Logical Positivism

(Also known as logical empiricism, logical neopositivism, neopositivism). A school of philosophy which arose in Austria and Germany during 1920s, primarily concerned with the logical analysis of scientific knowledge. Among its members were Moritz Schlick, founder of the Vienna Circle, Rudolf Carnap, the leading figure of logical positivism, Hans Reichenbach, founder of the Berlin Circle, Herbert Feigl, Philipp Frank, Kurt Grelling, Hans Hahn, Carl Gustav Hempel, Victor Kraft, Otto Neurath, Friedrich Waismann.

Logical positivists denied the soundness of metaphysics and traditional philosophy; they asserted that many philosophical problems are indeed meaningless. During 1930s the most important representatives of logical positivism emigrated to USA, where they influenced American philosophy. Until 1950s logical positivism was the leading philosophy of science; today its influence persists especially in the way of doing philosophy, in the great attention given to the analysis of scientific thought and in the definitely acquired results of the technical researches on formal logic and the theory of probability.


According to logical positivism, there are only two sources of knowledge: logical reasoning and empirical experience. The former is analytic a priori, while the latter is synthetic a posteriori; hence synthetic a priori does not exist.

The fundamental thesis of modern empiricism [i.e. logical positivism] consists in denying the possibility of synthetic a priori knowledge.
(H. Hahn, O. Neurath, R. Carnap, Wissenschaftliche Weltauffassung. Der Wiener Kreis, 1929).
Logical knowledge includes mathematics, which is reducible to formal logic. Empirical knowledge includes physics, biology, psychology, etc. Experience is the only judge of scientific theories; however, logical positivists were aware that scientific knowledge does not exclusively rise from the experience: scientific theories are genuine hypotheses that go beyond the limits of finite human experience.
It is not possible to establish a logically durable building on verifications [a verification is an observational statement about immediate perception], for they are already vanished when the building begins. If they were, with respect to time, at the beginning of the knowledge, then they would be logically useless. On the contrary, there is a great difference when they are at the end of the process: with their help the test is performed... From a logical point of view, nothing depends on them: they are not premises but a firm end point.
(M. Schlick, '&Uumlber das Fundament der Erkenntnis', in Erkenntnis, 4, 1934).
A statement is meaningful if and only if it can be proved true or false, at least in principle, by means of the experience -- this assertion is called the verifiability principle [aka the "verifiability criterion of meanng"]. The meaning of a statement is its method of verification; that is we know the meaning of a statement if we know the conditions under which the statement is true or false.
When are we sure that the meaning of a question is clear? Obviously if and only if we are able to exactly describe the conditions in which it is possible to answer yes, or, respectively, the conditions in which it is necessary to answer with a no. The meaning of a question is thus defined only through the specification of those conditions...
The definition of the circumstances under which a statement is true is perfectly equivalent to the definition of its meaning.
... a statement has a meaning if and only if the fact that it is true makes a verifiable difference.
(M. Schlick, 'Positivismus und Realismus' in Erkenntnis, 3, 1932).
Metaphysical statements are not empirically verifiable and are thus forbidden: they are meaningless. The only role of philosophy is the clarification of the meaning of statements and their logical interrelationships.  There is no distinct "philosophical knowledge" over and above the analytic knowledge provided by the formal disciplines of logic and mathematics and the empirical knowledge provided by the sciences.
Philosophy is the activity by means of which the meaning of statements is clarified and defined.
(M. Schlick, 'Die Wende der Philosophie' in Erkenntnis, 1, 1930).
A scientific theory is an axiomatic system that obtains an empirical interpretation through appropriate statements called rules of correspondence, which establish a correlation between real objects (or real processes) and the abstract concepts of the theory. The language of a theory includes two kinds of terms: observational and theoretical. The statements of a theory are divided in two groups: analytic and synthetic. Observational terms denote objects or properties that can be directly observed or measured, while theoretical terms denote objects or properties we cannot observe or measure but we can only infer from direct observations. Analytic statements are a priori and their truth is based on the rules of the language; on the contrary, synthetic statements depend on experience, and their truth can be acknowledged only by means of the experience. This conception about the structure of scientific theories is perhaps the most durable philosophical principle of the logical positivism.
Its main points are:
  • the distinction between observational and theoretical terms
  • the distinction between synthetic and analytic statements
  • the distinction between theoretical axioms and rules of correspondence
  • the deductive nature of scientific theories
  • These four points are linked together. Rules of correspondence give an empirical meaning to theoretical terms and are analytic, while theoretical axioms express the observational portion of the theory and are synthetic. A theory must be a deductive system; otherwise, a formal distinction between the various kinds of sentences and terms is impossible.

    The distinction between observational and theoretical terms depends on the verifiability criterion of meaning. A statement is meaningful only if it is verifiable; but, in scientific theories, there are many statements which are not verifiable -- for example, assertions dealing with quantum particles or relativistic gravitational fields. These statements are too "theoretical" for a direct test; strictly speaking, they are meaningless.

    To avoid such a consequence, one could either deny that these were statements, or one could try to "reduce" the "theoretical terms" to "observational terms." The theoretical terms which belong to the abstract language of a scientific theory are explicitly definable in a restricted language whose terms describe only that which is directly observable.  So a distinction between observational and theoretical terms arose. But soon Carnap realized that theoretical terms are not definable by observational ones. In a first time, he proposed a partial reducibility of theoretical to observational terms ('Testability and meaning', in Philosophy of science, 3, 1936 and 4, 1937). Later, it was supposed that all theoretical terms were removable from a scientific theory. This hypothesis was supported by two outcomes of formal logic: Craig's theorem and Ramsey statements.

