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Modal logics with possible worlds semantics can be used to represent mental states such as belief, goal, and intention, allowing one to formally describe the rational behavior of agents. Agent??s beliefs and goals are typically represented in these logics by primitive modal operators. However, the representation of agent??s intentions varies greatly between theories. Some logics characterize intention as a primitive operator, while others define intention in terms of more primitive constructs. Taking the latter approach is a theory due to Philip Cohen and Hector Levesque, under which intentions are a special form of commitment or persistent goal. The theory has motivated theories of speech acts and joint intention and innovative applications in multiagent systems and industrial robotics. However, Munindar Singh shows the theory to have certain logical inconsistencies and permit certain absurd scenarios. This thesis presents a modification of the theory that preserves the desirable aspects of the original while addressing the criticism of Singh. This is achieved by the introduction of an additional operator describing the achievement of expectations, refined assumptions, and new defi- nitions of intention. The modified theory gives a cogent account of the rational balance between agents?? action and deliberation, and suggests the use of meansends reasoning in agent implementations. A rule-based reasoner in Jess facilitates evaluation of the predictiveness and intuitiveness of the theory, and provides a prototypical agent based on the theory.

The main content of this thesis concerns a tableau method for solving the satisfiability problem for the modal μ-calculus. A sound and complete tableau system for the modal μ-calculus is given. Since every tableau in such tableau system is finite and bounded by the length of the formula, the tableau system may be used as a decision procedure for determining the satisfiability of the formula. An alternative proof of the small model property is obtained: every satisfiable formula has a model of size singleexponential in the length of the formula. Contrary to known proofs in literature, the results presented here do not rely on automata theory. Two simplifications of the tableau system are given. One is for the class of aconjunctive formulae. The resulting tableau system has been used to prove the completeness of Kozen’s axiomatisation with respect to the aconjunctive fragment of the modal μ- calculus. Another is for the formulae in the class Πμ 2 . In addition to the tableau method, the thesis explores some model-surgery techniques with the aim that such techniques may be used to directly prove the small model theorem. The techniques obtained so far have been used to show the small model property for Πμ 2 -formulae and for formulae with linear models.

A concurrent system is a computer system with components that run in parallel and interact with each other. Such systems are ubiquitous and are notably responsible for supporting the infrastructure for transport, commerce and entertainment. They are very difficult to design and implement correctly: many different modeling languages and verification techniques have been devised to reason about them and verifying their correctness. However, existing languages and techniques can only express a limited range of systems and properties. In this dissertation, we address some of the shortcomings of established models and theories in four ways: by introducing a general modal logic, extending a modelling language with types and a more general operation, providing an automated tool support, and adapting an established behavioural type theory to specify and verify systems with unreliable communication. A modal logic for transition systems is a way of specifying properties of concurrent system abstractly. We have developed a modal logic for nominal transition systems. Such systems are common and include the pi-calculus and psi-calculi. The logic is adequate for many process calculi with regard to their behavioural equivalence even for those that no logic has been considered, for example, CCS, the pi-calculus, psi-calculi, the spi-calculus, and the fusion calculus. The psi-calculi framework is a parametric process calculi framework that subsumes many existing process calculi. We extend psi-calculi with a type system, called sorts, and a more general notion of pattern matching in an input process. This gives additional expressive power allowing us to capture directly even more process calculi than was previously possible. We have reestablished the main results of psi-calculi to show that the extensions are consistent. We have developed a tool that is based on the psi-calculi, called the psi-calculi workbench. It provides automation for executing the psi-calculi processes and generating a witness for a behavioural equivalence between processes. The tool can be used both as a library and as an interactive application. Lastly, we developed a process calculus for unreliable broadcast systems and equipped it with a binary session type system. The process calculus captures the operations of scatter and gather in wireless sensor and ad-hoc networks. The type system enjoys the usual property of subject reduction, meaning that well-typed processes reduce to well-typed processes. To cope with unreliability, we also introduce a notion of process recovery that does not involve communication. This is the first session type system for a model with unreliable communication.

