Hans Reichenbach

Life and Intellectual Context

Hans Reichenbach (1891–1953) was a central figure in twentieth‑century philosophy of science and one of the leading representatives of logical empiricism. Trained as both an engineer and a physicist before turning fully to philosophy, he became known for methodologically rigorous and scientifically informed work on relativity theory, quantum mechanics, and the foundations of probability.

Reichenbach studied in Stuttgart, Munich, and Berlin, attending lectures by figures such as Max Planck, David Hilbert, and Ernst Cassirer. His early exposure to both mathematical physics and neo‑Kantian philosophy shaped his lifelong attempt to reconcile the success of modern physics with a revised account of a priori knowledge.

He completed a doctorate in 1915 on the theory of probability but was soon drawn into active service during the First World War. After the war, his academic career developed rapidly. In 1926 he became professor at the University of Berlin, where he established a seminar that soon evolved into the Berlin Circle, a key counterpart to the Vienna Circle in the emerging logical empiricist movement.

Reichenbach was of Jewish descent and politically liberal; with the rise of National Socialism he was dismissed from his position in 1933. He emigrated first to Turkey, accepting a post at the University of Istanbul, where he helped build a modern philosophy curriculum. In 1938 he moved to the United States, joining the University of California, Los Angeles (UCLA). There he taught until his death in 1953, becoming an important conduit through which logical empiricism influenced American philosophy.

Logical Empiricism and the Berlin Circle

Reichenbach’s mature work developed within the broader current of logical empiricism, but it displayed distinctive emphases and divergences from better‑known Vienna Circle figures such as Rudolf Carnap.

Like other logical empiricists, Reichenbach endorsed an anti‑metaphysical program: he held that meaningful statements must be, in principle, empirically testable or analytically true. However, he was skeptical of overly strict verificationist criteria. Instead, he stressed the role of probability and approximate confirmation, arguing that science deals with graded support rather than all‑or‑nothing verification.

The Berlin Circle, organized by Reichenbach in the mid‑1920s, gathered scientists and philosophers to discuss relativity, quantum theory, and the logic of science. While sharing the Vienna Circle’s commitment to scientific philosophy, it tended to be more empirically oriented and more directly engaged with physical theory. Reichenbach’s seminars and publications advanced a style of philosophy that combined technical knowledge of physics with logical analysis, a model that later became standard in analytic philosophy of science.

Reichenbach also introduced the influential distinction between the context of discovery and the context of justification. He maintained that philosophers should not attempt to prescribe or analyze the psychological processes by which scientists arrive at hypotheses (the context of discovery). Instead, philosophy of science should focus on the logical and methodological relations that justify accepting or rejecting hypotheses (the context of justification). This distinction shaped mid‑century debates about scientific method, though later historians and philosophers of science questioned its sharpness and adequacy.

Philosophy of Space, Time, and Relativity

A significant part of Reichenbach’s work concerned the foundations of space and time, especially in light of Einstein’s theories of relativity. His early books, including Relativitätstheorie und Erkenntnis a priori (1920) and Axiomatik der relativistischen Raum-Zeit-Lehre (1924), aimed to show how relativity undermines the traditional synthetic a priori of neo‑Kantian philosophy.

Reichenbach argued that many principles previously regarded as necessary truths about space and time—for example, the Euclidean structure of geometry or absolute simultaneity—should instead be understood as conventions or coordinative definitions. A coordinative definition is a rule that links mathematical concepts (such as the metric of a manifold) with empirical procedures (such as measuring rods and clocks). These rules are not empirical hypotheses themselves, but they make it possible to apply mathematical structures to the physical world. As empirical knowledge changes, so too may our preferred coordinative definitions.

In his influential English‑language work Philosophy of Space and Time (1958, posthumous, based on earlier German writings), Reichenbach distinguished between:

• Topological properties of space‑time (qualitative relations such as continuity and order),
• Projective properties (structure preserved under projection),
• Metric properties (quantitative measures of distance and duration).

He claimed that empirical evidence from relativity theory supports specific choices of metric and topology for physical space‑time, thereby replacing Kantian necessary truths with empirically grounded structures.

Reichenbach also developed a detailed analysis of simultaneity and clock synchronization. He formalized the idea that assigning a simultaneity relation depends on conventions about the speed and behavior of light signals. His ε‑definition of simultaneity showed that, within special relativity, there is some flexibility in how one defines simultaneity at a distance, so long as certain constraints are satisfied. This analysis influenced later conventionalist accounts of geometry and time, as well as discussions in philosophy of physics about the conventional versus factual components of physical theories.

Critics have debated how far Reichenbach’s conventionalism extends. Some contend that his approach risks blurring the line between empirical content and mere choice of description, while proponents argue that it clarifies the subtle interplay between theory, measurement, and geometry in modern physics.

Probability, Induction, and Causality

Reichenbach’s most enduring contributions lie in the foundations of probability and the analysis of induction and causality. His books Wahrscheinlichkeitstheorie (1935) and The Theory of Probability (English 1949) developed a distinctive frequency interpretation of probability. According to this view, probability statements refer to limiting relative frequencies within long (in principle, infinite) sequences of events. For example, to say that the probability of heads on a coin is 0.5 is to say that, in the long run, the relative frequency of heads in repeated tosses will converge to 0.5.

Reichenbach linked this interpretation to a pragmatic justification of induction. He introduced the idea of a “principle of the limiting frequency”, holding that if stable limiting frequencies exist in nature, then inductive methods based on observed frequencies will, in the long run, converge to the correct probabilities. If no such regularities exist, then no method could succeed, and induction is no worse off than any rival strategy. On this basis, he proposed a pragmatic vindication of induction: adopting inductive rules is rational because they are our best (and only) chance of aligning beliefs with stable patterns in the world, should those patterns exist.

This approach has been both influential and controversial. Supporters see it as a sophisticated alternative to traditional justifications of induction, avoiding circular appeals to past success. Critics argue that it only offers a conditional or “if‑then” vindication, not a genuine justification, and that it presupposes, rather than explains, why the world should exhibit the regularities needed for induction to work.

In his later work, especially The Direction of Time (1956, posthumous), Reichenbach extended his probabilistic analysis to causality and temporal asymmetry. He aimed to reconstruct the concept of cause in terms of probabilistic relations among events rather than deterministic laws. A cause, on this view, is something that changes the probability of its effect relative to background conditions, and causal structure can be inferred from patterns of statistical dependence and independence.

Reichenbach also offered a probabilistic account of the arrow of time. He suggested that the temporal direction from past to future is tied to the existence of branch systems—subsystems of the universe (such as clouds of gas or macroscopic bodies) that evolve from relatively low‑entropy, non‑equilibrium states into higher‑entropy, more disordered ones. On his view, the asymmetry of causal processes and of our knowledge (we remember the past, not the future) is linked to these thermodynamic and probabilistic features. Later work in philosophy of physics and probability theory has drawn on, refined, or challenged these ideas, particularly in the context of statistical mechanics and causal modeling.

Across these domains, Reichenbach’s program aimed to replace opaque, metaphysical notions of necessity and causation with empirically interpretable, mathematically precise concepts anchored in scientific practice. His combination of technical competence in physics and mathematics with philosophical analysis helped set the agenda for much subsequent work in analytic philosophy of science. While many details of his theories have been revised or rejected, his influence persists in contemporary debates over confirmation, interpretation of probability, the structure of space‑time, and the logic of causal inference.