Gerhard Heinzmann

During a conference entitled “Un logicien consciencieux. La philosophie de Evert Willem Beth,” held in Nancy in 1998 and published in this journal, volume 3, cahier 4 [Barth, Heinzmann et al. 1998-1999], the participants regretted that a reasoned biography of Beth was not available. In view of the efforts made since then in this direction, in particular by Paul van Ulsen in his PhD thesis in Dutch [2000], this article attempts to provide an overview of Beth’s work for a broader community of philosophers of science. However, my focus is not so much on Beth’s widely discussed and important contributions to mathematical logic, where his most creative merits reside. My aim is to emphasize Beth’s philosophical and cultural background. Both are also discussed in the first two chapters of Ulsen’s thesis, which globally focuses on Beth’s work in logic. Of course, philosophical and scientific aspects cannot be systematically separated, but Beth’s influence on the philosophy of mathematics between the 1950s and 1960s seems to me large enough to highlight it here. Beth is one of the very few philosophers who also presented highly technically relevant results in logic, or one of the few logicians who also wrote philosophical papers, not only in his youth or old age, but throughout his career. In his 741-page magnum opus The Foundations of Mathematics [1959], he combines the treatment of philosophical and logico-mathematical problems under the perspective that the traditional deductive procedure is an essential and not merely incidental element in mathematical proof so that “logic must play the principal role in a philosophical examination of mathematical thought” [1959, XI]. By 1937, Beth was “convinced that the only possibility of founding mathematics in a truly satisfactory way is to apply intuitive and formal methods simultaneously in such a way that they can control and verify each other” (“L’évidence intuitive dans les mathématiques modernes” [1937, 165]). In 1956, he still justified the emphasis on the intuitive element of proof by “certain complications that the development of these [formalist] tendencies would later give rise to” (“Poincaré et la philosophie” [1956b, 238, transl. G. H.]).

There is no substantive agreement about the philosophical views of analytic philosophers. Nevertheless, for much of them logical analysis is widely recognized to be important. It is true that Poincare used no logical analysis but refused nevertheless the old metaphysics. Indeed, the analytic tradition of philosophy of science is perhaps better characterized by several overlapping similarities, which are the clincher for my main thesis: Analytic philosophy of science has one of its origins in the philosophical network in France around 1900 and, especially, in Poincare.

The goal of this paper is to answer the following question: Does it make sense to speak of thought experiments not only in physics, but also in mathematics, to refer to an authentic type of activity? One may hesitate because mathematics as such is the exercise of reasoning par excellence, an activity where experience does not seem to play an important role. After reviewing some results of the research on thought experiments in the natural sciences, we turn our attention to experiments and thought experiments in mathematics, especially in fundamental mathematics, and investigate what thought experiments can teach us there. If we accept the principle that mathematical practice can sometimes be best described by a pragmatist approach where the concept of experience in mathematics is considered from a new perspective, thought experiments can in some cases be a useful instrument different from both ‘mathematical experiments’ and ‘formal’ proofs: they are mathematical experiments using deviant methods.

This open access book examines the many contributions of Paul Lorenzen, an outstanding philosopher from the latter half of the 20th century. It features papers focused on integrating Lorenzen's original approach into the history of logic and mathematics. The papers also explore how practitioners can implement Lorenzen’s systematical ideas in today’s debates on proof-theoretic semantics, databank management, and stochastics. Coverage details key contributions of Lorenzen to constructive mathematics, Lorenzen’s work on lattice-groups and divisibility theory, and modern set theory and Lorenzen’s critique of actual infinity. The contributors also look at the main problem of Grundlagenforschung and Lorenzen’s consistency proof and Hilbert’s larger program. In addition, the papers offer a constructive examination of a Russell-style Ramified Type Theory and a way out of the circularity puzzle within the operative justification of logic and mathematics. Paul Lorenzen's name is associated with the Erlangen School of Methodical Constructivism, of which the approach in linguistic philosophy and philosophy of science determined philosophical discussions especially in Germany in the 1960s and 1970s. This volume features 10 papers from a meeting that took place at the University of Konstanz.

