Peter (= Carl Gustav) Hempel and I projected something like this volume some six years ago as a representative selection of his earliest and latest philosophical writings, a companion piece to the familiar collection, Aspects of Scientific Explanation, of writings earlier and later than those. We began by translating his 1934 Berlin doctoral dissertation – a piece which I finally decided to leave out. (It is available in the Hempel collection in the Hillman Library of the University of Pittsburgh.) Included in its stead are two articles (chapters 6 and 7) that are based on that dissertation, which was published in the 1930s. The first one, a translation from the German by Christoph Ehrlenkamp, has been revised by Peter's old friend and collaborator, Olaf Helmer. The only other paper we managed to translate was Peter's 1937 “Le problème de la vérité,” chapter 4 here. Other translators of essays here were Wilfred Sellars (chapter 9) and Christian Piller (chapters 10 and 14).

In this book Peter speaks for himself; I have kept editorial comment to a minimum. But I must give some account of his long and remarkable life. The following brief biography is adapted from Erkenntnis 47 (1997) 181–83, a nd it is reprinted here with permission.

CARL GUSTAV HEMPEL JANUARY 8, 1905–NOVEMBER 9, 1997

He lived in “interesting” times, which drove him out of Germany, first to Belgium and then to the United States – where he died sixty years later in Princeton, his adoptive home, much loved and full of honors.

INTRODUCTION

The following inquiry attempts to contribute to the logical analysis of the concept of probability. Philosophical efforts to clarify this concept are as old as its history, and the basic ideas of the two main groups of theories which seek to determine its meaning go back to the beginnings of this history. For a first orientation, these may be contrasted as the “aprioristic” and “empiricist” views of probability.

On the aprioristic views, probability theory rests on a priori judgments of proportions in empirical or logical space. On the empiricist views, probability theory concerns the relative frequencies with which certain results occur in statistical series of empirical observations.

At present, these two views are no longer on an equal footing. Instead, the progressive refinement of the logical analysis of scientific concept formations has made it increasingly clear that an adequate logico-descriptive theory of the probability concept can be found only in the direction in which the empiricist interpretation seeks it.

A consistent realization of the empiricist view, however, leads to certain logical difficulties, which have not been completely overcome even in the latest articulations of the empiricalstatistical probability theory. The present study examines these difficulties and develops a proposal for their removal on the basis of a “finitistic” turn of the statistical interpretation of the probability concept.

LOGICAL DIFFICULTIES OF THE STATISTICAL INTERPRETATION

The basic problem of the logical analysis of the probability concept – the problem in response to which the theory groups just mentioned differ from each other – is the question of the meaning of probability statements (i.e., of statements which attribute a certain probability to an event).

INTRODUCTORY REMARKS ON THE TWO EMPIRICIST GROUPS

The central ideas of logical, or scientific, empiricism as it developed during the twenties and early thirties in Vienna and in Berlin grew out of collaborative efforts of scientifically interested philosophers and philosophically interested scientists. Those thinkers noted that while the claims made by the physical sciences were amenable to objective test by experiment and observation, the pronouncements put forward by metaphysics were incapable of any such objective critical appraisal. And while hypotheses advanced in the physical sciences would eventually be accepted or rejected and thus lead to the growth of a body of objective scientific knowledge, the problems and pronouncements of metaphysics, inaccessible to objective appraisal, kept reappearing over and over again.

Moritz Schlick, the founding father of the Vienna Circle, addressed this situation in an article entitled “Die Wende der Philosophie,” which appeared in 1930 in the first issue of the empiricist periodical Erkenntnis. Rejecting “the anarchy of philo-sophical opinions,” he solemnly declared, “We stand in the middle of a definitive turn of philosophy and are justified in regarding the fruitless strife of philosophical opinions as finished.” The means for avoiding all such fruitless strife he saw in the clear and precise thinking exemplified by the exact sciences and informed by the principles of exact formal, or symbolic, logic.