    Craig's theorem is an unquestionable result of formal logic. According to this theorem, it is possible to translate a scientific theory in a purely observational language without any loss of deductive power.  Ramsey sentences, named after English philosopher Frank Plumpton Ramsey (1903-1930), were used by Carnap for dividing the axioms of a theory in two sets, say A and R.  Set R contains only statements which contain purely observational terms and expresses the empirical portion of the theory, the "observational data."  Set A consists of analytic statements and defines the meaning of theoretical terms.  Given a typical scientific theory T containing both observational and theoretical terms, it is thus possible to rationally reconstruct that theory as theory T* which contains no theoretical terms,  such that T and T* are equivalent with respect to all observational statements that can be deduced from the avxioms of T*.

    [While the analysis of relationships between the two kinds of terms began the object of many logical and philosophical studies, the distinction itself was criticized. According to Popper all scientific concepts are theoretical, for every assertion not only entails hypotheses but also is hypothetical, that is not sure and always falsifiable. Quine ('Two dogmas of empiricism' in The Philosophical Review, 60, 1951) criticized both observational-theoretical and analytic-synthetic distinction. Hempel ('The theoretician's dilemma' in Minnesota studies in the philosophy of science, II, 1958) noted that the theory T* without theoretical terms, in spite of the equivalence (with respect to the observational language) to the original theory T, is not useful as T. In fact, from an inductive point of view, T and T* are very different. Usually the original theory T suggests certain relations between its concepts, while in T* these concepts are forbidden. The discovery of laws is almost impossible in T*, while it is a natural consequence in T. Moreover, while the number of the axioms of T usually is finite, Craig's theorem does not assure us of the existence of a theory T* with a finite number of axioms. So T* is almost useless. Theoretical terms are thus necessary in science.]

    In Philosophical foundations of physics, 1966, Carnap proposed a slightly different approach to observational-theoretical distinction. Now the starting-point is the difference between empirical and theoretical laws. It is possible to directly confirm (or disprove) an empirical law, while a theoretical law can be tested only through the empirical laws that are among its consequences. Moreover, an empirical law explains facts while a theoretical law explains empirical laws. Thus there are three levels:

        a) Empirical facts: these are expressed by direct "observation reports"
        b) Empirical laws: Simple generalizations we can directly confirm by observation. They explain facts and are employed to predict empirical  facts by deducing observation statements from laws and statements of initial conditions..
        c) Theoretical laws: General principles we can use to explain empirical laws:al laws by deducing the empirical laws from such theoretical statements.
    Empirical laws include only observational terms, while theoretical terms occur in theoretical laws.

        The distinction between analytic and synthetic statements is another consequence of the verifiability principle and it is linked with the observational-theoretical as well as axioms-rules of correspondence distinction. According to the verifiability principle, an alleged synthetic a priori statement does not have a meaning; thus there are only two kinds of assertions: synthetic a posteriori and analytic a priori. What is the role of analytic sentences in a scientific theory? Only two possibilities are allowed: an analytic statement is a logical-mathematical theorem (thus it has no empirical significance) or it is a convention that defines the meaning of theoretical terms.

        There is an explicit assumption in logical positivism's analysis of science: a theory is a deductive system. This means that pragmatic aspects are not considered. Moreover, neopositivism was not interested in the real process of discovering, but it was concerned with the rational reconstruction of scientific knowledge, that is it dealt with logical (formal) relationships between statements in a given theory.

        According to logical positivism, there is not any method of discovering a hypothesis prior to its test by deducing empirical consequences, and therefore a scientist can propose any hypothesis he prefers; only logical relationships between the hypothesis and the given empirical evidence are relevant. But there were some problems with this conception of science. First of all, the relation between empirical experience and theoretical principles is not a deductive one: observational statements do not imply theoretical axioms. Carnap argues that the relation is explicable with the help of the inductive logic.

    Subsequent History of Logical Positivism:

        The spread of logical positivism in USA came in the early 1930s. In 1929 and in 1932 Schlick was Visiting Professor at Stanford, while Feigl emigrated to USA in 1930, where he became lecturer (1931) and professor (1933) at the University of Iowa and afterwards at the University of Minnesota (1940). In 1932 the American Philosophical Association organized a discussion on the philosophy of logical positivism. In the same years several articles about logical positivism were published in American philosophical journals.  In 1936 Schlick was murdered by a Nazi student at the University of Vienna. Between 1936 and 1940 several German and Austrian philosophers emigrated to USA: Carnap moved in 1936 to the University of Chicago, Reichenbach in 1938 to UCLA, Frank in 1938 (he became professor at Harvard University in 1939), Hempel in 1939 (City College of New York and in 1940 Queens College). Logical positivists found a favourable terrain in USA. They established solid relationships with American pragmatism; particularly Charles Morris took part to several neopositivist's projects. One of them was the International Encyclopedia of Unified Science, primarily promoted by Neurath. Although the original project was never fully realized, many works were indeed published.

    The English philosopher Alfred Jules Ayer (1910-1989) played an important role in spreading logical positivism. His work Language, Truth and Logic, 1936, gained an immediate success. In that book, Ayer completely accepted both the verifiability principle and the distinction between analytic and synthetic statements; hence he asserted that metaphysical sentences are meaningless. A direct influence was exerted by Waismann and Neurath who emigrated in England in 1937 and 1940 respectively.