Hans Reichenbach (September 26, 1891 – April 9, 1953) was a leading philosopher of science, educator, and proponent of logical positivism, or logical empiricism, as he preferred calling it. Reichenbach made important contributions to the philosophy of space and time, the study of geometry of space, the study of the theory of motion, the philosophical meaning of the theory of relativity, symbolic logic, a mathematical formulation of quantum mechanics, meaning and verifiability, and science and philosophy.
Reichenbach is perhaps best known for founding the Berlin Circle, a companion group to the Vienna Circle, and devoted to approximately the same interests (although Reichenbach's view of verification was significantly different from that of the members of the Vienna Circle), and for his logical empiricism.
After completing secondary school in Hamburg, Reichenbach studied civil engineering at the Technische Hochschule in Stuttgart, and physics, mathematics, and philosophy at various universities, including Berlin, Erlangen, Göttingen, and Munich. Among his teachers were Ernst Cassirer, David Hilbert, Max Planck, Max Born, and Arnold Sommerfeld. Reichenbach was active in youth movements and student organizations, and published articles about university reform, the freedom of research, and against anti-Semitic infiltrations in student organizations.
Reichenbach received a degree in philosophy from the University of Erlangen in 1915; his dissertation on the theory of probability, supervised by Paul Hensel and Emmy Noether, was published in 1916. Reichenbach served during World War I on the Russian front, in the German army radio troops. In 1917, he was removed from active duty, due to an illness, and returned in Berlin. While working as a physicist and engineer, Reichenbach attended Albert Einstein's lectures on the theory of relativity in Berlin from 1917 to 1920.
In 1920, Reichenbach began teaching at the Technische Hochschule at Stuttgart as Privatdozent. In the same year, he published his first book on the philosophical implications of the theory of relativity, The Theory of Relativity and A Priori Knowledge, which criticized the Kantian notion of a synthetic a priori. He subsequently published Axiomatization of the Theory of Relativity (1924), From Copernicus to Einstein (1927), and The Philosophy of Space and Time (1928), the last stating the logical positivist view (Reichenbach preferred the term "logical empiricist" because of his differences with the positivists about verification and verifiability) on the theory of relativity.
In 1926, with the help of Albert Einstein, Max Planck, and Max von Laue, Reichenbach became assistant professor in the physics department of Berlin University.
He gained notice for his methods of teaching. Specifically, he was easily approached by students and his courses were open to discussion and debate. This was highly unusual at the time, although the practice is a common one presently.
In 1928, Reichenbach founded the Berlin Circle (In German: Die Gesellschaft für empirische Philosophie; in English: "Society for Empirical Philosophy"). Among its members were Carl Gustav Hempel, Richard von Mises, David Hilbert, and Kurt Grelling. In 1930 he and Rudolf Carnap began editing the journal Erkenntnis ("Knowledge"), which can be thought of as the house journal for the logical positivists and logical empiricists.
In 1933, when Adolf Hitler became Chancellor of Germany, Reichenbach emigrated to Turkey, where he headed the Department of Philosophy at the University of Istanbul. He introduced interdisciplinary seminars and courses on scientific subjects, and in 1935, he published The Theory of Probability.
In 1938, with the help of Charles Morris, he moved to the United States to take up a professorship at the University of California, Los Angeles. His work on the philosophical foundations of quantum mechanics was published in 1944, followed by Elements of Symbolic Logic and The Rise of Scientific Philosophy. Hilary Putnam may have been his most prominent student. He helped establish UCLA as a leading philosophy department in the U.S. in the post-war period.
Reichenbach died on April 9, 1953, in Los Angeles, while working on problems in the philosophy of time and on the nature of scientific laws. This work resulted in two books published posthumously: The Direction of Time and Nomological Statements and Admissible Operations.
In theory of knowledge, Reichenbach accepted the verifiability principle, but rejected a truth theory of meaning, as had been advocated by members of the Vienna Circle, and advocated instead a probability theory of meaning. This means that a proposition is meaningful if it is possible to determine a probability for it.
In probability and induction, Reichenbach held to a frequency interpretation, by which the probability of an event is the limit of the relative frequency of that event within an infinite series.
In logic, Reichenbach proposed that the conventional two-valued logic of truth and falsity should be replaced with with a probability logic that would have a range of values including all real numbers from "0" to "1," not simply the values of "true" and "false" of conventional logic.
Concerning space and time, Reichenbach explored the implications of changing the parallel postulate of Euclid (given a line and a point outside that line, exactly one parallel can be drawn through that point) to a postulate that no parallel can be drawn to a line through a point outside that line, or a postulate that an infinite number of parallels can be drawn through that point. Each of those produces a new and internally consistent geometry. He thus distinguished between mathematical geometry—a purely mathematical-logical construction— and physical geometry, which deals with the physical world. Moreover, Reichenbach held, it is possible to determine which of those applies to the actual world through experimental observation. He used an example of two dimensional being living on the surface of a sphere. They could draw a circle on their sphere and draw a diameter of that circle and then measure both. If the ratio of circle to diameter exactly equals Π (pi), then they are living in a Euclidian world. If it is less than Π the surface is a sphere. If it were greater than П they would be living in a world of hyperbolic geometry. If one assumes normal causality, then topology becomes an empirical theory, not an a priori one. Thus, Reichenbach claimed, he had refuted Kant's notion that Euclidian geometry is synthetic a priori.
In his work on time, Reichenbach distinguished between the order and direction of time. Temporal events can be ordered similar to points on a line, but this does not necessarily show the direction of time. The laws of mechanics specify or define an order of events—event A causes B which in turn causes C, and so on—but not a temporal direction, so, from the laws of temporal ordering, it would be perfectly possible for that order to be reversed, or for time to "run backwards," so to speak. But the actual direction of time is discovered through irreversible processes of nature, so, again, theoretical developments yield to empirical determinations.
In philosophy of science, Reichenbach distinguished between the "context of discovery" and the "context of justification." He claimed that the context of discovery is psychological and has no importance for philosophy, but the context of justification provides for a rational reconstruction of the hypothesis or law, so that an investigator's knowledge of it increases as the justification of it moves forward. He also distinguished between conventionalism and empiricism, and opted for empiricism as the correct approach to science.
In his work on quantum theory, Reichenbach pointed out that there is no interpretation of quantum mechanics that does not have causal anomalies, meaning that the principle of local action is refuted and action seems to take place over a distance. The only two theories that do not have this problem are the Bohr-Heisenberg formulation, based on the principle of indeterminacy, in which there is no way to measure both the position and the momentum of elementary particles. Heisenberg thought that this interpretation was faulty. The other possibility is the adoption of a three-valued logic, instead of the usual two-valued, true or false one; that three-valued logic would have three truth values: True, false, and indeterminate. He also worked out a mathematical formulation and axiomatization of quantum mechanics.
In 1938, the year he moved to the United States and became a professor at the University of California at Los Angeles, Reichenbach's Experience and Prediction was published. His work on quantum mechanics, Philosophic Foundations of Quantum Mechanics, was published in 1944. He then wrote two popular books: Elements of Symbolic Logic (1947) and The Rise of Scientific Philosophy (1951)—the latter of those strongly attacked and attempted to discredit pre-scientific philosophy. In 1949, Reichenbach contributed an essay on "The Philosophical Significance of the Theory of Relativity" to the book Albert Einstein: Philosopher-Scientist, part of the Library of Living Philosophers series, edited by Paul Arthur Schilpp. When Reichenbach died he was working on the philosophy of time, and two books that were products of that study, Nomological Statements and Admissible Operations (1954) and The Direction of Time (1956) were published posthumously.
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