Defeasible Theories Research Articles

Stable acceptance for mighty knowledge

Drawing on the puzzling behavior of ordinary knowledge ascriptions that embed an epistemic (im)possibility claim, we tentatively conclude that it is untenable to jointly endorse (i) an unfettered classical logic for epistemic language, (ii) the general veridicality of knowledge ascription, and (iii) an intuitive ‘negative transparency’ thesis that reduces knowledge of a simple negated ‘might’ claim to an epistemic claim without modal content. We motivate a strategic trade-off: preserve veridicality and (generalized) negative transparency, while abandoning the general validity of contraposition. We criticize various approaches to incorporating veridicality into domain semantics, a paradigmatic ‘information-sensitive’ framework for capturing negative transparency and, more generally, the non-classical behavior of sentences with epistemic modals. We then present a novel information-sensitive semantics that successfully executes our favored strategy: stable acceptance semantics, extending a vanilla bilateral state-based semantics for epistemic modals with a knowledge operator loosely inspired by the defeasibility theory of knowledge.

Justified belief, knowledge, and the topology of evidence

We propose a new topological semantics for evidence, evidence-based justifications, belief, and knowledge. Resting on the assumption that an agent’s rational belief is based on the available evidence, we try to unveil the concrete relationship between an agent’s evidence, belief, and knowledge via a rich formal framework afforded by topologically interpreted modal logics. We prove soundness, completeness, decidability, and the finite model property for the associated logics, and apply this setting to analyze key epistemological issues such as “no false lemma” Gettier examples, misleading defeaters, undefeated justification versus undefeated belief, as well as the defeasibility theories of knowledge.

Defeasible Reasoning via Datalog¬

AbstractWe address the problem of compiling defeasible theories to Datalog¬ programs. We prove the correctness of this compilation, for the defeasible logic DL(∂||), but the techniques we use apply to many other defeasible logics. Structural properties of DL(∂||) are identified that support efficient implementation and/or approximation of the conclusions of defeasible theories in the logic, compared with other defeasible logics. We also use previously well-studied structural properties of logic programs to adapt to incomplete Datalog¬ implementations.

What Does It Take to Know that You Know?

In some recent work, (Williams Analysis, 75(2), 213–217, 2015, Logos and Episteme, 7(1), 83–94, 2016) John N.Williams defends a new objection to the defeasibility theory of knowledge. But the objection is of wider interest, since Williams also suggests that this style of objection may undermine other theories of knowledge. I distinguish two versions of Williams’ objection. I then show that the first version relies on false conceptual principles, and the second relies on a specific and dubious conception of the goal of the analysis of knowledge.

A Defeasible Logic-based Framework for Contextualizing Deployed Applications

In human to human communication, context increases the ability to convey ideas. However, in human to application and application to application communication, this property is difficult to attain. Context-awareness becomes an emergent need to achieve the goal of delivering more user-centric personalized services, especially in ubiquitous environments. However, there is no agreed-upon generic framework that can be reused by deployed applications to support context-awareness. In this paper, a defeasible logic-based framework for context-awareness is proposed that can enhance the functionality of any deployed application. The nonmonotonic nature of defeasible logic has the capability of attaining justifiable decisions in dynamic environments. Classical defeasible logic is extended by meta-rules to increase its expressiveness power, facilitate its representation of complex multi-context systems, and permit distributed reasoning. The framework is able to produce justified decisions depending on both the basic functionality of the system that is itself promoted by contextual knowledge and any cross-cutting concerns that might be added by different authorities or due to further improvements to the system. Active concerns that are triggered at certain contexts are encapsulated in separate defeasible theories. A proof theory is defined along with a study of its formal properties. The framework is applied to a motivating scenario to approve its feasibility and the conclusions are analyzed using argumentation as an approach of reasoning.