O presente trabalho tem como objetivo proporcionar aos estudantes que precisem da lógica modal como ferramenta um texto conciso mas suficientemente completo. Embora seja um texto de cunho matemático, procura-se manter o equilíbrio entre os conceitos matemáticos e suas motivações filosóficas, pela crença de que tal equilíbrio é essencial para situar o pensamento em um texto introdutório. O primeiro capítulo começa com um breve histórico filosófico e trabalha os conceitos fundamentais de um ponto de vista sintático. O segundo capítulo retoma os conceitos do primeiro capítulo de um ponto de vista semântico e faz a conexão entre sintaxe e semântica. O terceiro capítulo trabalha o conceito de bissimulação e apresenta ferrametas que abrirão caminho para aplicações. / The goal of this work is to provide the studens who need to deal with modal logic as a tool with a text which might be concise but complete enough at the same time. Although this is a rather mathematical text, an effort is made in order to maintain the equilibrium between mathematical concepts and their philosophical origins for believing this equilibium is of great importance for clarifing the ideas in a work for beginners. The first chapter starts with a brief historical approach of logic and then discusses some fundamental concepts from a syntactical point of view. The second chapter discusses the same concepts from a semantical point of view and links syntact and semantics. The third chapter presents the concept of bisimulation and paves the way for working with applications.

O presente trabalho tem como objetivo proporcionar aos estudantes que precisem da lógica modal como ferramenta um texto conciso mas suficientemente completo. Embora seja um texto de cunho matemático, procura-se manter o equilíbrio entre os conceitos matemáticos e suas motivações filosóficas, pela crença de que tal equilíbrio é essencial para situar o pensamento em um texto introdutório. O primeiro capítulo começa com um breve histórico filosófico e trabalha os conceitos fundamentais de um ponto de vista sintático. O segundo capítulo retoma os conceitos do primeiro capítulo de um ponto de vista semântico e faz a conexão entre sintaxe e semântica. O terceiro capítulo trabalha o conceito de bissimulação e apresenta ferrametas que abrirão caminho para aplicações. / The goal of this work is to provide the studens who need to deal with modal logic as a tool with a text which might be concise but complete enough at the same time. Although this is a rather mathematical text, an effort is made in order to maintain the equilibrium between mathematical concepts and their philosophical origins for believing this equilibium is of great importance for clarifing the ideas in a work for beginners. The first chapter starts with a brief historical approach of logic and then discusses some fundamental concepts from a syntactical point of view. The second chapter discusses the same concepts from a semantical point of view and links syntact and semantics. The third chapter presents the concept of bisimulation and paves the way for working with applications.

Things could have been different, but could it also have been different what things there are? It is natural to think so, since I could have failed to be born, and it is natural to think that I would then not have been anything. But what about entities like propositions, properties and relations? Had I not been anything, would there have been the property of being me? In this thesis, I formally develop and assess views according to which it is both contingent what individuals there are and contingent what propositions, properties and relations there are. I end up rejecting these views, and conclude that even if it is contingent what individuals there are, it is necessary what propositions, properties and relations there are. Call the view that it is contingent what individuals there are first-order contingentism, and the view that it is contingent what propositions, properties and relations there are higher-order contingentism. I bring together the three major contributions to the literature on higher-order contingentism, which have been developed largely independently of each other, by Kit Fine, Robert Stalnaker, and Timothy Williamson. I show that a version of Stalnaker's approach to higher-order contingentism was already explored in much more technical detail by Fine, and that it stands up well to the major challenges against higher-order contingentism posed by Williamson. I further show that once a mistake in Stalnaker's development is corrected, each of his models of contingently existing propositions corresponds to the propositional fragment of one of Fine's more general models of contingently existing propositions, properties and relations, and vice versa. I also show that Stalnaker's theory of contingently existing propositions is in tension with his own theory of counterfactuals, but not with one of the main competing theories, proposed by David Lewis. Finally, I connect higher-order contingentism to expressive power arguments against first-order contingentism. I argue that there are intelligible distinctions we draw with talk about "possible things", such as the claim that there are uncountably many possible stars. Since first-order contingentists hold that there are no possible stars apart from the actual stars, they face the challenge of paraphrasing such talk. I show that even in an infinitary higher-order modal logic, the claim that there are uncountably many possible stars can only be paraphrased if higher-order contingentism is false. I therefore conclude that even if first-order contingentism is true, higher-order contingentism is false.

