Carl Gustav Hempel

Carl Gustav Hempel, a leading member of logical positivism, was born in Orianenburg, Germany, in 1905.

Carl G. Hempel studied at the Realgymnasium at Berlin and, in 1923, he was admitted at the University of Gottingen where he studied mathematics with David Hilbert and Edmund Landau and symbolic logic with Heinrich Behmann. Hempel was very impressed with Hilbert’s program of proving the consistency of mathematics by means of elementary methods; he also studied philosophy, but he found mathematical logic more interesting than traditional logic. The same year he moved to the University of Heidelberg, where he studied mathematics, physics and philosophy.

In 1924, Hempel started to study at Berlin, where he met Hans Reichenbach who introduced him to the Berlin Circle.

Hempel attended Hans Reichenbach’s courses on mathematical logic, the philosophy of space and time, the theory of probability. He studied physics with Max Planck and logic with John von Neumann.

In 1929, he took part in the first congress on scientific philosophy organized by logical positivists. He met Rudolf Carnap and was very impressed.

He moved to Vienna where he attended three courses with Rudolf Carnap, Moritz Schlick and Friedrich Waismann, and took part to the meetings of the Vienna Circle.

In the same years Hempel qualified as teacher in the secondary school and eventually, in 1934, he gained the doctorate in philosophy at Berlin, with a dissertation on the theory of probability. After then, he emigrated to Belgium, with the help of a friend of Hans Reichenbach, Paul Oppenheim.

Two years later Hempel and Paul Oppenheim published the book Der Typusbegriff im Lichte der neuen Logik on the logical theory of classifier, comparative and metric scientific concepts.

In 1937 Hempel was invited to the University of Chicago as Research Associate in Philosophy.

After an another brief period in Belgium, Hempel emigrated to the United States in 1939. He taught in New York, at the City College (1939-1940) and at the Queens College (1940-1948).

In those years he was interested in the theory of confirmation and explanation, and published several articles on that subject:

– A purely syntactical definition of confirmation, The Journal of Symbolic Logic, 8, 1943;
– Studies in the logic of confirmation, Mind, 54, 1945;
– A definition of Degree of confirmation, (with P. Oppenheim), Philosophy of science, 12, 1945;
– A note on the paradoxes of confirmation, in Mind, 55, 1946;
– Studies in the logic of explanation (with P. Oppenheim), Philosophy of science, 15, 1948.

Between 1948 and 1955, Hempel taught at Yale University. His work ‘Fundamentals of concept formation in empirical science’ was published in 1952 in the International Encyclopedia of Unified Science.

In 1955 he started to teach at the University of Princeton. Aspects of scientific explanation and Philosophy of natural science were published in 1965 and 1966 respectively.

After the pensionable age he continued in teaching at Berkley, Irvine, Jerusalem and, from 1976 to 1985, at Pittsburgh.

In the meantime, his philosophical perspective was changing and he detached from logical positivism. However, he remained affectionately joined to logical positivism: in 1975 he undertook the editorship (with W. Stegmüller and W.K. Essler) of the new series of the journal Erkenntnis.

In ‘The meaning of theoretical terms’, 1973, Hempel criticizes an aspect of logical positivism’s theory of science: the distinction between observational and theoretical terms and the related problem about the meaning of theoretical terms.

According to Hempel, there is an implicit assumption in neopositivist analysis of science, that is the meaning of theoretical terms can be explained by means of linguistic methods. Therefore the very problem is how can be determined a set of statements that gives a meaning to theoretical terms. Hempel analyzes the various theories proposed by logical positivism.

According to Schlick, the meaning of theoretical concepts is determined by the axioms of the theory; that axioms thus play the role of implicit definitions. Therefore theoretical terms must be interpreted in a way that makes the theory true. But according to such interpretation – Hempel objects – a scientific theory is always true, for it is true by convention, and thus every scientific theory is a priori true. This proves – Hempel says – that Schlick’s interpretation of the meaning of theoretical terms is not tenable. Also the thesis which asserts that the meaning of a theoretical term depends on the theory in which that term is used is, according to Hempel, untenable.

Another solution to the problem of the meaning of theoretical terms is based on the rules of correspondence (also known as meaning postulates). They are statements in which observational and theoretical terms occur. Theoretical terms thus gain a partial interpretation by means of observational terms. Hempel raises two objections to this theory. First of all, he asserts that observational concepts do not exist. When a scientific theory introduces new theoretical terms, they are linked with other old theoretical terms that usually belong to another already consolidated scientific theory. Therefore the interpretation of new theoretical terms is not based on observational terms but it is given by other theoretical terms that, in a sense, are more familiar than the new ones.