O Paradoxo do Prefácio Generalizado

O paradoxo do prefácio é um presumido contra-exemplo à tese de que a consistência é uma condição necessária da racionalidade ou justificação. O artigo visa desenvolver a ideia, na esteira de uma sugestão Earl Conee (2009), de que as presumidas crenças inconsistentes e racionais podem ser formadas a partir de fontes evidenciais radicalmente diferentes. A execução de tal tarefa está dividida em quatro partes. Na primeira delas, eu apresento sumariamente o paradoxo propriamente dito. Logo em seguida, eu apresento os outros exemplos: o evento, viagem de avião, encontro de colegas e memória parcial. Após isto, tento identificar a estrutura subjacente aos exemplos, e sugiro que a teoria da anulabilidade é uma ferramenta útil para pensar os contraexemplos. Por fim, ofereço uma análise diacrônica de um dos exemplos e tento reforçar a dificuldade de encontrar uma solução satisfatória para o problema.

Is the Truth Condition Superfluous for Defeasibility Theories of Knowledge?

Sarsilabilirlik kuramlari makul bir bilgi tanimina ulasabilmek icin hatali gerekcelendirmelere dayali dogru inanclari eleme stratejisini guder. Farkli felsefeciler kendilerine ozgu sarsilabilirlik anlayislari gelistirmistir. Sarsilmaz gerekcelendirmeler hicbir zaman yanlis inanclarin dayanagi olmadigindan Zagzebski (1994) kati sarsilabilirlik sartinin inancin dogrulugu ile gerekcelendirilmesi arasindaki bagimsizligi ortadan kaldirdigini ifade eder. Bunu takiben, Zagzebski ve yine benzer gorusleri savunan bazi felsefeciler (or. Merricks, 1995) sarsilmaz gerekcelendirmenin zorunlu olarak dogrulugu beraberinde getirdigi sonucuna varir. Bu makalede, sarsilmaz gerekcelendirmelerin her zaman dogru inanca ulasmamasi durumuna ornek gostererek, dogruluk kosulunun luzumsuz olmadigini iddia ediyorum. Iddiama gore metafizik sorulara (ornegin: Tanri var midir?) iliskin inanclar sarsilmaz gerekcelendirmelere sahip olabilirler. Buna ragmen soz konusu gerekcelendirmeler destekledikleri inanclara dogruluk atfedemez.

The Reasoning View and Defeasible Practical Reasoning

According to the Reasoning View about normative reasons, facts about normative reasons for action can be understood in terms of facts about the norms of practical reasoning. I argue that this view is subject to an overlooked class of counterexamples, familiar from debates about Subjectivist theories of normative reasons. Strikingly, the standard strategy Subjectivists have used to respond to this problem cannot be adapted to the Reasoning View. I think there is a solution to this problem, however. I argue that the norms of practical reasoning, like the norms of theoretical reasoning, are characteristically defeasible, in a sense I make precise. Recognizing this property of those norms makes space for a solution to the problem. The resulting view is in a way analogous to the familiar defeasibility theory of knowledge, but it avoids a standard objection to that theory.

Sorte, Virtude, e Anulabilidade Epistêmica

http://dx.doi.org/10.5007/1808-1711.2016v20n2p179 Duncan Pritchard has suggested that anti-luck epistemology and virtue epistemology are the best options to solve the Gettier problem. Nonetheless, there are challenging problems for both of them in the literature. Pritchard holds that his anti-luck virtue epistemology puts together the correct intuitions from both anti-luck epistemology and virtue epistemology and avoids their problems. Contra Pritchard, we believe that there is already a satisfactory theory on offer, namely, the defeasibility theory of knowledge. In this essay we intend (i) to examine Pritchard’s anti-luck virtue epistemology, and (ii) to defend the defeasibility theory of knowledge as an alternative to Pritchard’s theory. We will provide the reader with reasons for believing that the defeasibility theory is better than Pritchard’s theory because the former is more economic and more ecumenical than the latter, since it goes without non-epistemic notions and remains neutral as for the internalism vs. externalism debate.

