Michael Detlefsen

(1948 - 2019)

Hide Ishiguro, Leibniz’s philosophy of logic and language. 2nd ed. Cambridge:Cambridge University Press, 1990. x + 246pp. £27.50/$49.50 ; £10.95/$16.95 Massimo Mugnai, Leibniz’ theory of relations. Stuttgart:Franz Steiner Verlag, 1992. 291 pp. 96 DM W. A. Wallace, Galileo’s logic of discovery and proof The background, content, and use of his appropriated treatises on Aristotle’s posterior analytics. Dordrecht, Boston, and London:Kluwer, 1992. xxiii + 323 pp. £84, $139, DF1240 W. A. Wallace, Galileo’s logical treatises. A translation, with notes and commentary, of his appropriated Latin questions on Aristotle’s posterior analytics. Dordrecht, Boston, and London:Kluwer, 1992. xix + 239 pp. £65, $;99, DF1185 W. A. Wallace, Galileo’s early notebooks:the physical questions ; Prelude to Galileo: essays on medieval and sixteenth-century sources of Galileo’s thought ; Galileo and his sources: the heritage of the Collegio Romano in Galileo’s science ; and Galileo Galilei:Tractatio de Precognitionibus et Praecognitis and Tractatio de Démonstration Raymond M. Smullyan, GödeVs incompleteness theorems, New York, Oxford:Oxford University Press, 1992. xiii + 139. pp. $29.95 Raymond M. Smullyan, Recursion theory for metamathematics, New York, Oxford:Oxford University Press, 1993. xiv + 163 pp. $29.95 D. MiéVille, Kurt Gödel:Actes du Colloque, Neuchâtel 13-14 juin 1991. Neuchâtel, Switzerland: Centre de Recherches Sémiologiques, Université de Neuchâtel, 1992. iv +200pp. 15 Sfr Geoffrey Hellman, Mathematics without numbers. Oxford:Clarendon Press, 1989. xi + 154 pp. £22.50 Michael Detlefsen Proof, logic and formalization. London and New York:Routledge, 1992. x + 241 pp. £40.00 J. Cavaillès, Método axiomâtico y formalismo. Foreword by S. Ramirez. Translation from the French by C. Alvarez and S. Ramirez. Mexico:Servicios Editoriales de la Facultad de Ciencias, UNAM, 1992. 199pp. No price stated Roberto Poli, Ontologia formale. Genoa:Marietti, 1992, 542 pp. No price stated Jon Barwise and John Etchemendy, The language of first-order logic, 3rd edition, Stanford, CA:CLSI Lecture Notes No. 34, 1992, xiv + 329pp. US $34.95

This volume of essays addresses the main problem confronting an epistemology for mathematics; namely, the nature and sources of mathematical justification. Attending to both particular and general issues, the essays, by leading philosophers of mathematics, raise important issues for our current understanding of mathematics. Is mathematical justification a priori or a posteriori? What role, if any, does logic play in mathematical reasoning or inference? And of what epistemological importance is the formalizability of proof? The editor, Michael Detlefsen, has brought together an outstanding collection of essays, only one of which has previously appeared. It will be essential for philosophers and historians of mathematics, as well as philosophically inclined logicians and philosophers interested in the nature of reasoning and justification. A companion volume entitled Proof, Logic and Formalization, edited by Michael Detlefsen, is also available from Routledge.

Formalism in the philosophy of mathematics has taken a variety of forms and has been advocated for widely divergent reasons. In Sects. 1 and 2, I briefly introduce the major formalist doctrines of the late nineteenth and early twentieth centuries. These are what I call empirico-semantic formalism, game formalism and instrumental formalism. After describing these views, I note some basic points of similarity and difference between them. In the remainder of the paper, I turn my attention to Hilbert’s instrumental formalism. My primary aim there will be to develop its formalist elements more fully. These are, in the main, its rejection of the axiom-centric focus of traditional model-construction approaches to consistency problems, its departure from the traditional understanding of the basic nature of proof and its distinctively descriptive or observational orientation with regard to the consistency problem for arithmetic. More specifically, I will highlight what I see as the salient points of connection between Hilbert’s formalist attitude and his finitist standard for the consistency proof for arithmetic. I will also note what I see as a significant tension between Hilbert’s observational approach to the consistency problem for arithmetic and his expressed hope that his solution of that problem would dispense with certain epistemological concerns regarding arithmetic once and for all.