Abstract This thesis takes an algorithmic perspective on the correspondence between modal and hybrid logics on the one hand, and first-order logic on the other. The canonicity of formulae, and by implication the completeness of logics, is simultaneously treated. Modal formulae define second-order conditions on frames which, in some cases, are equiv- alently reducible to first-order conditions. Modal formulae for which the latter is possible are called elementary. As is well known, it is algorithmically undecidable whether a given modal formula defines a first-order frame condition or not. Hence, any attempt at delineating the class of elementary modal formulae by means of a decidable criterium can only consti- tute an approximation of this class. Syntactically specified such approximations include the classes of Sahlqvist and inductive formulae. The approximations we consider take the form of algorithms. We develop an algorithm called SQEMA, which computes first-order frame equivalents for modal formulae, by first transforming them into pure formulae in a reversive hybrid language. It is shown that this algorithm subsumes the classes of Sahlqvist and inductive formulae, and that all formulae on which it succeeds are d-persistent (canonical), and hence axiomatize complete normal modal logics. SQEMA is extended to polyadic languages, and it is shown that this extension succeeds on all polyadic inductive formulae. The canonicity result is also transferred. SQEMA is next extended to hybrid languages. Persistence results with respect to discrete general frames are obtained for certain of these extensions. The notion of persistence with respect to strongly descriptive general frames is investigated, and some syntactic sufficient conditions for such persistence are obtained. SQEMA is adapted to guarantee the persistence with respect to strongly descriptive frames of the hybrid formulae on which it succeeds, and hence the completeness of the hybrid logics axiomatized with these formulae. New syntactic classes of elementary and canonical hybrid formulae are obtained. Semantic extensions of SQEMA are obtained by replacing the syntactic criterium of nega- tive/positive polarity, used to determine the applicability of a certain transformation rule, by its semantic correlate—monotonicity. In order to guarantee the canonicity of the formulae on which the thus extended algorithm succeeds, syntactically correct equivalents for monotone formulae are needed. Different version of Lyndon’s monotonicity theorem, which guarantee the existence of these equivalents, are proved. Constructive versions of these theorems are also obtained by means of techniques based on bisimulation quantifiers. Via the standard second-order translation, the modal elementarity problem can be at- tacked with any second-order quantifier elimination algorithm. Our treatment of this ap- proach takes the form of a study of the DLS-algorithm. We partially characterize the for- mulae on which DLS succeeds in terms of syntactic criteria. It is shown that DLS succeeds in reducing all Sahlqvist and inductive formulae, and that all modal formulae in a single propositional variable on which it succeeds are canonical.

Neste trabalho, estudamos algumas formas de combinar sistemas de Lógica Modal, analisando quando a combinação preserva propriedades como correção, completude e decidibilidade. Estendemos um estudo já realizado sobre combinações de sistemas de Lógica Modal Normal para sistemas de Lógica Modal Não-normal. O principal resultado deste trabalho é a preservação de completude da aplicação externa de um sistema de Lógica Modal Não-normal M em um sistema lógico L. Outro resultado importante é um exemplo de interação forte na combinação independente, ou fusão, de dois sistemas de Lógica Modal Não-normal. / In this work, we study a few ways of combining Modal Logic systems, analysing when the combination preserves properties like soundness, completeness and decidability. We extend a study of the combination of Normal Modal Logic systems to Non-normal Modal Logic systems. The main result of this work is the completeness preservation in the external application of a Non-normal Modal Logic system M to a logic system L. Another important result is an example of strong interations arising in the fusion of two Non-normal Modal Logic system.

Šiame darbe mes apžvelgėme modalumo logiką S4 bei kvantorinę modalumo logiką S4. Taip pat jų taisykles, aksiomas ir naudojamus skaičiavimus. Pateikėme kelias sekvencijų išvedimo pavyzdžių. Taip pat, apžvelgėme kai kurių, atskirų šių logikų klasių išsprendžiamumą. Taipogi šiame darbe buvo nagrinėjama labai įdomi tema – išsprendžiamumo klasių formavimas, naudojantis formulių transformavimų į klasikinę predikatų logiką. / Logic is the branch of mathematics that deals with the formal principles, methods and criteria of validity of inference, reasoning and knowledge. Logic is concerned with what is true and how we can know whether something is true. This involves the formalization of logical arguments and proofs in terms of symbols representing propositions and logical connectives. The goal of this work is to learn more about modal logic S4 and to consider some it decidable classes of formulas. It’s important, because decidable classes helps the substantiation of different formulas. In this work we will consider the formulas of modal logic without functional symbols.