L'histoire des fondements des mathématiques du xxe siècle montre qu'il nous faut réviser la signification des notions philosophiques traditionnelles comme « évidence », « existence », « expérience » ou « rationalité ». On expose comment le logicien Paul Bernays, familier des conceptions de la philosophie de Jacob Friedrich Fries et de Léonard Nelson, donne aux résultats techniques une interprétation philosophique dont il s'inspire de plus en plus - à partir du milieu du siècle - de la « philosophie ouverte » de Ferdinand Gonseth. Bien avant Thomas Kuhn, Bernays conçoit la révision envisagée non pas simplement comme une question de vérité ou de fausseté, mais comme un problème exigeant l'introduction d'un nouveau système conceptuel. Cependant, l'affaiblissement du concept de vérité est pour lui une conséquence épistémologique provenant de la théorie de la connaissance adoptée et non une conséquence méthodologique issue d'un changement de paradigme et donc d'une incommensurabilité.

Pascal Engel distingue globalement deux tendances de concevoir la vérité : les uns défendent une théorie substantialiste selon laquelle la vérité exprime une « caractéristique réelle », les autres, déflationnistes, soutiennent que la vérité n’est pas une propriété authentique des porteurs de vérités. Prenant le réalisme comme option par défaut, Engel cherche une position médiane stable, telle que la vérité est un concept plus substantiel que le déflationnisme ne le prétend et moins substantiel que les théories réalistes ne disent qu’elle est. Dans cet article on développe une solution médiane alternative à la position proposée par Engel en se basant sur une option anti-réaliste et pragmatique (mais non utilitariste). On montre, plus exactement, dans quelle mesure la conception dialogique de la vérité, avancée par Kuno Lorenz, s’oppose aux thèses défendues par Engel tout en poursuivant le même but que lui.

Pascal Engel distingue globalement deux tendances de concevoir la vérité : les uns défendent une théorie substantialiste selon laquelle la vérité exprime une « caractéristique réelle », les autres, déflationnistes, soutiennent que la vérité n’est pas une propriété authentique des porteurs de vérités. Prenant le réalisme comme option par défaut, Engel cherche une position médiane stable, telle que la vérité est un concept plus substantiel que le déflationnisme ne le prétend et moins substantiel que les théories réalistes ne disent qu’elle est. Dans cet article on développe une solution médiane alternative à la position proposée par Engel en se basant sur une option anti-réaliste et pragmatique. On montre, plus exactement, dans quelle mesure la conception dialogique de la vérité, avancée par Kuno Lorenz, s’oppose aux thèses défendues par Engel tout en poursuivant le même but que lui.

RésuméSelon un pragmatiste, la réflexion du philosophe porte sur le lien entre la construction et la description des objets mathématiques. Grâce à sa conception?un raisonnement dit ≫theorematique≪, Peirce a réussi à etablir ľesquisse?une interprétation pragmatique?une structure mathématique. Cette dernière reste néanmoins vague quant à la catégoricité de la structure. – lci, il est utile de recourir à Gonseth et & dcar;analyser sa reconstruction?un système axiomatique: la genèse logique?une structure selon les principes de ľidonéisme correspond en effet au programme de Peirce, à savoir àľexplication de la signification?un concept àľaide?une séquence cumulative? interprétants.SummaryAccording to a pragmatist, philosophical thought bears on the link between the construction and the description of mathematical objects. Thanks to his development of ≫theorematic reasoning)), Peirce succeeded in bringing forward a pragmatic interpretation of a mathematical structure, exhibiting however an inherent vagueness with regard to the categoricalness of the structure. ‐ In this matter, Gonseth's model of reconstruction of an axiomatic system can be resorted to: the logical genesis of a structure considered in his ≫ldoneism≪ corresponds to Peirce's program of explaining the meaning of a concept by means of a cumulative sequence of interpretants.