n.6
- Herbert Feigl and Charles Morris, Bibliography and index, 1969, vol.2 n.10
Perhaps the most famous work published in the International Encyclopedia of Unified Science is Thomas Kuhn's The Structure of Scientific Revolutions. However, every entry in the encyclopedia is of substantial scientific and philosophical value.
Monographs on the Scientific World-Conception
A third collection was published by the Vienna Circle from 1928 to 1937. This collection was entitled Schriften zur wissenschaftlichen Weltauffassung (Monographs on the Scientific World-Conception), and was edited by Schlick and Frank. Scientists and philosophers such as Karl Popper contributed. The contributors and monographs were:
- Richard von Mises, Wahrscheinlichkeit, Statistik und Wahrheit, 1928 (Probability, Statistics, and Truth, New York: MacMillan Company, 1939)
- Rudolf Carnap, Abriss der Logistik, 1929
- Moritz Schlick, Fragen der Ethik, 1930 (Problems of Ethics, New York: Prentice-Hall, 1939)
- Otto Neurath, Empirische Soziologie, 1931
- Philipp Frank, Das Kausalgesetz und seine Grenzen, 1932 (The Law of Causality and its Limits, Dordrecht; Boston: Kluwer, 1997)
- Otto Kant, Zur Biologie der Ethik, 1932
- Rudolf Carnap, Logische Syntax der Sprache, 1934 (The Logical Syntax of Language, New York: Humanities, 1937)
- Karl Popper, Logik der Forschung, 1934 (The Logic of Scientific Discovery, New York: Basic Books, 1959)
- Josef Schächeter, Prolegomena zu einer kritischen Grammatik, 1935 (Prolegomena to a Critical Grammar, Dordrecht; Boston: D. Reidel Pub. Co., 1973)
- Victor Kraft, Die Grundlagen einer wissenschaftliche Wertlehre, 1937 (Foundations for a Scientific Analysis of Value, Dordrecht; Boston: D. Reidel Pub. Co., 1981)
Philosophers associated with logical positivism
There is an extensive list of philosophers who are associated to some degree with logical positivism.
The Vienna Circle
- The physicist and philosopher Moritz Schlick, one of the first philosophers interested in the theory of relativity. He taught at the University of Vienna, where he held the chair of theory of inductive science. In Vienna, he organized the discussion group known as the Vienna Circle. Schlick can be regarded as the father of logical positivism, both for his organizational skills and for his philosophical ideas. He formulated the verifiability principle.
- Rudolf Carnap, one of the major philosophers of the twentieth century, a leading exponent of logical positivism, and co-author of the Vienna Circle manifesto. He made contributions to the philosophy of science, the philosophy of language, the theory of probability, and classical, inductive and modal logic. Since ordinary language is ambiguous, Carnap asserted the necessity to study philosophical issues in artificial languages, governed by the rules of logic and mathematics. In such languages he dealt with problems like the meaning of a statement, the distinction between analytic and synthetic, a priori and a posteriori, necessity and contingency, the different interpretations of probability, and the nature of explanation. Carnap taught at the University of Prague, the University of Vienna, the University of Chicago, Harvard University, and the University of California at Los Angeles.
- The philosopher of science Herbert Feigl, who encouraged Karl Popper to write the book Logik der Forschung. His 1931 article with A. E. Blumberg, "Logical Positivism: A New Movement in European Philosophy" in The Journal of Philosophy, was one of the first reports on logical positivism published in the United States, and promoted the spread of logical positivism. He taught at the University of Iowa and at the University of Minnesota, where in 1953 he founded the Minnesota Center for the Philosophy of Science, the oldest center for the philosophy of science in the world.
- The physicist Philipp Frank, a student of David Hilbert and Ludwig Boltzmann, who was an editor of the series Monographs on the Scientific World-Conception and Unified Science; he contributed to the International Encyclopedia of Unified Science with the 1946 work "Foundations of physics".. He taught at Harvard University and wrote about the theory of relativity.
