Relations between logic and mathematics in the work of Benjamin and Charles S. Peirce.

Charles Peirce (1839-1914) was one of the most important logicians of the nineteenth century. This thesis traces the development of his algebraic logic from his early papers, with especial attention paid to the mathematical aspects. There are three main sources to consider. 1) Benjamin Peirce (1809-1880), Charles's father and also a leading American mathematician of his day, was an inspiration. His memoir Linear Associative Algebra (1870) is summarised and for the first time the algebraic structures behind its 169 algebras are analysed in depth. 2) Peirce's early papers on algebraic logic from the late 1860s were largely an attempt to expand and adapt George Boole's calculus, using a part/whole theory of classes and algebraic analogies concerning symbols, operations and equations to produce a method of deducing consequences from premises. 3) One of Peirce's main achievements was his work on the theory of relations, following in the pioneering footsteps of Augustus De Morgan. By linking the theory of relations to his post-Boolean algebraic logic, he solved many of the limitations that beset Boole's calculus. Peirce's seminal paper `Description of a Notation for the Logic of Relatives' (1870) is analysed in detail, with a new interpretation suggested for his mysterious process of logical differentiation. Charles Peirce's later work up to the mid 1880s is then surveyed, both for its extended algebraic character and for its novel theory of quantification. The contributions of two of his students at the Johns Hopkins University, Oscar Mitchell and Christine Ladd-Franklin are traced, specifically with an analysis of their problem solving methods. The work of Peirce's successor Ernst Schröder is also reviewed, contrasting the differences and similarities between their logics. During the 1890s and later, Charles Peirce turned to a diagrammatic representation and extension of his algebraic logic. The basic concepts of this topological twist are introduced. Although Peirce's work in logic has been studied by previous scholars, this thesis stresses to a new extent the mathematical aspects of his logic - in particular the algebraic background and methods, not only of Peirce but also of several of his contemporaries

Une archéologie de la logique du sens : arithmétique et contenu dans le processus de mathématisation de la logique au XIXe siècle

This work aims at providing a new general interpretation of the logic that was born with the work of Gottlob Frege, in order to make explicit one of the most decisive conditions of contemporary philosophy: the one that concerns the relation of philosophy to formal practices and knowledge. Its initial hypothesis states that Frege’s primary and most constant project was that of building a logic of content. However, the intelligibility thus gained does not intend to unearth a new underlying unity of Frege’s thought; it rather aims at localising the real gaps within Frege’s formulations that have not been identified as such until now. Still, those gaps do not require to be filled, for Frege’s logic is indeed effective despite this indeterminacy. Rather than the gaps, it is this ungrounded effectiveness that needs to be explained. Our answer to this question is that the effectiveness of Frege’s logic as a logic of content comes from a certain relationship with Arithmetic; in fact, Frege’s logic is constructed on the template of Arithmetic, before it becomes capable of constructing Arithmetic in turn. The task then arises to characterise precisely, at this constitutive and non-foundational level, the nature of the relation between a logic of content as a specific form of logic in the framework of its mathematization, and Arithmetic as a particular mathematical domain. From the meticulous study of the constitution of the Fregean system, an idea can be drawn that constitutes the central argument of this thesis: the various mathematical or formalised logical systems rest upon mathematics only through an intermediary dimension consisting in the practice, the reflection and the elaboration of signs, where the circulations between these two contemporary domains of formal knowledge (mathematics and logic) are constructed and justified. From this point of view, we then lay out a detailed study of the rise of the two most significant projects for formalizing logic in the nineteenth century: Frege’s and Boole’s (and the Booleans’). In the space leading from mathematical practices to logical systematisations through semiotic functioning, two general schemes or semiotic formal regimes can be drawn: “Symbolic Abstraction”, leading from abstract Algebra to Boolean propositional logic; and “Expressionism”, leading from Arithmetic to Predicate Calculus, associated to Frege’s work. More deeply, our research reveals a deep connexion between logical content and Arithmetic (understood as the theory of integers), which horizontally crosses the different semiotic regimes. Following the multiple dimensions of this nexus – which is responsible for the introduction of the category of sense in the framework of mathematized logic – a formal theory of expression can be drawn, which defines the conditions for the actual development of a logic of sense.Ce travail s’engage dans la reconstitution d’une intelligibilité globale nouvelle pour la logique qui est née avec Frege afin de restituer l’une des conditions décisives pour la philosophie contemporaine, à savoir celle qui concerne son rapport aux pratiques et aux savoirs formels. Son hypothèse initiale affirme que le projet premier et constant de Frege a été celui d’une logique du contenu. Pourtant, il ne s’agit pas de réinvestir l’œuvre de Frege d’une cohérence nouvelle dans le but de rétablir une unité stable. Car l’intelligibilité procurée par cette reconstitution permet de localiser dans les formulations de Frege de véritables lacunes qui ne semblent pas avoir été identifiées comme telles jusqu’ici. Que la logique de Frege soit efficace malgré ces lacunes, voilà ce qu’il faut expliquer. La réponse que nous donnons à ces questions est que l’efficacité de la logique de Frege en tant que logique du contenu provient d’un certain rapport à l’Arithmétique, à savoir celui par lequel c’est la logique qui est construite d’après les principes de l’Arithmétique, avant qu’elle ne soit capable de la construire à son tour. La question se pose alors de caractériser avec précision à ce niveau constitutif, non « fondationnel », la nature du rapport entre une logique du contenu comme forme spécifique de la logique dans le cadre de sa mathématisation, et l’Arithmétique comme domaine mathématique particulier. De l’analyse minutieuse de la constitution du système logique frégéen, une idée se dégage qui constitue la thèse centrale de notre travail : les différents systèmes de la logique mathématisée ou formelle ne reposent sur les mathématiques que par l’intermédiaire d’une dimension d’exercice, de réflexion et d’élaboration de signes, où les circulations et les emprunts entre ces deux savoirs formels contemporains que sont les mathématiques et la logique se construisent et se justifient. C’est donc cette thèse qu’il s’agit de démontrer, par une étude détaillée des processus d’émergence des deux plus grands projets de formalisation de la logique du XIXe siècle : celui de Frege et celui de Boole et des Booléens. Dans cet espace qui mène des pratiques mathématiques aux systématisations logiques à travers les fonctionnements des signes, deux régimes généraux se dessinent : celui d’ « Abstraction symbolique » qui mène de l’Algèbre abstraite à la Logique propositionnelle booléenne ; et celui de l’ « Expressionnisme », qui mène de l’Arithmétique au Calcul logique des prédicats, associée aux travaux de Frege. Mais plus profondément, par l’effet d’une lecture symptomale au plus près des dynamiques internes à ces processus, le présent travail décèle un lien transversal entre le contenu logique d’une part et l’Arithmétique comme ensemble des déterminations du nombre de l’autre. En suivant ce lien, qui s’avère le responsable de l’introduction de la catégorie de sens dans le cadre de la logique mathématisée, une théorie de l’expression formelle se dessine, définissant les conditions pour le développement d’une logique du sens