Broadly speaking, the Vienna Circle held that the purported problems of metaphysics constitute no genuine problems at all and that in an inquiry making use of an appropriately precise conceptual and linguistic apparatus, metaphysical questions could not even be formulated.

1. 6 [5] “Über den Gehalt von Wahrscheinlichkeitsaussagen,” Erkenntnis 5 (1935), 228–60. Translation by Christoph Erlenkamp, revised by Olaf Helmer.

2. 7 [14] “On the Logical Form of Probability-Statements,” Erkenntnis 7 (1937), 154–60. [16] “Supplementary Remarks on the Form of Probability-Statements, Suggested by the Discussion. (Participants: Braithwaite, Clay, v. Dantzig, Ditchburn, Lindenbaum, Mannoury, Storer, Waismann.), Erkenntnis 7 (1937), 360–3.

3. 8 [24] With P. Oppenheim, “A Definition of ‘Degree of Confirmation,’” Philosophy of Science 12 (1945), 98–115.

Essay 6 is a translation of an article based on Hempel's Ph.D. dissertation. The essay argues for a view of probability as determined by empirical frequencies, while offering sympathetic but ultimately destructive criticism of the particular frequentist theories floated by Reichenbach and von Mises. The positive contribution of the article is a “finitization” of Reichenbach's approach.

The brief critical essay 7 also derives from the dissertation. It was read at the Fourth International Congress for the Unity of Science at Cambridge University in 1938. There, in a friendly spirit, Hempel identified an intractable difficulty that vitiates both von Mises's theory and Reichenbach's.

In the early 1940s, collaborating with Olaf Helmer and Paul Oppenheim, Hempel saw the project of logical empiricism as urgently needing a satisfactory account of how sentences can confirm or disconfirm other sentences. Unlike Carnap, the H2O trio sought an explication of degree of confirmation in partially empirical terms. The high-water mark in their hopes for the project (1945) is marked by essay 8 and by a parallel publication by Helmer and Oppenheim.

1. The Problem. The concept of confirmation of a hypothesis by empirical evidence is of fundamental importance in the methodology of empirical science. For, first of all, a sentence cannot even be considered as expressing an empirical hypothesis at all unless it is theoretically capable of confirmation or disconfirmation, that is, unless the kind of evidence can be characterized whose occurrence would confirm, or disconfirm, the sentence in question. And second, the acceptance or rejection of a sentence which does represent an empirical hypothesis is determined, in scientific procedure, by the degree to which it is confirmed by relevant evidence.

The preceding remarks, however, are meant only as accounts of methodological tendencies and are not intended to imply the existence of clear-cut criteria by means of which the scientist can decide whether – or, in quantitative terms, to what degree – a given hypothesis is confirmed by certain data. For indeed, no general and objective criteria of this kind are at present available; in other words, no general definition of the concept of confirmation has been developed so far. This is a remarkable fact in view of the importance of the concept concerned, and the question naturally suggests itself whether it is at all possible to set up adequate general criteria of confirmation, or whether it may not rather be necessary to leave the decision in matters of confirmation to the intuitive appraisal of the scientist.

One of the most important and most discussed problems of contemporary philosophy is that of determining how psychology should be characterized in the theory of science. This problem, overflowing the limits of epistemological analysis and leading to heated controversy in metaphysics itself, is brought to a focus by this familiar disjunction: “Is psychology a natural science, or is it one of the sciences of mind and culture (Geisteswissenchaften)?”

The present article attempts to sketch the general lines of a new analysis of psychology, one which makes use of rigorous logical tools and which has made possible decisive advances toward the solution of the above problem. This analysis was successfully undertaken by the Vienna Circle (Wiener Kreis), the members of which (M. Schlick, R. Carnap, Phillipp Frank, o. Neurath, F. Waismann, H. Feigl, etc.) have, during the past ten years, developed an extremely fruitful method for the epistemological examination and critique of the various sciences, based in part on the work of L. Wittgenstein. We shall limit ourselves essentially to the examination of psychology as carried out by Carnap and Neurath.

