Three Essays in Intuitionistic Epistemology

We present three papers studying knowledge and its logic from an intuitionistic viewpoint. An Arithmetic Interpretation of Intuitionistic Verification Intuitionistic epistemic logic introduces an epistemic operator to intuitionistic logic which reflects the intended BHK semantics of intuitionism. The fundamental assumption concerning intuitionistic knowledge and belief is that it is the product of verification. The BHK interpretation of intuitionistic logic has a precise formulation in the Logic of Proofs and its arithmetical semantics. We show here that this interpretation can be extended to the notion of verification upon which intuitionistic knowledge is based. This provides the systems of intuitionistic epistemic logic extended by an epistemic operator based on verification with an arithmetical semantics too. This confirms the conception of verification incorporated in these systems reflects the BHK interpretation. Intuitionistic Verification and Modal Logics of Verification The systems of intuitionistic epistemic logic, IEL, can be regarded as logics of intuitionistic verification. The intuitionistic language, however, has expressive limitations. The classical modal language is more expressive, enabling us to formulate various classical principles which make explicit the relationship between intuitionistic verification and intuitionistic truth, implicit in the intuitionistic epistemic language. Within the framework of the arithmetic semantics for IEL we argue that attempting to base a general verificationism on the properties of intuitionistic verification, as characterised by IEL, yields a view of verification stronger than is warranted by its BHK reading. Intuitionistic Knowledge and Fallibilism Fallibilism is the view that knowledge need not guarantee the truth of the proposition known. In the context of a classical conception of truth fallibilism is incompatible with the truth condition on knowledge, i.e. that false propositions cannot be known. We argue that an intuitionistic approach to knowledge yields a view of knowledge which is both fallibilistic and preserves the truth condition. We consider some problems for the classical approach to fallibilism and argue that an intuitionistic approach also resolves them in a manner consonant with the motivation for fallibilism

Complexity Issues in Justification Logic

Justification Logic is an emerging field that studies provability, knowledge, and belief via explicit proofs or justifications that are part of the language. There exist many justification logics closely related to modal epistemic logics of knowledge and belief. Instead of modality □ in pure justification logics, or in addition to modality □ in hybrid logics, which has an existential epistemic reading \u27there exists a proof of F,\u27 all justification logics use constructs t:F, where a justification term t represents a blueprint of a Hilbert-style proof of F. The first justification logic, LP, introduced by Sergei Artemov, was shown to be a justification counterpart of modal logic S4 and serves as a missing link between S4 and Peano arithmetic, thereby solving a long-standing problem of provability semantics for S4 and Int. The machinery of explicit justifications can be used to analyze well-known epistemic paradoxes, e.g. Gettier\u27s examples of justified true belief that can hardly be considered knowledge, and to find new approaches to the concept of common knowledge. Yet another possible application is the Logical Omniscience Problem, which reflects an undesirable property of knowledge as described by modality when an agent knows all the logical consequences of his/her knowledge. The language of justification logic opens new ways to tackle this problem. This thesis focuses on quantitative analysis of justification logics. We explore their decidability and complexity of Validity Problem for them. A closer analysis of the realization phenomenon in general and of one procedure in particular enables us to deduce interesting corollaries about self-referentiality for several modal logics. A framework for proving decidability of various justification logics is developed by generalizing the Finite Model Property. Limitations of the method are demonstrated through an example of an undecidable justification logic. We study reflected fragments of justification logics and provide them with an axiomatization and a decision procedure whose complexity (the upper bound) turns out to be uniform for all justification logics, both pure and hybrid. For many justification logics, we also present lower and upper complexity bounds

Subset Semantics for Justifications

Justification logic is a variant of modal logic where the modal operators are replaced be justification terms. So we deal with formulas like t:A where t is a term denoting some justification that justifies the formula A. There are many justification logics among which the Logic of Proof established by Artemov was the first. However, since a long time the framework of justification logic is also used in a wide range of epistemic logics. In this field justification terms represent reasons to belief or know something. A standard interpretation of a justification term t is then the set of formulas that are supported by the reason t. This thesis establishes in the first part another way to interpret terms, namely as sets of worlds. We use so-called subset models in which t:A is true in a normal world, when the interpretation of t in this world is a subset of the truthset of A. These models are shown to be sound and complete towards a whole family of justification logics, including the Logic of Proof. As is shown in the second part of this thesis, subset models can easily be adapted to model new kinds of justification terms and operations: finer distinctions between several variants of combining justifications, justifications with presumptions, probabilistic evidence. Furthermore, it is shown, how subset models can be used to model dynamic reasoning and forgetting

Relating Justification Logic Modality and Type Theory in Curry–Howard Fashion

This dissertation is a work in the intersection of Justification Logic and Curry--Howard Isomorphism. Justification logic is an umbrella of modal logics of knowledge with explicit evidence. Justification logics have been used to tackle traditional problems in proof theory (in relation to Godel\u27s provability) and philosophy (Gettier examples, Russel\u27s barn paradox). The Curry--Howard Isomorphism or proofs-as-programs is an understanding of logic that places logical studies in conjunction with type theory and -- in current developments -- category theory. The point being that understanding a system as a logic, a typed calculus and, a language of a class of categories constitutes a useful discovery that can have many applications. The applications we will be mainly concerned with are type systems for useful programming language constructs. This work is structured in three parts: The first part is a a bird\u27s eye view into my research topics: intuitionistic logic, justified modality and type theory. The relevant systems are introduced syntactically together with main metatheoretic proof techniques which will be useful in the rest of the thesis. The second part features my main contributions. I will propose a modal type system that extends simple type theory (or, isomorphically, intuitionistic propositional logic) with elements of justification logic and will argue about its computational significance. More specifically, I will show that the obtained calculus characterizes certain computational phenomena related to linking (e.g. module mechanisms, foreign function interfaces) that abound in semantics of modern programming languages. I will present full metatheoretic results obtained for this logic/ calculus utilizing techniques from the first part and will provide proofs in the Appendix. The Appendix contains also information about an implementation of our calculus in the metaprogramming framework Makam. Finally, I conclude this work with a small ``outro\u27\u27, where I informally show that the ideas underlying my contributions can be extended in interesting ways