- The logician and mathematician Kurt Gödel, who proved the completeness of first-order logic and the incompleteness of formal arithmetic. Gödel also worked on set theory and non-classical logics, such as intuitionistic logic and modal logic. He proved that the continuum hypothesis is consistent with the axioms of classical set theory. He was interested in the mathematical aspects of the theory of relativity, and proved the existence of solutions of Einstein’s relativistic equations in which time travel to the past is possible.
- The mathematician Hans Hahn, co-author of the Vienna Circle manifesto, who taught at Innsbruck, Bonn and Vienna; among his students there were Karl Popper and Kurt Gödel.
- The philosopher and sociologist Otto Neurath, who played an important role in the development of logical positivism. He was co-author of the manifesto of the Vienna Circle (it is supposed that he was indeed the principal author), planned and directed the International Encyclopedia of Unified Science, was an editor of the journal Erkentnnis and of the series Unified Science, and founded and directed the International Foundation for Visual Education.
- The philosopher Friedrich Waismann, who was one of the few members of the Vienna Circle admitted to the meetings with Wittgenstein. He taught philosophy of mathematics and philosophy of science at the University of Cambridge and at the University of Oxford.
Germany and the Berlin Circle
In Germany, members of the Berlin Circle contributed in an essential way to the development of logical positivism:
- The physicist and philosopher Hans Reichenbach, the founder of the Berlin Circle. He studied with Albert Einstein, Arnold Sommerfeld, Ernst Cassirer, David Hilbert, Max Planck, and Max Born. He is one of the fathers of the frequency interpretation of probability. He taught physics at Technische Hochschule in Stuttgart, philosophy of physics at the University of Berlin; he was chief of the department of philosophy at the University of Istanbul; he taught philosophy at the University of California at Los Angeles. He wrote about the philosophical meaning of the theory of relativity, which he studied under the teaching of Albert Einstein, and quantum mechanics.
- The logician and philosopher Kurt Grelling, a victim of Nazism; it is supposed that he died with his wife in the Auschwitz concentration camp in 1942, although it also has been reported that Grelling was killed in 1941 at the border between France and Spain while he was trying to escape in Spain. Grelling was a teacher in secondary school and was interested in logical problems. A semantic paradox is named after him, the Grelling paradox, formulated in 1908 by Grelling and Leonard Nelson. Grelling collaborated with Kurt Gödel and in 1936 he published an article in which he defended Gödel's theorem of incompleteness against an erroneous interpretation, according to which Gödel's theorem is indeed a paradox like Russell's paradox.
- The philosopher Carl Gustav Hempel, a leading member of logical positivism. He studied philosophy with Hans Reichenbach, physics with Max Planck, logic with von Neumann and mathematics with David Hilbert. He taught in New York, at the City College and at Queens College. Later, Hempel taught at Yale University, at Princeton University and, well on in years, he continued in teaching at the University of California at Berkeley and Irvine, the Hebrew University of Jerusalem, and the University of Pittsburgh. He contributed to the International Encyclopedia of Unified Science. He is well-known for his studies on the logic of confirmation and explanation.
- As early as 1930 Scandinavian philosophers were interested in logical positivism. Two of them, Swedish Ake Petzäll and Finnish Eino Kaila, employed for the first time the expression "logical neopositivism" for denoting the new philosophical movements (A. Petzäll, Der logistische Neupositivismus, 1930; and E. Kaila, "Der logistische Neupositivismus" in Annales Universitatis Aboensis, 1930). Petzäll was mainly influenced by the Vienna Circle and in 1930 or 1931 he went to Vienna, where he took part in theVienna Circle's meetings. Later he founded a new journal, Theoria, published in Göteborg; in that journal Hempel published his very first description of the paradoxes of confirmation (Le problème de la vérité, 1937). Eino Kaila published in 1939 a work pervaded by the principles of logical positivism (The human knowledge, in Finnish). He taught philosophy at the University of Helsinki. Among his students was Georg Henrik von Wright, who published a study about logical positivism (The Logical Empiricism, 1943, in Finnish). Wright contributed to the development of modal logic and deontic logic. Finn Jaakko Hintikka, who had Wright as a teacher, pursued Carnap's studies on inductive logic. Hintikka's article "A two-dimensional continuum of inductive methods" in Aspects of inductive logic (eds. J. Hintikka and P. Suppes, 1966), extended the methods Carnap used in The Continuum of Inductive Methods, 1952.