The method characteristic of the studies of the Vienna Circle can be briefly defined as a logical analysis of the language of science. This method became possible only with the development of an extremely subtle logical apparatus which makes use, in particular, of all the formal procedures of modern logistics.

1. 13 [70] Rudolf Carnap, “Logical Empiricist,” Synthese 25 (1973), 256–68.

2. 14 [89] “Der Wiener Kreis und die Metamorphosen seines Empirismus,” in Norbert Leser, ed., Das geistige Leben Wiens in der Zwischenkriegszeit (Wien: Bundesverlag, 1981), pp. 205–15. Translated by Christian Piller.

3. 15 [106] “Hans Reichenbach Remembered,” Erkenntnis 35 (1991), 5–10.

4. 16 [109] “Empiricism in the Vienna Circle and in the Berlin Society for Scientific Philosophy: Recollections and Reflections,” Scientific Philosophy: Origins and Developments, in Friedrich Stadler, ed., Kluwer (Dordrecht: Academic Publishers, 1993), pp. 1–9.

In the end, Carnap and Neurath had more influence than his Doktorvater, Reichrenbach, on Hempel's thought; and although the interaction with Carnap went on much longer, it was Neurath whose influence proved to be the more enduring. Hempel tells the story here in memorial essays (13, 15) on Carnap and Reichenbach – and in two historical essays (14, 16) in which Neurath figures prominently on the career of logical empiricism.

1. In the following remarks, an attempt will be made to analyze the logical form of the concept of probability and of the statements in which it occurs, and to apply the result of this analysis to some problems concerning the language of empirical science. Our considerations will be based on the modern statistical theory of probability, which is, at present, certainly by far the most satisfactory and consistent basis for a theoretical account of the concept of probability as it is actually used in empirical science as well as in everyday language.

2. According to the statistical theory, each particular probability statement refers to an infinite sequence k of events and to a property P; the probability statement then has the form: “The probability for an element of k to have the property P is p,” which, by the statistical interpretation, is equipollent to “The relative frequency of those among the first n elements of k which have the property P converges toward the limit p as n goes toward infinity.” Thus, according to the statistical theory, probability is a three-termed relation between a property (or class), a sequence, and a real number; and each particular probability statement may be given the form “Prob (P, k, p)” (“The probability of P in k is p”). Here, “p” is a real number constant with o ≦ p ≦ 1; “k” is a functor whose arguments are the natural numbers and whose values are tetrads of real numbers which may be considered as determining the spatio-temporal coordinates of events.

1. 9 [3] “Analyse logique de la psychologie,” Revue de Synthese 10 (1935), 27–42. Wilfrid Sellars's translation, “The Logical Analysis of Psychology,” appeared in Herbert Feigl and Wilfrid Sellars, eds., Readings in Philosophical Analysis (New York: Appleton-Century-Crofts, 1949), pp. 373–84.

2. 10 [92] “Schlick und Neurath: Fundierung vs. Köharenz in der wissenschaftlichen Erkenntnis,” Grazer philosophische Studien, vol. 16/17 (1983), 1–18; Christian Piller, trans., “Schlick and Neurath: Foundation vs. Coherence in Scientific Knowledge.”

3. 11 [102] “On the Cognitive Status and the Rationale of Scientific Methodology,” Poetics Today 9 (1988), 5–27. Hempel notes that Part I is an annotated and slightly revised version of [95] and that Part II contains passages from [97].

4. 12 [100] “Provisoes: A Problem Concerning the Inferential Function of Scientific Theories,” in Adolf Grunbaum and Wesley Salmon, eds., The Limitations of Deductivism (Berkeley: University of California Press, 1988), pp. 19–36. Also in Erkenntnis 28 (1988), 147–64.

