The core pluralist thesis about logic, broadly construed, is the claim that two or more logics are correct. In this thesis I discuss a uniquely interesting variant of logical pluralism that I call logical contextualism. Roughly, the logical contextualists’ thought is that, for fixed values p and q, the statement “p entails q” and its cognates such as “q is a logical consequence of p” or “the argument from p to q is logically valid,” are true in some contexts and false in others. After developing a contextualist account of logical pluralism I proceed to examine implications that, if true, logical contextualism would have on discussions about reasonable disagreement among epistemic peers and on discussions about the aim and purpose of argumentation. I show that logical contextualism allows for the possibility of logically-based reasonable disagreements among epistemic peers. In the face of such disagreements there is no obligation to revise one’s belief, nor is there any obligation to degrade the peer status of the agent with whom one stands in disagreement. The possibility of logically-based reasonable disagreements, it will be argued, suggests a reconceptualization of the aims and purpose of argumentation. Most accounts of the purpose of argumentation hold that argumentation’s primary purpose is to achieve rational agreement on a contested issue. Such an agreement is thought to require that at least one of the parties in the argumentation change their beliefs or commitments. However, the existence of logically-based reasonable disagreements, I argue, implies that there are some argumentations that ought not to resolve with agreement. Therefore, rather than understanding argumentation as purely an effort to convince an opponent, or as a means to reach consensus, I claim that argumentation ought to be understood as an effort to gain a better understanding of divergent and perhaps irreconcilable perspectives.

The core pluralist thesis about logic, broadly construed, is the claim that two or more logics are correct. In this thesis I discuss a uniquely interesting variant of logical pluralism that I call logical contextualism. Roughly, the logical contextualists’ thought is that, for fixed values p and q, the statement “p entails q” and its cognates such as “q is a logical consequence of p” or “the argument from p to q is logically valid,” are true in some contexts and false in others. After developing a contextualist account of logical pluralism I proceed to examine implications that, if true, logical contextualism would have on discussions about reasonable disagreement among epistemic peers and on discussions about the aim and purpose of argumentation. I show that logical contextualism allows for the possibility of logically-based reasonable disagreements among epistemic peers. In the face of such disagreements there is no obligation to revise one’s belief, nor is there any obligation to degrade the peer status of the agent with whom one stands in disagreement. The possibility of logically-based reasonable disagreements, it will be argued, suggests a reconceptualization of the aims and purpose of argumentation. Most accounts of the purpose of argumentation hold that argumentation’s primary purpose is to achieve rational agreement on a contested issue. Such an agreement is thought to require that at least one of the parties in the argumentation change their beliefs or commitments. However, the existence of logically-based reasonable disagreements, I argue, implies that there are some argumentations that ought not to resolve with agreement. Therefore, rather than understanding argumentation as purely an effort to convince an opponent, or as a means to reach consensus, I claim that argumentation ought to be understood as an effort to gain a better understanding of divergent and perhaps irreconcilable perspectives.

Esta tese tem como objetivo estudar a paraconsistentização de lógicas, que consiste em encontrar, para uma dada lógica, sua contraparte paraconsistente. A estratégia geral utilizada para tal tarefa é: ao encontrar premissas inconsistentes, faça inferências a partir de seus subconjuntos consistentes. Para isso, foram desenvolvidos dois métodos de paraconsistentização. O primeiro consiste na utilização do arcabouço teórico da Teoria de Categorias, enquanto que o segundo faz uso da teoria da prova e da noção de estruturas valorativas. Após a apresentação dos métodos, algumas propriedades formais de ambas as propostas foram investigadas. Em particular, provou-se que a paraconsistentização preserva correção e completude, isto é, se a lógica inicial for correta e completa, sua contraparte paraconsistente também o será. Também foram estabelecidas as condições suficientes que uma lógica deve satisfazer para poder ser paraconsistentizada. A partir destes resultados, os dois métodos propostos foram comparados com outras abordagens desenvolvidas para raciocinar a partir de conjuntos inconsistentes. Por fim, a tese analisa o debate entre pluralismo e monismo lógico investigando, especificamente, as contribuições da paraconsistentização para tal discussão. / This thesis aims to study the paraconsistentization of logics, which consists in finding, for a given logic, its paraconsistent counterpart. The general strategy used for this task is the following: when finding inconsistent premises, one must draw inferences from its consistent subsets. For this, two methods of paraconsistentization were developed. The first consists in using the framework of Category Theory, while the second one makes use of proof theory and the notion of valuation structure. After their presentation, some formal properties of both proposals were investigated. In particular, it was proved that paraconsistentization preserves soundness and completeness, that is, if the initial logic is sound and complete, its paraconsistent counterpart will also be. The sufficient conditions that a logic must satisfy in order for it to be paraconsistentized were also established. From these results, the two proposed methods were compared with other approaches developed to reason from inconsistent sets. Finally, the thesis analyses the debate between pluralism and logical monism investigating, specifically, the contributions of paraconsistentization to this discussion.

