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Summary Article: Reichenbach, Hans (1891–1953)
from Blackwell Companions to Philosophy: A Companion to Metaphysics

Known chiefly as a philosopher of science, he was one of the two principal founders of logical EMPIRICISM (See logical positivism). Unlike CARNAP, the other principal founder, Reichenbach was never a member of the Vienna Circle and never in any strict sense a logical positivist; indeed, he considered his major epistemological treatise (1938) a refutation of LOGICAL POSITIVISM. After a short period as a Kantian early in his career, he became a dedicated empiricist and thereafter emphatically rejected the possibility of synthetic a priori knowledge or any form of speculative metaphysics (See kantianism).

Among the issues involved in Reichen-bach's rejection of logical positivism, two are especially relevant in the present context: (1) he adopted a physicalistic (as opposed to phenomenalistic) basis for common-sense and scientific knowledge (See phenomenalism; physicalism, materialism); and (2) he maintained that the existence of such un-observable entities as atoms could be established empirically; that is, he affirmed scientific REALISM (as opposed to instrumentalism). As these considerations show, he advocated views that would today be called metaphysical, but his metaphysics was thoroughly scientific.

Throughout his career, Reichenbach was deeply concerned with philosophical problems of SPACE AND TIME. A devoted student of Einstein, he argued in his first book on the subject (1920) that the theory of relativity is logically inconsistent with KANT'S total set of synthetic a priori propositions about space, time, and causality. Because relativity theory is scientifically well-founded, at least some of Kant's synthetic a priori principles must be relinquished.

Reichenbach (1928) is the classic work on philosophy of space and time in the first half of the twentieth century. A fundamental problem in this area is the ascertainment of the geometrical structure of physical space. For this purpose, he maintains, we must use some sort of measuring instrument – for example, solid measuring rods – but we face an immediate problem because we cannot ascertain empirically whether such rods retain the same length as they are moved from one place to another. To deal with this situation we introduce coordinative definitions to establish a relationship between physical entities (measuring rods) and an abstract geometrical relation (congruence). In the absence of such a stipulation we cannot determine whether physical space is Euclidean or non-Euclidean, whether it is curved or not. Alternatively, he maintains, one could choose a geometry – say Euclidean – by convention, in which case the behavior of the measuring instruments would become a matter of empirical fact. Either way, ascertainment of the geometric structure of physical space involves one matter of convention and one matter of empirical fact.

A central problem about the nature of time concerns the relation of simultaneity within any inertial reference frame. If we wish to synchronize two clocks, at rest with respect to one another but not in spatial proximity, we can send messages back and forth between them. Since, according to relativity theory, no signal or causal influence can travel faster than light, Reichenbach argues, there is a degree of conventionality in the simultaneity relation. This conventionality is distinct from, and logically prior to, Einstein's celebrated relativity of simultaneity.

At the time of his death, Reichenbach was working on The Direction of Time (1956, published posthumously), which he had completed except for a final chapter. In the completed chapters he argued that, although the fundamental laws of nature (See law of nature) are time symmetric (the violation of time symmetry in elementary particle physics had not been discovered, and does not seriously undermine his main arguments), there is objective temporal asymmetry in the world. It is based on de facto conditions rather than nomological necessities. He also argues for the objectivity of temporal becoming.

After giving a microphysical analysis of the direction of time in terms of entropy and the Second Law of Thermodynamics, he extends the argument to the macrophysical level, and offers an analysis of temporal asymmetry on the basis of causal considerations. He enunciates the principle of the common cause and maintains that improbable coincidences can be explained in terms of common causes and not by reference to common effects.

In all of his treatments of space and time, Reichenbach maintained a causal theory -i.e., that all spatial and all temporal relations are grounded in causal relations. In The Direction of Time he defined a number of causal concepts statistically, and, in so doing, laid the foundations for a theory of probabilistic causality. This topic is under active investigation by a large number of philosophers at present.

  • The Direction of Time (University of California Press Berkeley and Los Angeles, 1956).
  • Experience and Prediction (University of Chicago Press Chicago, 1938).
  • Philosophie der Raum-Zeit-Lehre (Berlin and Leipzig, 1928); trans. M. Reichenbach; J. Freund, The Philosophy of Space and Time (Dover New York, 1958).
  • Relativatätstheorie und Erkenntnis Apriori (Berlin, 1920); trans. Reichenbach, M. , The Theory of Relativity and A Priori Knowledge (University of California Press Berkeley and Los Angeles, 1969).
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