Forest logging: A trace-based analysis of large rule-based computations

Knowledge representation systems based on the well-founded semantics can offer the degree of scalability required for semantic web applications and make use of ex- pressive semantic features such as Hilog, frame-based rea- soning, and defeasibility theories. Such features can be com- piled into Prolog tabling engines that have good support for indexing and memory management. However, due both to the power of the semantic features and to the declarative style typical of knowledge representation rules, the resources needed for query evaluation can be unpredictable. In such a situation, users need to understand the overall structure of a computation and examine problematic portions of it. This problem, of profiling a computation, differs from debug- ging and justification which address why a given answer was or wasn't derived, and so profiling requires different tech- niques. In this paper we present a trace-based analysis tech- nique called forest logging which has been used to profile large, heavily tabled computations. In forest logging, criti- cal aspects of a tabled computation are logged; afterwards the log is loaded and analyzed. As implemented in XSB, for- est logging slows down execution of practical programs by a constant factor that is often small; and logs containing tens or hundreds of millions of facts can be loaded and analyzed in minutes.

Not knowing you know: a new objection to the defeasibility theory of knowledge

Foley (2012: 93�98) and Turri (2012: 215�19) have recently given objections to the defeasibility theory of propositional knowledge. Here, I give an objection of a quite different stripe by looking at what the theory must say about knowing that you know. I end with some remarks on how this objection relates to rival theories and how this might be a worry for some of these. What might be called the early defeasibility theory of propositional knowledge may be formulated as follows. You know that p just in case you have a justified true belief that p and there is no defeater D of your justification for believing that p where This theory deals nicely with Gettier cases such Lamborghini, offered recently by Turri (2012: 215) and adapted as follows. Wishing to impress you, your student tells you that he owns a Lamborghini and affords you excellent evidence that does (he gives you a ride, tells you about the purchase, parks it in his garage and so on). Reasoning that he behaves just like a Lamborghini-owner, you believe on that basis that he owns one. Unbeknownst to you, he has promised to take care of his cousin�s Lamborghini while she is on a trip. But unbeknownst to you and him, she has died just after this promise, leaving him the Lamborghini in an unopened will.Most would agree that you have a justified belief that your student owns a Lamborghini that is luckily true and thus does not amount to knowledge. Because you justify that belief by inducing it from the truth that he behaves just like �

Instantiating rule-based defeasible theories in abstract dialectical frameworks and beyond

Instantiating rule-based defeasible theories in abstract dialectical frameworks and beyond

Defeasibility and Gettierization: A Reminder

For some of us, the defeasibility theory of knowledge remains the most plausible approach to the Gettier Problem. Epistemological fashion and faded memories notwithstanding, persuasive objections to the theory are very hard to find. The most impressive of those objections to the theory that have hitherto gone unanswered are examined and rejected here. These are objections put forward by Richard Feldman, Richard Foley, and John Turri. While these are all interesting, the objection recently put forward by Turri is, we think, by far, the most serious threat to the theory that we have seen in a long time. A successful reply to it requires a surprising amount of care, as it turns out. If tenable, Turri's objection deals a devastating blow to the theory developed by Roderick Chisholm, Keith Lehrer, Peter Klein, Marshall Swain, Risto Hilpinen, John Pollock, and Paul Moser, among others. Under scrutiny, however, the threat proves illusory. It results from inattention to a crucial, but relatively subtle, aspect of the theory. Interestingly, there is only one source in the defeasibilist literature for a precise account of this crucial feature of the theory: one of the most neglected passages in Peter Klein's work on the issue. That crucial feature is put under the spotlight here. Our response to three major objections to the defeasibility theory requires a brief introduction to the theory as an anti-Gettier weapon, an introduction aimed at countering the numbing simplicity that characterizes most introductions to the topic. Following this brief introduction, those three objections are tackled. We conclude, on that basis, that anybody who fails to notice how resilient the defeasibility theory has proven to be for the last fifty years has a seriously deficient understanding of the current state of play in the debate over the Gettier Problem.