The dissertation aims to set out a methodological framework conducive to further research into the demarcation problem of logical constants through a critical examination of the principal proposals for the problem. Logical constants should be characterised so as the essential values of logic, i.e., necessity, normativity and formality, are secured. Formality is central to the proposed framework in consideration of its relation to validity / necessity and normativity are established with reference to formality. Logical constanthood is analysed into logicality and constanthood to explore the conditions and constraints on logical form. On the purpose of their determination, a Wittgensteinian stance is endorsed, focusing on the view of language as a reflexive autonomous realm. The autonomy of language unfolds a specific viewpoint that indicates that logical form is existentially grounded in the possibilities presented by the autonomy. v Gentzen&rsquo / s natural deduction and sequent calculi are adopted as the proper perspectives to discuss the relations of logicality and constanthood to formalisation. Logical constants are required to be fixed so that the resultant logical form is sterile of content and semantically inert with respect to argument content. In addition to the conditions of harmony in logical form, mutual disharmony is introduced. Analytic and grammatical truths are specified as constraints to logical form. In particular, the conservativeness condition is found to be irrelevant to logical form. The framework incorporates methodological pluralism as a probe into the understanding of logicality. The dissertation suggests a bidimensional programme of research related to formal conditions and the Wittgensteinian grammatical constraints.

In broadest terms, this thesis is concerned to answer the question of whether the view that arithmetic is analytic can be maintained consistently. Lest there be much suspense, I will conclude that it can. Those who disagree claim that accounts which defend the analyticity of arithmetic are either unable to give a satisfactory account of the foundations of mathematics due to the incompleteness theorems, or, if steps are taken to mitigate incompleteness, then the view loses the ability to account for the applicability of mathematics in the sciences. I will show that this criticism is not successful against every view whereby arithmetic is analytic by showing that the brand of "conventionalism" about mathematics that Rudolf Carnap advocated in the 1930s, especially in Logical Syntax of Language, does not suffer from these difficulties. There, Carnap develops an account of logic and mathematics that ensures the analyticity of both. It is based on his famous "Principle of Tolerance", and so the major focus of this thesis will to defend this principle from certain criticisms that have arisen in the 80 years since the book was published. I claim that these criticisms all share certain misunderstandings of the principle, and, because my diagnosis of the critiques is that they misunderstand Carnap, the defense I will give is of a primarily historical and exegetical nature. Again speaking broadly, the defense will be split into two parts: one primarily historical and the other argumentative. The historical section concerns the development of Carnap's views on logic and mathematics, from their beginnings in Frege's lectures up through the publication of Logical Syntax. Though this material is well-trod ground, it is necessary background for the second part. In part two we shift gears, and leave aside the historical development of Carnap's views to examine a certain family of critiques of it. We focus on the version due to Kurt Gödel, but also explore four others found in the literature. In the final chapter, I develop a reading of Carnap's Principle - the `wide' reading. It is one whereby there are no antecedent constraints on the construction of linguistic frameworks. I argue that this reading of the principle resolves the purported problems. Though this thesis is not a vindication of Carnap's view of logic and mathematics tout court, it does show that the view has more plausibility than is commonly thought.

Partant des travaux séminaux de Boole, Frege et Russell, le mémoire cherche à clarifier l‟enjeu du pluralisme logique à l‟ère de la prolifération des logiques non-classiques et des développements en informatique théorique et en théorie des preuves. Deux chapitres plus « historiques » sont à l‟ordre du jour : (1) le premier chapitre articule l‟absolutisme de Frege et Russell en prenant soin de montrer comment il exclut la possibilité d‟envisager des structures et des logiques alternatives; (2) le quatrième chapitre expose le chemin qui mena Carnap à l‟adoption de la méthode syntaxique et du principe de tolérance, pour ensuite dégager l‟instrumentalisme carnapien en philosophie de la Logique et des mathématiques. Passant par l‟analyse d‟une interprétation intuitive de la logique linéaire, le deuxième chapitre se tourne ensuite vers l‟établissement d‟une forme logico-mathématique de pluralisme logique à l‟aide de la théorie des relations d‟ordre et la théorie des catégories. Le troisième chapitre délimite le terrain de jeu des positions entourant le débat entre monisme et pluralisme puis offre un argument contre la thèse qui veut que le conflit entre logiques rivales soit apparent, le tout grâce à l‟utilisation du point de vue des logiques sous-structurelles. Enfin, le cinquième chapitre démontre que chacune des trois grandes approches au concept de conséquence logique (modèle-théorétique, preuve-théorétique et dialogique) forme un cadre suffisamment général pour établir un pluralisme. Bref, le mémoire est une défense du pluralisme logique. / Starting from the seminal work of Boole, Frege and Russell, the dissertation seeks to clarify the issue of logical pluralism in the era of the proliferation of non-classical logics and the developments in theoretical computer science and proof theory. Two “historical” chapters are scheduled: the first chapter articulate the absolutism of Frege and Russell, taking care to show how it condemns the possibility to consider alternative structures and logics; the fourth chapter describes the path that led Carnap from the adoption of the syntactic method to the formulation of the principle of tolerance, then goes on to display Carnap‟s instrumentalism in philosophy of Logic and mathematics. Opening with the analysis of an intuitive interpretation of linear logic, the second chapter then turns to the establishment of a form of logico-mathematical pluralism with the help of order theory and category theory. The third chapter delineates the playground of revisionism (philosophical positions surrounding the debate between monism and pluralism) and then provides an argument against the thesis that denies the reality of the conflict between rival logics, all this being done by adopting the substructural logic point of view. The fifth chapter shows that each of the three main approaches to the concept of logical consequence (model-theoretic, proof-theoretic and dialogical) supplies a framework sufficiently general to establish pluralism. In short, the dissertation is a defence of logical pluralism.

