The Development of Mathematical Logic from Russell to Tarski: 1900–1935

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The Development of Mathematical Logic from Russell to Tarski: 1900–1935 The Development of Mathematical Logic from Russell to Tarski: 1900–1935 Paolo Mancosu Richard Zach Calixto Badesa The Development of Mathematical Logic from Russell to Tarski: 1900–1935 Paolo Mancosu (University of California, Berkeley) Richard Zach (University of Calgary) Calixto Badesa (Universitat de Barcelona) Final Draft—May 2004 To appear in: Leila Haaparanta, ed., The Development of Modern Logic. New York and Oxford: Oxford University Press, 2004 Contents Contents i Introduction 1 1 Itinerary I: Metatheoretical Properties of Axiomatic Systems 3 1.1 Introduction . 3 1.2 Peano’s school on the logical structure of theories . 4 1.3 Hilbert on axiomatization . 8 1.4 Completeness and categoricity in the work of Veblen and Huntington . 10 1.5 Truth in a structure . 12 2 Itinerary II: Bertrand Russell’s Mathematical Logic 15 2.1 From the Paris congress to the Principles of Mathematics 1900–1903 . 15 2.2 Russell and Poincar´e on predicativity . 19 2.3 On Denoting . 21 2.4 Russell’s ramified type theory . 22 2.5 The logic of Principia ......................... 25 2.6 Further developments . 26 3 Itinerary III: Zermelo’s Axiomatization of Set Theory and Re- lated Foundational Issues 29 3.1 The debate on the axiom of choice . 29 3.2 Zermelo’s axiomatization of set theory . 32 3.3 The discussion on the notion of “definit” . 35 3.4 Metatheoretical studies of Zermelo’s axiomatization . 38 4 Itinerary IV: The Theory of Relatives and Lowenheim’s¨ Theorem 41 4.1 Theory of relatives and model theory . 41 4.2 The logic of relatives . 44 4.3 Lowenheim’s¨ theorem . 46 4.4 Skolem’s first versions of Lowenheim’s¨ theorem . 56 i Contents 5 Itinerary V: Logic in the Hilbert School 59 5.1 Early lectures on logic . 59 5.2 The completeness of propositional logic . 60 5.3 Consistency and completeness . 61 5.4 Axioms and inference rules . 66 5.5 Grundzuge¨ der theoretischen Logik . 70 5.6 The decision problem . 71 5.6.1 The decision problem in the tradition of algebra of logic 72 5.6.2 Work on the decision problem after 1920 . 73 5.7 Combinatory logic and λ-calculus . 74 5.8 Structural inference: Hertz and Gentzen . 76 6 Itinerary VI: Proof Theory and Arithmetic 81 6.1 Hilbert’s Program for consistency proofs . 81 6.2 Consistency proofs for weak fragments of arithmetic . 82 6.3 Ackermann and von Neumann on epsilon substitution . 87 6.4 Herbrand’s Theorem . 92 6.5 Kurt Godel¨ and the incompleteness theorems . 94 7 Itinerary VII: Intuitionism and Many-valued Logics 99 7.1 Intuitionistic logic . 99 7.1.1 Brouwer’s philosophy of mathematics . 99 7.1.2 Brouwer on the excluded middle . 101 7.1.3 The logic of negation . 102 7.1.4 Kolmogorov . 103 7.1.5 The debate on intuitionist logic . 106 7.1.6 The formalization and interpretation of intuitionistic logic . 108 7.1.7 Godel’s¨ contributions to the metatheory of intuition- istic logic . 110 7.2 Many-valued logics . 111 8 Itinerary VIII: Semantics and Model-theoretic Notions 117 8.1 Background . 117 8.1.1 The algebra of logic tradition . 117 8.1.2 Terminological variations (systems of objects, mod- els, and structures) . 118 8.1.3 Interpretations for propositional logic . 119 8.2 Consistency and independence for propositional logic . 120 8.3 Post’s contributions to the metatheory of the propositional calculus . 123 8.4 Semantical completeness of first-order logic . 124 8.5 Models of first order logic . 129 8.6 Completeness and categoricity . 130 8.7 Tarski’s definition of truth . 134 ii Notes 141 Bibliography 149 Index of Citations 175 iii Introduction The following nine itineraries in the history of mathematical logic do not aim at a complete account of the history of mathematical logic during the period 1900–1935. For one thing, we had to limit our ambition to the technical developments without attempting a detailed discussion of issues such as what conceptions of logic were being held during the period. This also means that we have not engaged in detail with historiographical de- bates which are quite lively today, such as those on the universality of logic, conceptions of truth, the nature of logic itself etc. While of extreme interest these themes cannot be properly dealt with in a short space, as they often require extensive exegetical work. We therefore merely point out in the text or in appropriate notes how the reader can pursue the connection between the material we treat and the secondary literature on these debates. Second, we have not treated some important developments. While we have not aimed at completeness our hope has been that by fo- cusing on a narrower range of topics our treatment will improve on the existing literature on the history of logic. There are excellent accounts of the history of mathematical logic available, such as, to name a few, Kneale and Kneale (1962), Dimitriu (1977), and Mangione and Bozzi (1993). We have kept the secondary literature quite present in that we also