The type of completeness Whitehead and Russell aimed for in their Principia Mathematica was what I call descriptive completeness. This is completeness with respect to the propositions that have been proved in traditional mathematics. The notion of completeness addressed by Gödel in his famous work of 1930 and 1931 was completeness with respect to the truths expressible in a given language. What are the relative significances of these different conceptions of completeness for traditional mathematics? What, if any, effects does incompleteness of the type Gödel had in mind have for the evaluation of the Principia Mathematica project and other projects like it?

© The Author [2017]. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] author makes clear that he does not see this book as a contribution to the philosophy of mathematics as traditionally understood. He takes it instead to be an essay about the philosophy of mathematics, one whose purpose is to explain its existence and to make clear the limited extent to which its current and past forms are properly regarded as philosophies of mathematics per se.A little more exactly, the aim is to explain the seemingly ‘perennial’ interest1 western philosophers have had in mathematics 2 and how this became significant enough to give rise to and sustain an identified sub-discipline of philosophy....Hacking identifies two principal reasons for this, both of which he sees as having originated in what might broadly be termed our experience of mathematics. The first of...

I focus on three preoccupations of recent writings on proof. I. The role and possible effects of empirical reasoning in mathematics. Do recent developments (specifically, the computer-assisted proof of the 4CT) point to something essentially new as regards the need for and/or effects of using broadly empirical and inductive reasoning in mathematics? In particular, should we see such things as the computer-assisted proof of the 4CT as pointing to the existence of mathematical truths of which we cannot have a priori knowledge? 2. The role of formalization in proof. What are the patterns ofinference according to which mathematical reasoning naturally proceeds? Are they of 'local' character (i.e. sensitive to the subject-matter of the reasoning concerned) or 'global' character (i.e. invariant across all subject-matters)? Finally, what if any relationship is there (a) between the patterns of inference manifest in a proof and its explanatory capacity and (b) between explanatory capacity and rigor? 3. Diagrams and their role in mathematical reasoning. What essentially is diagrammatic reasoning, and what is the nature and basis of its usefulness? Can it play a justificative role in the development of mathematical knowledge and, more particularly, in genuine proof? Finally, does the use of diagrammatic reasoning force an adjustment either in our conception of rigor or in our view of its importance?

We consider a seemingly popular justification (we call it the Re-flexivity Defense) for the third derivability condition of the Hilbert-Bernays-Löb generalization of Godel's Second Incompleteness Theorem (G2). We argue that (i) in certain settings (rouglily, those where the representing theory of an arithmetization is allowed to be a proper subtheory of the represented theory), use of the Reflexivity Defense to justify the tliird condition induces a fourth condition, and that (ii) the justification of this fourth condition faces serious obstacles. We conclude that, in the types of settings mentioned, the Reflexivity Defense does not justify the usual ‘reading’ of G2—namely, that the consistency of the represented theory is not provable in the representing theory.

Au début du XXe siècle, Poincaré et Russell eurent un débat à propos de la nature du raisonnement mathématique. Poincaré, comme Kant, défendait l’idée que le raisonnement mathématique était de caractère non logique. Russell soutenait une conception contraire et critiquait Poincaré. Je défends ici l’idée que les critiques de Russell n’étaient pas fondées.In the early twentieth century, Poincare and Russell engaged in a discussion concerning the nature of mathematical reasoning. Poincare, like Kant, argued that mathematical reasoning was characteristically non-logical in character. Russell argued for the contrary view and criticized Poincare. I argue that Russell’s criticisms were largely mistaken