Partant des travaux séminaux de Boole, Frege et Russell, le mémoire cherche à clarifier l‟enjeu du pluralisme logique à l‟ère de la prolifération des logiques non-classiques et des développements en informatique théorique et en théorie des preuves. Deux chapitres plus « historiques » sont à l‟ordre du jour : (1) le premier chapitre articule l‟absolutisme de Frege et Russell en prenant soin de montrer comment il exclut la possibilité d‟envisager des structures et des logiques alternatives; (2) le quatrième chapitre expose le chemin qui mena Carnap à l‟adoption de la méthode syntaxique et du principe de tolérance, pour ensuite dégager l‟instrumentalisme carnapien en philosophie de la Logique et des mathématiques. Passant par l‟analyse d‟une interprétation intuitive de la logique linéaire, le deuxième chapitre se tourne ensuite vers l‟établissement d‟une forme logico-mathématique de pluralisme logique à l‟aide de la théorie des relations d‟ordre et la théorie des catégories. Le troisième chapitre délimite le terrain de jeu des positions entourant le débat entre monisme et pluralisme puis offre un argument contre la thèse qui veut que le conflit entre logiques rivales soit apparent, le tout grâce à l‟utilisation du point de vue des logiques sous-structurelles. Enfin, le cinquième chapitre démontre que chacune des trois grandes approches au concept de conséquence logique (modèle-théorétique, preuve-théorétique et dialogique) forme un cadre suffisamment général pour établir un pluralisme. Bref, le mémoire est une défense du pluralisme logique. / Starting from the seminal work of Boole, Frege and Russell, the dissertation seeks to clarify the issue of logical pluralism in the era of the proliferation of non-classical logics and the developments in theoretical computer science and proof theory. Two “historical” chapters are scheduled: the first chapter articulate the absolutism of Frege and Russell, taking care to show how it condemns the possibility to consider alternative structures and logics; the fourth chapter describes the path that led Carnap from the adoption of the syntactic method to the formulation of the principle of tolerance, then goes on to display Carnap‟s instrumentalism in philosophy of Logic and mathematics. Opening with the analysis of an intuitive interpretation of linear logic, the second chapter then turns to the establishment of a form of logico-mathematical pluralism with the help of order theory and category theory. The third chapter delineates the playground of revisionism (philosophical positions surrounding the debate between monism and pluralism) and then provides an argument against the thesis that denies the reality of the conflict between rival logics, all this being done by adopting the substructural logic point of view. The fifth chapter shows that each of the three main approaches to the concept of logical consequence (model-theoretic, proof-theoretic and dialogical) supplies a framework sufficiently general to establish pluralism. In short, the dissertation is a defence of logical pluralism.

Logical pluralism from historical perspective - Abstract The plurality of logics is understood as a challenge to seek a deeper understanding of the na- ture and import of logic. Two basic approaches to demarcation of logic are considered, the model-theoretic and the proof-theoretic one. Investigation of the history which led to these two appraoches identifies the postion of logic in Kant's epistemology as crucial for the devel- opment. An analogical development from Kant's conception of geometry to the plurality of geometric theories leads to a holistic view both of geometry and of logic. It furthermore proves essential to understand the pragmatic import of logic. Given the problems tied to the attempts to demarcate logic, inferentialism and logical expressivism are arrived at as jointly provid- ing the most appropriate account. These approaches are developed into a conception which stresses, in line with the historical perspective of the work, the ability of logic to develop.