wanted to write an essay that would strike a balance between covering material that was adequately discussed in the secondary literature and presenting new lines of investigation. This explains, for instance, why the reader will find a long and precise exposition of Lowenheim’s¨ ( 1915) theorem but only a short one on Godel’s¨ incompleteness theorem: Whereas there is hitherto no precise presentation of the first result, accounts of the second result abound. Finally, the treatment of the foundations of mathematics is quite restricted and it is ancillary to the exposition of the history of mathemati- cal logic. Thus, it is not meant to be the main focus of our exposition.1 Page references in citations are to the English translations, if available; or to the reprint edition, if listed in the bibliography. All translations are the authors’, unless an English translation is listed in the references. 1 1 Itinerary I. Metatheoretical Properties of Axiomatic Systems 1.1 Introduction The two most important meetings in philosophy and mathematics in 1900 took place in Paris. The First International Congress of Philosophy met in August and so did, soon after, the Second International Congress of Mathe- maticians. As symbolic, or mathematical, logic has traditionally been part both of mathematics and philosophy, a glimpse at the contributions in mathematical logic at these two events will give us a representative se- lection of the state of mathematical logic at the beginning of the twenti- eth century. At the International Congress of Mathematicians Hilbert pre- sented his famous list of problems ( Hilbert 1900a), some of which became central to mathematical logic, such as the continuum problem, the consis- tency proof for the system of real numbers, and the decision problem for Diophantine equations (Hilbert’s tenth problem). However, despite the at- tendance of remarkable logicians like Schroder,¨ Peano, and Whitehead in the audience, the only other talk that could be classified as pertaining to mathematical logic were two talks given by Alessandro Padoa on the ax- iomatizations of the integers and of geometry, respectively. The third section of the International Congress of Philosophy was de- voted to logic and history of the sciences ( Lovett 1900–01). Among the contributors of papers in logic we find Russell, MacColl, Peano, Burali- Forti, Padoa, Pieri, Poretsky, Schroder,¨ and Johnson. Of these, MacColl, Poretsky, Schroder,¨ and Johnson read papers that belong squarely to the algebra of logic tradition. Russell read a paper on the application of the theory of relations to the problem of order and absolute position in space and time. Finally, the Italian school of Peano and his disciples—Burali- Forti, Padoa and Pieri—contributed papers on the logical analysis of math- ematics. Peano and Burali-Forti spoke on definitions, Padoa read his fa- mous essay containing the “logical introduction to any theory whatever,” and Pieri spoke on geometry considered as a purely logical system. Al- though there are certainly points of contact between the first group of 3 1. Metatheoretical Properties of Axiomatic Systems logicians and the second group, already at that time it was obvious that two different approaches to mathematical logic were at play. Whereas the algebra of logic tradition was considered to be mainly an application of mathematics to logic, the other tradition was concerned more with an analysis of mathematics by logical means. In a course given in 1908 in Gottingen,¨ Zermelo captured the double meaning of mathemat- ical logic in the period by reference to the two schools: The word “mathematical logic” can be used with two different meanings. On the one hand one can treat logic mathematically, as it was done for instance by Schroder¨ in his Algebra of Logic; on the other hand, one can also investigate scientifically the logical components of mathematics. ( Zermelo 1908a, 1)2 The first approach is tied to the names of Boole and Schroder,¨ the second was represented by Frege, Peano and Russell.3 We will begin by focusing on mathematical logic as the logical analysis of mathematical theories but we will return later (see itinerary IV) to the other tradition. 1.2 Peano’s school on the logical structure of theories We have mentioned the importance of the logical analysis of mathemat- ics as one of the central motivating factors in the work of Peano and his school on mathematical logic. First of all, Peano was instrumental in em- phasizing the importance of mathematical logic as an artificial language that would remove the ambiguities of natural language thereby allowing a precise analysis of mathematics. In the words of Pieri, an appropriate ideographical algorithm is useful as “an instrument appropriate to guide and discipline thought, to exclude ambiguities, implicit assumptions, men- tal restrictions, insinuations and other shortcomings, almost inseparable from ordinary language, written as well as spoken, which are so damag- ing to speculative research.” ( Pieri 1901, 381).
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