Justification Logic Research Articles

A realization theorem for the Gödel-Löb provability logic

We present a new justification logic corresponding to the Gödel-Löb provability logic and prove the realization theorem connecting these two systems in such a way that all the realizations provided in the theorem are normal. Bibliography: 9 titles.

Labeled sequent calculus for justification logics

Justification logics are modal-like logics that provide a framework for reasoning about justifications. This paper introduces labeled sequent calculi for justification logics, as well as for combined modal-justification logics. Using a method due to Sara Negri, we internalize the Kripke-style semantics of justification and modal-justification logics, known as Fitting models, within the syntax of the sequent calculus to produce labeled sequent calculi. We show that all rules of these systems are invertible and the structural rules (weakening and contraction) and the cut rule are admissible. Soundness and completeness are established as well. The analyticity for some of our labeled sequent calculi are shown by proving that they enjoy the subformula, sublabel and subterm properties. We also present an analytic labeled sequent calculus for S4LPN based on Artemov–Fitting models.

Modal logics, justification logics, and realization

Justification logics connect with modal logics, replacing unstructured modal operators with justification terms explicitly representing interdependence and flow of reasoning. The number of justification logics quickly grew from an initial single instance to a handful to about a dozen examples. In this paper we provide very general, though partly non-constructive, methods that cover all previous examples, and extend to an infinite family of modal logics. The full range of the phenomenon is not known. The extent to which constructive methods apply is also not known, but it is related to the availability of cut-free proof methods for modal logics.

A new framework for justification logic

The logic of justification provides an in-depth analysis of the epistemic states of an agent. This paper aims at solving some of the problems to which the common interpretation of the operators of justification logic is subject by providing a framework in which a crucial distinction between potential and explicit justifiers is exploited. The paper is subdivided into three sections. The first section offers an introduction to a basic system LJ of justification logic and to the problems concerning its interpretation. In the second section, three new systems of justification logic are introduced and characterised with respect to an appropriate semantics. The final section shows why the highlighted problems do not afflict the new systems and how it is possible to interpret LJ in the new framework.

A logic of plausible justifications

In this work, we combine features from Justification Logics and Logics of Plausibility-Based Beliefs to build a logic for Multi-Agent Systems where each agent can explicitly state his justification for believing in a given sentence. Our logic is a normal modal logic based on the standard Kripke semantics, where we provide a semantic definition for the evidence terms and define the notion of plausible evidence for an agent, based on plausibility relations in the model. As we deal with beliefs, justifications can be faulty and unreliable. In our logic, agents can disagree not only over whether a sentence is true or false, but also on whether some evidence is a valid justification for a sentence or not. After defining our logic and its semantics, we provide a strongly complete axiomatic system for it, show that it has the finite model property, analyze the complexity of its Model-Checking Problem and show that its Satisfiability Problem has the same complexity as the one from basic modal logics. Thus, this logic seems to be a good first step for the development of a dynamic logic that can model the processes of argumentation and debate in Multi-Agent Systems.

Justification logic and audited computation

Justification logic and audited computation

First steps towards probabilistic justification logic

In this article, we introduce the probabilistic justification logic PJ, a logic in which we can reason about the probability of justification statements. We present its syntax and semantics, and establish a strong completeness theorem. Moreover, we investigate the relationship between PJ and the logic of uncertain justifications.

Practical Extraction of Evidence Terms From Common-knowledge Reasoning

Knowledge, belief, and evidence are fundamental notions which appear in a wide range of areas. Over the last decade epistemic reasoning with justifications has broadened even more the scope of applications of epistemic logic as agents gained the ability to not only reason about epistemic states of knowledge and belief of agents, but also to track their justifications and to sort those which are pertinent to given facts and sufficient for epistemic conclusions.This paper extends realization algorithm for S4-to-LP case to S4nJ-to-S4nLP case. It converts cut-free derivations in S4nJ into derivations in the corresponding Justification Logic S4nLP where witnesses of knowledge, the justification terms, are recovered for all instances of justified common knowledge. The algorithm was implemented in the MetaPRL framework and was tested on several well-known epistemic puzzles, such as Muddy Children, Surprise Examination Paradox, etc.

Lógicas da justificação e quase-verdade

Two kinds of justification logics are studied. Then, this article shows how the notion of quasi-truth can be defined in these systems.

Realizing public announcements by justifications

Modal public announcement logics study how beliefs change after public announcements. However, these logics cannot express the reason for a new belief. Justification logics fill this gap since they can formally represent evidence and justifications for an agent's belief. We present OPAL(K) and JPAL(K), two alternative justification counterparts of Gerbrandy–Groeneveld's public announcement logic PAL(K). We show that PAL(K) is the forgetful projection of both OPAL(K) and JPAL(K). We also establish that JPAL(K) partially realizes PAL(K). The question whether a similar result holds for OPAL(K) is still open.

A complexity question in justification logic

Bounds for the computational complexity of major justification logics were found in papers by Buss, N. Krupski, Kuznets, and Milnikel: logics J, J4, JT, LP and JD were established to be Σ2p-complete. A corresponding lower bound is also known for JD4, the system that includes the consistency axiom and positive introspection. However, no upper bound has been established so far for this logic. Here, the missing upper bound for the complexity of JD4 is established through an alternating algorithm. It is shown that using Fitting models of only two worlds is adequate to describe JD4; this helps to design an effective tableau procedure and essentially is what distinguishes the new algorithm from existing ones.