Essay 9 is a period piece from the early heyday of physicalism, when it could seem plausible both that any meaningful statement must be translatable into a sentence in the language of physics and that the possibility of such translation is a purely logical matter. In it, Hempel's proposed dissolution of the mind-body problem was a version of the identity theory according to which it is a matter of logic that “Otto has a toothache” will be equipol-lent to some physical sentence. Which physical sentence? The answer is an empirical matter.

This paper has been suggested by a recent discussion between Prof. Schlick and Dr. Neurath, made public in two articles which appeared in volume 4 of Erkenntnis, which mainly concerns the positivistic concept of verification and truth.

For the following exposition it may be advantageous to refer to the well-known crude classification which divides the different theories of truth into two main groups – that is, the correspondence theories and the coherence theories of truth. According to the correspondence-theories truth consists of a certain agreement or correspondence between a statement and the so-called facts or reality; while according to the coherence theories truth is a possible property of a whole system of statements (i.e., a certain conformity of statements with each other). In extreme coherence theories truth is even identified with the mutual compatibility of the elements of such a system.

The logical positivists' theory of truth developed step-by-step from a correspondence theory into a restrained coherence theory.

Let us shortly consider the most important logical phases of this process (which do not exactly correspond to the historical ones).

The philosophical ideas, which L. Wittgenstein has developed in his Tractatus Logico-Philosophicus and which represent the logical and historical starting point of the Vienna Circle's researches, are obviously characterized by a correspondencetheory of truth.

According to one of Wittgenstein's fundamental theses, a statement is to be called true if the fact or state of affairs expressed by it exists; otherwise the statement is to be called false.

The working title of this collection was “Philosophical Essays, Early and Late” – that is, earlier and later than the essays of Hempel's middle period, which are collected in his Aspects of Scientific Explanation and Other Essays in the Philosophy of Science [54]. They were published in two decades (roughly, 1945–1965), during which logical empiricism evolved from an arcane neo-Kantian or counter-Kantian sect into something that would seriously be called “The Received View” – a view whose shape and reception owed much to those very essays. They were essays in the sort of logical constructivism that Russell had framed in Our Knowledge of the External World as a Field for Scientific Method in Philosophy (1915). Russell's book was the inspiration for Carnap's Der logische Aufbau der Welt (1928), and for the Logische Syntax der Sprache (1934), which incorporated Carnap's version of “physicalism.” In those two decades Hempel could still regard the failings and limitations of that work as mere bumps on the road to logical empiricism. This book of Hempel's essays charts his course into and out of the methods and concerns of that middle period.

Having been one of the first logical empiricist graduate students, Hempel would become the last survivor of the Berlin and Vienna circles. Throughout, he was in close intellectual contact with the major figures in the movement – and a movement it was, defined for its activists by no doctrine but by a certain morale and discipline.

METHODOLOGY OF SCIENCE: DESCRIPTIVE AND PRESCRIPTIVE FACETS

Two Conceptions of the Methodology of Science

In the course of its long history – most strikingly in recent centuries – scientific inquiry has vastly broadened man's knowledge and deepened his understanding of the world he lives in, and the remarkable successes of predictions and technological applications based on those insights are widely acknowledged as eloquent testimony to the soundness or the “rationality” of scientific methods of research. Yet, there is no unanimity among students of the methodology of science as to whether or to what extent it is possible to formulate clear and precise rules of procedure which are characteristic of scientific research and which make science the exemplar of rationality in the pursuit of reliable knowledge. Nor is there unanimity concerning the grounds on which such rules of scientific inquiry could be established and how, accordingly, their cognitive status is to be understood: Is the methodology of science to be viewed as a descriptive study of actual scientific research or as a quasi-normative discipline aimed at formulating – in an analytic, aprioristic manner – standards of proper or rational scientific inquiry?

These questions have in recent years been the focus of an intense and fruitful debate between two schools of thought, which I will briefly refer to as the analytic-empiricist and the historic-sociological or pragmatist schools. By the former, I mean a body of ideas that, broadly speaking, has grown out of logical empiricism and the work of kindred thinkers.