- Danish philosopher Jorgen Jorgensen very actively collaborated with neopositivists. After Hans Hahn's death in 1934, Jorgensen became an editor of the Vienna Circle's series Unified Science; later he collaborated on the International Encyclopedia of Unified Science, to which he contributed the 1951 essay The Development of Logical Empiricism.
- English philosopher Alfred Jules Ayer played an important role in the spread of logical positivism. His 1936 work Language, Truth and Logic gained immediate success. In that book, he completely accepted both the verifiability principle and the distinction between analytic and synthetic statements; hence he asserted that metaphysical sentences are meaningless.
- Logical positivism had extensive contacts with the group of Polish logicians who developed several branches of contemporary logic. Polish philosophy was greatly influenced by Kazimierz Twardowski, who studied at the University of Vienna and taught at Lwow; he is the founder of Polish analytic philosophy. He taught several Polish philosophers and logicians. Among them were:
- Jan Lukasiewicz, who developed both the algebra of logic and many-valued propositional calculus, which influenced Carnap's inductive logic and Reichenbach's interpretation of quantum mechanics, in which Reichenbach employed a three-valued propositional calculus. He contributed to Erkenntnis, the journal of logical positivism, edited by Carnap and Reichenbach.
- Stanislaw Lesniewski, who was interested in the logical antinomies.
- Kazimierz Ajdukiewicz, who taught philosophy of language, epistemology and logic, and contributed to Erkenntnis.
- Tadeusz Kotarbinski, who asserted that many alleged philosophical problems in fact are scientific problems, i.e. they are the object of empirical science and not of philosophy, which deals with logical and ethical problems only.
- The Polish logician Alfred Tarski, who developed the theory of semantics in formal language, took part in the congresses on scientific philosophy organized by the Vienna and Berlin Circles; he greatly influenced Carnap's philosophy of language. Carnap was interested in logical syntax but, after the publication of Tarski's works, he turned to semantics.
Early critics of logical positivism said that its fundamental tenets could not themselves be formulated in a way that was clearly consistent. The verifiability criterion of meaning did not seem verifiable; but neither was it simply a logical tautology, since it had implications for the practice of science and the empirical truth of other statements. This presented severe problems for the logical consistency of the theory. Another problem was that, while positive existential claims ("there is at least one human being") and negative universals ("not all ravens are black") allow for clear methods of verification (find a human or a non-black raven), negative existential claims and positive universal claims do not allow for verification.
Universal claims could apparently never be verified: How can you tell that all ravens are black, unless you've hunted down every raven ever, including those in the past and future? This led to a great deal of work on induction, probability, and "confirmation", which combined verification and falsification.
Karl Popper, a well-known critic of logical positivism, published the book Logik der Forschung in 1934 (translated by himself as The Logic of Scientific Discovery published 1959). In it he presented an influential alternative to the verifiability criterion of meaning, defining scientific statements in terms of falsifiability. First, though, Popper's concern was not with distinguishing meaningful from meaningless statements, but distinguishing "scientific" from "metaphysical" statements. He did not hold that metaphysical statements must be meaningless; neither did he hold that a statement that in one century was "metaphysical" while unfalsifiable (like the ancient Greek philosophy about atoms), could not in another century become "falsifiable" and thus "scientific". About psychoanalysis he thought something similar: in his day it offered no method for falsification, and thus was not falsifiable and not scientific. However, he did not exclude it being meaningful, nor did he say psychoanalysts were necessarily "wrong" (it only couldn't be proven either way: that would have meant it was falsifiable), nor did he exclude that one day psychoanalysis could evolve into something falsifiable, and thus "scientific". He was, in general, more concerned with scientific practice than with the logical issues that troubled the positivists. Second, although Popper's philosophy of science enjoyed great popularity for some years, if his criterion is construed as an answer to the question the positivists were asking, it turns out to fail in exactly parallel ways. Negative existential claims ("there are no unicorns") and positive universals ("all ravens are black") can be falsified, but positive existential and negative universal claims cannot.