Beyond Flows, Fluids and Networks: Social Theory and the Fetishism of the Global Informational Economy

In this paper we critically explore some of the arguments made by those social theorists who claim that we live in a new global economy defined by informational and technological flows, fluids and networks. By recourse to Marx's concept of fetishism we argue that these theorists often fetishise the very social changes in the global economy they are trying to describe. As a result, they articulate a ‘flat ontology’ of concrete and contingent relations that mistakenly claims to capture the most important dynamics of global capitalism. We reject this approach, preferring instead to see global capitalism as a dialectical flux between concrete and more abstract processes. These critical points are developed by drawing on Marxism to explore how these social theorists often reproduce unhelpful dualisms in social theory, how they fetishise technology, how some of their arguments run parallel to a management justification logic of the market world and finally how they present a limited explanation of global finance.

J-Calc: A Typed Lambda Calculus for Intuitionistic Justification Logic

In this paper we offer a system J-Calc that can be regarded as a typed λ-calculus for the {→,⊥} fragment of Intuitionistic Justification Logic. We offer different interpretations of J-Calc, in particular, as a two phase proof system in which we proof check the validity of deductions of a theory T based on deductions from a stronger theory T′ and computationally as a type system for separate compilations. We establish some first metatheoretic results.

Prehistoric Graph in Modal Derivations and Self-Referentiality

By terms-allowed-in-formulas capacity, Artemov's Logic of Proofs LP Artemov includes self-referential formulas of the form t:?(t) that play a crucial role in the realization of modal logic S4 in LP, and in the Brouwer---Heyting---Kolmogorov semantics of intuitionistic logic via LP. In an earlier work appeared in the Proceedings of CSR 2010 the author defined prehistoric loop in a sequent calculus of S4, and verified its necessity to self-referentiality in S4?LP realization. In this extended version we generalize results there to T and K4, two modal logics smaller than S4 that yet call for self-referentiality in their realizations into corresponding justification logics JT and J4.

The Logic of Uncertain Justifications

In Artemovʼs Justification Logic, one can make statements interpreted as “t is evidence for the truth of formula F.” We propose a variant of this logic in which one can say “I have degree r of confidence that t is evidence for the truth of formula F.” After defining both an axiomatic approach and a semantics for this Logic of Uncertain Justifications, we will prove the usual soundness and completeness theorems.

Logical omniscience as infeasibility

Logical theories for representing knowledge are often plagued by the so-called Logical Omniscience Problem. The problem stems from the clash between the desire to model rational agents, which should be capable of simple logical inferences, and the fact that any logical inference, however complex, almost inevitably consists of inference steps that are simple enough. This contradiction points to the fruitlessness of trying to solve the Logical Omniscience Problem qualitatively if the rationality of agents is to be maintained. We provide a quantitative solution to the problem compatible with the two important facets of the reasoning agent: rationality and resource boundedness. More precisely, we provide a test for the logical omniscience problem in a given formal theory of knowledge. The quantitative measures we use are inspired by the complexity theory. We illustrate our framework with a number of examples ranging from the traditional implicit representation of knowledge in modal logic to the language of justification logic, which is capable of spelling out the internal inference process. We use these examples to divide representations of knowledge into logically omniscient and not logically omniscient, thus trying to determine how much information about the reasoning process needs to be present in a theory to avoid logical omniscience.

The logic of justified belief, explicit knowledge, and conclusive evidence

We present a complete, decidable logic for reasoning about a notion of completely trustworthy (“conclusive”) evidence and its relations to justifiable (implicit) belief and knowledge, as well as to their explicit justifications. This logic makes use of a number of evidence-related notions such as availability, admissibility, and “goodness” of a piece of evidence, and is based on an innovative modification of the Fitting semantics for Artemovʼs Justification Logic designed to preempt Gettier-type counterexamples. We combine this with ideas from belief revision and awareness logics to provide an account for explicitly justified (defeasible) knowledge based on conclusive evidence that addresses the problem of (logical) omniscience.

Distributed Knowledge Justification Logics

Justification logics are a family of modal epistemic logics which enables us to reasoning about justifications and evidences. In this paper, we introduce evidence-based multi-agent distributed knowledge logics, called distributed knowledge justification logics. The language of our justification logics contain evidence-based knowledge operators for individual agents [InlineEquation not available: see fulltext.] and for distributed knowledge [InlineEquation not available: see fulltext.], which are interpreted respectively as "t is a justification that agent i accepts for F", and "t is a justification that all agents accept for F if they combine their knowledge and justifications". We study basic properties of our logics and prove the conservativity of distributed knowledge justification logics over multi-agent justification logics. We present Kripke style models, pseudo-Fitting and Fitting models, as well as Mkrtychev models (single world Fitting models) and prove soundness and completeness theorems. We also find a class of Fitting models which satisfies the principle of full communication. Finally, we establish the realization theorem, which states that distributed knowledge justification logics can be embedded into the modal distributed knowledge logics, and vise versa.

Realization using the model existence theorem

Justification logics refine modal logics by replacing the usual necessity operator with a family of justification terms that embody reasons for the necessity of a formula, rather than simply recording the fact of necessity. Many common modal logics have justification counterparts. The connection between a modal logic and its justification counterpart is through a Realization Theorem, which says that modal operators can be replaced in a precise way with justification terms so that modal theorems turn into justification logic theorems. In this paper we present a new proof of Realization. We use the familiar machinery of consistency properties to prove a weak version, we call it Quasi-Realization. Then we show how to convert Quasi-Realizations into Realizations proper. Unlike most other treatments in the literature, the work here is not propositional, but firstorder. Only one modal/justification logic is discussed, but the methods easily extend to other standard systems.

Justification logic, inference tracking, and data privacy

Internalization is a key property of justification logics. It states that justification logics internalize their own notion of proof which is essential for the proof of the realization theorem. The aim of this note is to show how to make use of internalization to track where an agent’s knowledge comes from and how to apply this to the problem of data privacy.