INTRODUCTORY REMARKS

The Vienna Circle was made the topic of this lecture series for good reasons. The philosophical ideas of that group, which started to blossom during the years between the wars and soon gained international influence, belong, without doubt, to the most important and most fruitful manifestations of Vienna's intellectual life of that time.

That I have been given the honorable task of speaking here about the Vienna Circle and about the evolution and influence of its ideas is certainly due in part to the fact that I belong to the nowadays small group of people who personally knew or know the leading thinkers of the Vienna Circle and similar-minded groups.

Furthermore, my intellectual development was strongly influenced by the thinkers of the Vienna Circle, as well as by Hans Reichenbach, the leading personage of the Berlin Group of empiricist philosophers. I feel a deep respect for and gratitude toward those thinkers; but gradually, I came to question their basic ideas, as I have indicated in my writings. But this was entirely in the spirit of my teachers, who always encouraged their students toward independent and critical thinking.

In view of my close connection with the movement of logical positivism or, as it was later called, logical empiricism, I hope it will be seen as appropriate if I start with some personal recollections about the leading representatives of the Vienna Circle.

Hans Reichenbach's far-ranging and influential contributions to epistemology and the philosophy of science will be acknowledged and appraised in many contexts on the occasion of the one-hundredth anniversary 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 Göttingen and Heidelberg. I promptly enrolled in a course with Reichenbach and was startled by his ideas on causality and determinism, 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 presenting. I did indeed change my mind on this subject and on many others; 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.

1. In a recent article in Analysis (vol. 2, no. 5), Prof. Schlick traces the outlines of his view concerning the relationship of propositions to “facts.” In this account, Prof. Schlick makes a contribution for which we must be grateful by elucidating some essential points of his article “Das Fundament der Erkenntnis” (Erkenntnis 4 [1933], 79), which occasioned a logical controversy, the fundamental ideas of which I tried to characterize in my note “On the Logical Positivists' Theory of Truth” (Analysis, vol. 2, no. 4).

In his paper, Prof. Schlick raises certain objections to some of the considerations which I sketched in my article and which correspond to Dr. Neurath and Prof. Carnap's view; he gives his objections the form of questions, which may be summed up by asking: What harm is there in saying that propositions are compared with “facts” and that true propositions express “facts”? To this question I shall try to reply.

2. Prof. Schlick illustrates the character of the comparison between a proposition and “facts” by means of a very instructive example. But I think that in an essential respect his account is not quite adequate.

For on the one hand Prof. Schlick expressly describes propositions as empirical objects (of a special kind) which may be compared with any other empirical object. So far I fully agree with him.

In this note, I want to deal with the objections which Dr. v. Juhos has raised in this periodical to certain considerations concerning the language of science, which are mainly due to Catnap, Neurath and Popper, and some which I have outlined in an earlier paper.

We may distinguish three main points to which v. Juhos's criticism refers: (I) The “behavioristic” interpretation of psychological statements such as “I see blue”: (II) the view that in our empirical science even statements of the kind just mentioned might be “altered” or “abandoned” under certain conditions; (III) the proposal to express “epistemological” considerations in the formal mode of speech.

Von Juhos maintains that each of these points is a thesis or a postulate of physicalism: He speaks of “the physicalist mode of speech which demands the alterability of all statements” (loc. cit., p. 89; in this passage, elements of each of the three independent points are combined), and he declares the recommendation of the formal mode to be “a special postulate of Physicalism” (loc. cit., p. 90). For the sake of clarity, it ought to be noticed that only (I) is a thesis of physicalism, whilst (II) and (III) are fully independent of it.

Indeed, physicalism asserts that the language of physics is a universal language of science – that is, “every sentence of any branch of scientific language is equipollent to some sentence of the physical language and can therefore be translated into the physical language without changing its content.”