Logical positivists' response to the first criticism is that logical positivism is a philosophy of science, not an axiomatic system that can prove its own consistency (see Gödel's incompleteness theorem). Secondly, a theory of language and mathematical logic were created to answer what it really means to make statements like "all ravens are black".
As response to the second criticism Carnap made a turn, which he called the liberalisation of empiricism, and changed to confirmation instead of verification. This means that meaningful statements can be confirmed more and more, until they can be accepted without reasonable doubt. Later he tried to develop a calculus of degree of confirmation.
Another response by A. J. Ayer in Language, Truth and Logic, in which he sets out the distinction between "strong" and "weak" verification. "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." (Ayer 1946:50) It is this sense of verifiable that causes the problem of verification with negative existential claims and positive universal claims. However, the weak sense of verification states that a proposition is "verifiable... if it is possible for experience to render it probable" (ibid.). After establishing this distinction, Ayer goes on to claim that "no proposition, other than a tautology, can possibly be anything more than a probable hypothesis" (Ayer 1946:51), and therefore can only be subject to weak verification. This defense was controversial among logical positivists, some of whom stuck to strong verification, and claimed that general propositions were indeed nonsense.
Quine criticised the analytic-synthetic distinction of logical positivism and offered a naturalized, holistic view of science. Holism was not something, logical positivists were not aware of. By the time of the critique of Quine they already accepted that sentences do not face "reality" separately, but are embedded in theories. Logical positivism never stated explicitly atomism and accepted quite early holism by itself. On the question of the distinction of analytic and synthetic Quine had a debate with Carnap until the death of the later one. This question is still open in philosophy.
Works of Thomas Kuhn has convinced many that it is not possible to provide truth conditions for science independent of its historical paradigm. But even this criticism was not unknown to the logical positivists: Otto Neurath compared science to a boat which we must rebuild on the open sea. Thomas Kuhn admitted himself that he did not know the views of logical positivism in detail, when he wrote his Structure.
Reconsidering logical positivism
In the last decade George Reisch, John Earman, Michael Friedman and Gürol Irzik started a re-evaluation of logical empiricism, especially of Carnap's work. A main issue is the comparison of Carnap and Kuhn and the transition of logical empiricism to post-positivism. The standard account assumes that Kuhn "killed" logical empiricism by refuting their main theses, like reductionism, verificationism, atomism, logicism and an a-historical view; and justifying completely new and opposite theses, like incommensurability, holism, theory laddenness of observations, and a historical and social view.
Despite to this standard account, the re-evaluated view is that Carnap's linguistic frameworks are quite similar to Kuhn's paradigms and his view about the pragmatical nature of the external questions is close to Kuhn's values of theory choice. According to the re-evaluated view Carnap's theory includes some kind of incommensurability, holism, theory ladenness of observation, so his theory is quite near to Kuhn's theory of scientific revolutions, normal science and paradigms. Reisch states that Kuhn's Structure is based on logical positivism. He shows that some of the philosophers in the Vienna Circle, like Otto Neurath, for instance, emphasized the historical viewpoint, just like Kuhn.
- Kazimierz Ajdukiewicz
- Alfred Jules Ayer
- Richard Bevan Braithwaite
- Rudolf Carnap
- Herbert Feigl
- Philipp Frank
- Kurt Gödel
- Kurt Grelling
- Hans Hahn
- Carl Gustav Hempel
- Tadeusz Kotarbinski
- Thomas Kuhn
- Stanislaw Lesniewski
- Jan Lukasiewicz
- Ernest Nagel
- Otto Neurath
- Karl Raimund Popper
- Hans Reichenbach
- Moritz Schlick
- Alfred Tarski
- Kazimierz Twardowski
- Friedrich Waismann
- Ludwig Wittgenstein