CARL G. HEMPEL

HANS REICHENBACH REMEMBERED

Hans Reichenbach’s far-ranging and influential contributions to episte- mology and the philosophy of science will be acknowledged and ap- praised in many contexts on the occasion of the one-hundredth anniver- sary of his birth. The following pages, however, are simply meant to record some personal recollections of Reichenbach as he affected one of his many students.

Reichenbach came to the University of Berlin in 1926, over strong philosophical opposition, but with stronger scientific support, especially by Einstein. I had by then been a student there for a year, after previous studies in Gettingen and Heidelberg. I promptly enrolled in a course with Reichenbach and was startled by his ideas on causality and deter- minism, which contrasted sharply with what I had been taught earlier, especially in courses on Kant’s Critique of Pure Reason. Early on, I told him that I considered the principle of causality as true a priori and thus found it impossible even to imagine that it might be false. With a characteristic smile, and an aside on how limited one’s imagination could be, Reichenbach said that evidently I had swallowed Kant’s conception hook, line, and sinker; but not to worry: he was confident I would change my mind in light of the arguments he would be pre- senting. I did indeed change my mind on this subject and on many another; Reichenbach’s ways of exploring philosophical issues came to exert a strong influence on the development of my thinking and led me into the arms of empiricism.

Reichenbach tended to be quite sure of his ideas, and occasionally too readily dismissive of dissenting views, but this did not affect his refreshingly receptive and informal way with students. At the time, many of the outstanding professors at German universities were vir- tually unapproachable to their students, specially beginners. I recall once running after Max Planck to ask whether I might put a brief question to him about a point he had made in his lecture. He did not even turn around. “Ask my assistant” he said, vanishing into his office. That was not rudeness or arrogance; it simply wasn’t the job of a professor to bother with such questions.

Erkenntnis 35: 5-10, 1991. @ 1991 Carl G. Hempel. Printed in the Netherlands.

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It was quite different in Reichenbach‘s classes - and, happily, in those of some of my other academic teachers, among them Paul Bernays in Gottingen, Wolfgang Kijhler and Kurt Lewin in Berlin, and many members of the Vienna Circle.

Reichenbach had a felicitous way of giving his students a sense of being engaged in a joint effort to solve problems of importance and even of being potential contributors to progress in the field. I am quite sure that he did not always have an answer to the questions he encouraged his students to explore. One of the seminars I took with him was devoted to an attempt at formalizing Hilbert’s axiomatization of Euclidean geometry in the notation of Principiu Muthematicu. We encountered great difficulties with the axiom of completeness - Reich- enbach himself did not know whether it could be formalized with the contemplated means. We tried hard and eventually concluded, I think, that it was indeed impossible.

But Reichenbach’s interaction with his students was not restricted to the classroom. He took a strong interest in some of the personal prob- lems, the plans, hopes, and doubts, of his students, and occasionally he joined some of them on an excursion to the lakes and woods in the environment of Berlin. My friend and fellow student Olaf Helmer, who took several of Reichenbach’s courses, has spoken to me with much appreciation of this aspect of Reichenbach’s sympathy and concern for his students.

Reichenbach’s seminars sometimes adjourned to a nearby cafe, where broader issues of social and political concern were debated as well. The cafe also provided a welcome setting for ideological and often political conversations among Reichenbach and some of his colleagues at the Gesellschaft fur empirische Philosophie, including Walter Dubislav of the Technische Hochschule and Kurt Grelling, the discoverer of the paradox concerning the term ‘impredicable’, a gifted mathematician and philosopher who taught at a high school in Berlin. This kind and humane man, who was of Jewish ancestry, was to die later in the gas chambers of Auschwitz.