The second objection is about the conventional nature of rules of correspondence. A meaning postulate defines the meaning of a concept and therefore, from a logical point of view, it must be true. But every statement in a scientific theory is falsifiable, and thus there is not any scientific statement which is beyond the jurisdiction of the experience. So also a meaning postulate can be false; hence it is not conventional and thus it does not define the meaning of a concept but it is a genuine physical hypothesis. So meaning postulate do not exist.

‘Provisoes: a problem concerning the inferential function of scientific theories’ published in Erkenntnis, 1988, criticizes another aspect of logical positivism’s theory of science: the deductive nature of scientific theories. It is very interesting that a philosopher who is famous for his deductive model of scientific explanation moved a criticism to the deductive model of science. At least this fact shows the open views of Hempel. He argues that it is impossible to derive observational statements from a scientific theory. For example, Newton’s theory of gravitation cannot determine the position of planets, even if the initial conditions are known, for Newton’s theory deals with the gravitational force, and thus the theory cannot forecast the influences exerted by other kinds of force. In other words, Newton’s theory requires an explicit assumption – a provisoe, according to Hempel – which assures that the planets are subjected only to the gravitational force. Without such hypothesis it is impossible to apply the theory to the study of planetary motion. But this assumption does not belong to the theory. Therefore the position of planets is not determined by the theory, but it is implied by the theory plus appropriate assumptions. Accordingly, not only observational statements are not entailed by the theory, but also there are no deductive links between observational statements. Hence it is impossible that an observational statement is a logical consequence of a theory (unless the statement is logically true). This fact has very important outcomes.

One of them is that the empirical content of a theory does not exist. Neopositivists defined it as the class of observational statements implied by the theory; but this class is an empty set.

Another consequence is that theoretical terms are not removable from a scientific theory. Known methods employed to accomplish this task assert that, for every theory T, it is possible to find a theory T* without theoretical terms so that an observational statement O is a consequence of T* if and only if it is a consequence of T. Thus it is possible to eliminate theoretical terms from T without loss of deductive power. But — Hempel argues — no observational statement O is derivable from T, so that T* lacks empirical consequence.

Suppose T is a falsifiable theory; therefore there is an observational statement O so that ~O ~T. Hence T ~O; so T entails an observational statement ~O. But no observational statement is a consequence of T. Thus the theory T is not falsifiable. The consequence is that every theory is not falsifiable. (Note: Hempel’s argument is evidently wrong, for according to Karl Popper the negation of an observational statement usually is not an observational statement).

Finally, the interpretation of science due to instrumentalism is not tenable. According to such interpretation, scientific theories are rules of inference, that is they are prescriptions according to which observational statements are derived. Hempel’s analysis shows that these alleged rules of inference are indeed void.

Carl Gustav Hempel died November 9, 1997, in Princeton Township, New Jersey.

Major Works of Carl Gustav Hempel

– The function of general laws in hystory, The journal of philosophy, 39, 1942
– A purely syntactical definition of confirmation, The journal of symbolic logic, 8, 1943
– Studies in the logic of confirmation, Mind, 54, 1945
– A definition of Degree of confirmation, (with Paul Oppenheim), Philosophy of science, 12, 1945
– Studies in the logic of explanation, (with Paul Oppenheim), Philosophy of science, 15, 1945
– Fundamentals of concept formation in empirical science, University of Chicago Press, 1945
– The theoretician’s dilemma, in Minnesota studies in the philosophy of science, II (edit by H. Feigl, M. Scriven, G. Maxwell) University of Minnesota Press, 1958
– Deductive-nomological vs. statistical explanation, Minnesota studies in the philosophy of science, III (edit by H. Feigl, G. Maxwell) University of Minnesota Press, 1962
– Aspects of scientific explanation, and other essays in the philosophy of science, New York: Free Press, 1965
– Philosophy of natural science, Englewood Cliffs, N.J.: Prentice-Hall, 1966
– The meaning of theoretical term: a critique to the standard empiricist construal, Logic, methodology and philosophy of science IV, North Holland Publishing Company, 1973
– Turns in the evolution of the problem of induction, Synthese, 46, 1981
– Valutation and objectivity in science, Physics, philosophy and psychoanalysis (ed. by R.S. Cohen and L. Laudan), Dordrecth, Holland: D. Reidel Pub. Co., 1983
– Thoughts on the limitation of discovery by computer, Logic of discovery and diagnosis in medicine (edited by Kenneth F. Schaffner), University of California Press, 1985
– Provisoes: a problem concerning the inferential function of scientific theories, Erkenntnis, 28, 1988