The Gesellschaft fur empirische Philosophie - in contrast to the Vienna Circle, which was a small closed discussion group of scholars - imposed no membership restrictions. Reichenbach, Dubislav, and Grelling were the leading figures. The group organized, each academic year, a series of lectures, open to the general public, which were held in the large, amphitheatrical auditorium of the Charite, the University’s

HANS REICHENBACH REMEMBERED 7

Medical School. The lectures covered a wide range of topics, reflecting a distinct openness to diverse issues and approaches. The speakers included traditional philosophers as well as members of the Vienna and the Polish groups of methodologists and logicians, among them Camap, Tarski, and Neurath. Some of the lectures dealt with psychoanalysis and other topics in psychiatry; the Berlin psychiatrist Alexander Herz- berg was among the more influential members of the Gesellschaft. But I recall also a lecture-performance of an academically quite unorthodox kind. It was given by a man offering to duplicate and explain the various feats of a psychic then much in the public eye, named Hanussen. The occasion attracted an audience much larger than even the Charite could accommodate. I recall Dubislav, who could be quite brusque, scurrying about and urging the persons sitting on the steps between the aisles to leave voluntarily in conformance with fire-safety regulations: or would they rather have him call in the police? The speaker performed and explained a number of feats, such as finding hidden objects and reading the thoughts of persons in the audience.

In a letter to a fellow-student, dated November 6, 1929, I wrote about a lecture that Carnap gave at the Gesellschaft fiir empirische Philosophie on questions concerning Wittgenstein’s logical atomism. The discussion lasted for four hours, the final two of them at a cafe, where the intensely involved participants - among them Reichenbach, Scholz, Bernays, Kurt Lewin, Grelling, Dubislav - became so agitated and noisy that they practically caused a public nuisance and made young couples at neighboring tables break off their tender exchanges.

The Gesellschaft ftir wissenschaftliche Philosophie in co-operation with its sister-organization, the Verein Ernst Mach in Vienna, spons- ored the publication of the periodical Erkenntnis, which succeeded the Annalen der Philosophic, with Carnap and Reichenbach as editors. The first issue, which bears no publication date, must have appeared in 1930. Reichenbach wrote the introduction and played a leading role in guiding the further development of the journal.

The first issue contained articles by Schlick, Carnap, Dubislav, and Reichenbach. They clearly reflected what Schlick, in the title of his contribution, called ‘Die Wende der Philosophie’. The next issue of volume 1 was devoted to a ‘Report on the first convention on the epistemology of the exact sciences’; subsequent numbers addressed philosophical questions concerning biology, and the first issue of volume 2 included an article by the astronomer Erwin F. Freundlich on the

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question of the finitude of the universe, treated as an astronomical problem. All this bespoke a keen concern to avoid very general philo- sophical discussions, always seen as threatening a slide into ‘meaning- lessness’, in favor of problems concerning specific aspects of scientific inquiry.

In 1928, I became acquainted with Carnap’s Der Zogische Aufuu der Welt and his less technical writings criticizing metaphysics. I decided to study in Vienna for a term 1 a plan that was reinforced when I met Camap in person at the first Tagung fiir die Erkenntnislehre der exakten Wissenschuften in Prague, 1929, which was an important milestone in the development of logical empiricism. Reichenbach supported my idea, and his letter of introduction to S&lick produced an immediate invitation to join the discussions of the Vienna Circle, which, together with various courses I attended, had a lasting effect on my philosophical development.

It was not long after my return from Vienna, I think, that Reichen- bath introduced me to Paul Oppenheim, an acquaintance of his who was a chemist by training, but who had also published a book and some articles on logical facets of concept formation in science. He wanted a critical appraisal of his ideas and advice on ways to improve and expand his conceptions. That was the beginning of a collaboration that was to continue for many decades, dealing with a variety of topics beyond those that started it; it was also the beginning of a deep friendship. With the advent of National Socialism, Oppenheim moved to Brussels; at his invitation, I joined him there in 1934. This made it possible for me to leave Germany and, with Camap’s help, to go to the United States in 1937. I hate to think of what might have become of me if Reichenbach had not put me in touch with Oppenheim.

After my return from Vienna, I began work on my doctoral disserta- tion, which dealt with the statistical concept of probability. Reichenbach agreed to be my thesis adviser (‘Doktor-Vater’), and I began work with him. But in 1933 he was relieved of his position in Berlin because of his Jewish ancestry. He promptly accepted a professorship at the University of Istanbul, where he maintained his professional links to Erkenntnis and other enterprises by correspondence. He joined the faculty of the University of California in Los Angeles in 1938.

Reichenbach’s dismissal made it necessary for me to seek new spon- sors and examiners for my dissertation project; I am grateful to Wolf- gang Kbhler, with whom I had done course work, and to Nicolai

HANS REICHENBACH REMEMBERED 9

Hartmann, who did not know me at all, for having agreed to take on that role. I completed my dissertation, on problems of the frequentist conception of probability, on my own. But Reichenbach, who had introduced me to the subject, showed great interest in what I had written, and, although I had expressed reservations about details of his own view, he saw to it that a condensed version of my study was published in Erkenntnis in 1935. He added a response of his own, welcoming some of my ideas, but rejecting my critical doubts about his own conception of the subject.

The files of my correspondence with Reichenbach run from 1929 to 1951. They begin with an account I gave him in December of 1929 of my first impressions of Vienna and especially of the discussions in the Vienna Circle and of the courses I was taking with various philosophers at the University. Reichenbach had expressed a keen interest in these matters, and I wrote him at his request. Our subsequent correspondence ranged over many topics, among them editorial matters concerning Erkenntnis; requests for me to write reviews and to do extensive proof- reading - the latter also for some of his books; discussions of each other’s manuscripts, information about mutual friends - among them Grelling, for whom Reichenbach wrote letters supporting his efforts to be admitted to the United States. In a letter written in December 1933, when Reichenbach was already in Istanbul, I reported to him about a meeting of the Gesellschaft fur empirische Philosophie at which Dubis- lav had lectured on laws of nature: the meeting, I said, was less well attended than on earlier occasions, but the level of the discussion was still good. I also gave him accounts of disturbing events set in motion by National Socialism: the eminent mathematician Ludwig Bieberbach strutting about in a Nazi ‘uniform, greeting his classes with the Hitler salute, and talking of the racial facets of mathematics.

Initially, the language of our correspondence was German, of course; in the mid-forties we changed to English. Reichenbach’s first English letter informed me that he had recommended me for a vacancy at Vanderbilt University. Nothing came of the recommendation, but I had by then obtained a satisfying position at Queens College in New York.

In 1951, when I spent some time in Los Angeles, Reichenbach im- mediately offered to put me up at his house; when this proved impracti- cable for me, he and his wife extended warm hospitality to me during a short visit.

Reichenbach’s letters to me were predominantly concerned with phi-

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losophical and editorial issues. This was so even when he had gone to Istanbul, which meant a radical change in his life. He did, however, make a few incidental remarks about his daily life and his teaching experiences. In November 1933, for example, he writes that he has to keep his lectures rather elementary, but hopes to be able gradually to raise the level of his courses. The interest of his students, he says, is touching, so that his work gives him much pleasure. He remarks on the surprises awaiting him and his wife on visits to mosques and bazaars. Once, they had just been to a bazaar and asked for a camel hair blanket for their bed whereupon the merchant brought them a heavy cover of the kind put on a camel’s back for riding on it.

In subsequent letters, he mentions having a whole row of assistants with whom he can alternately speak German, French, and English, but whose philosophical training is rather weak. He also tells of lecturing on the history of philosophy, dealing with the thinkers in question more from a psychological and sociological perspective, which he finds quite an interesting mode of approach. Additionally, he refers to a collo- quium held by a small circle of scholars speaking German. But, he adds, that group is only a weak substitute for the circle in Berlin.

In all, I remember Hans Reichenbach as a powerfully creative philo- sophical and scientific thinker and writer, as a dedicated and enlight- ened educator, and as a man deeply concerned with the human con- dition and dedicated to its enhancement. He was a Mensch.

Department of Philosophy Princeton University Princeton, NJ 08.544 U.S.A.