THERE IS NO KNOWLEDGE FROM FALSEHOOD

AbstractA growing number of authors defend putative examples of knowledge from falsehood (KFF), inferential knowledge based in a critical or essential way on false premises, and they argue that KFF has important implications for many areas of epistemology (whether evidence can be false, the Gettier debate, defeasibility theories of knowledge, etc.). I argue, however, that there is no KFF, because in any supposed example either the falsehood does not contribute to the knowledge or the subject lacks knowledge. In particular, I show that if the subject actually has knowledge in putative KFF cases, then there is always a veridical evidential path meeting the basing conditions that accounts for her knowledge; if there is no such path, then the subject is in a type of Gettier case. All the recent arguments that rely on KFF are therefore based on a mistake.

Incremental Tabling in Support of Knowledge Representation and Reasoning

AbstractResolution-based Knowledge Representation and Reasoning (KRR) systems, such as Flora-2, Silk or Ergo, can scale to tens or hundreds of millions of facts, while supporting reasoning that includes Hilog, inheritance, defeasibility theories, and equality theories. These systems handle the termination and complexity issues that arise from the use of these features by a heavy use of tabled resolution. In fact, such systems table by default all rules defined by users, unless they are simple facts.Performing dynamic updates within such systems is nearly impossible unless the tables themselves can be made to react to changes. Incremental tabling as first implemented in XSB (Saha 2006) partially addressed this problem, but the implementation was limited in scope and not always easy to use. In this paper, we introducetransparent incremental tablingwhich at the semantic level supports updates in the 3-valued well-founded semantics, while guaranteeing full consistency of all tabled queries. Transparent incremental tabling also has significant performance improvements over previous implementations, including lazy recomputation, and control over the dependency structures used to determine how tables are updated.

Interdefinability of defeasible logic and logic programming under the well-founded semantics

AbstractWe provide a method of translating theories of Nute's defeasible logic into logic programs, and a corresponding translation in the opposite direction. Under certain natural restrictions, the conclusions of defeasible theories under the ambiguity propagating defeasible logic ADL correspond to those of the well-founded semantics for normal logic programs, and so it turns out that the two formalisms are closely related. Using the same translation of logic programs into defeasible theories, the semantics for the ambiguity blocking defeasible logic NDL can be seen as indirectly providing an ambiguity blocking semantics for logic programs. We also provide antimonotone operators for both ADL and NDL, each based on the Gelfond–Lifschitz (GL) operator for logic programs. For defeasible theories without defeaters or priorities on rules, the operator for ADL corresponds to the GL operator and so can be seen as partially capturing the consequences according to ADL. Similarly, the operator for NDL captures the consequences according to NDL, though in this case no restrictions on theories apply. Both operators can be used to define stable model semantics for defeasible theories.

Contextual Defeasible Logic and Its Application to Ambient Intelligence

The imperfect nature of context in ambient intelligence environments, and the special characteristics of the entities that possess and share the available context information render contextual reasoning a very challenging task. The accomplishment of this task requires formal models that handle the involved entities as autonomous logic-based agents, and provide methods for handling the imperfect and distributed nature of context. We propose a solution based on the multi-context systems (MCS) formalism, in which local context knowledge of ambient agents is encoded in rule theories (contexts), and information flow between agents is achieved through mapping rules, associating concepts used by different contexts. To handle the imperfect nature of context, we extend MCS with non-monotonic features-local defeasible theories, defeasible mappings, and a preference ordering on the system contexts. In this paper, we present the novel representation model, called contextual defeasible logic, describe how its elements are used to derive distributed conclusions through a proof theory, and propose an algorithm for distributed query evaluation that implements the proof theory of contextual defeasible logic. The application of the proposed approach in a scenario from the ambient intelligence domain demonstrates how its distinct features overcome the challenges imposed by the special characteristics of ambient intelligence environments.

Defeasible Contextual Reasoning with Arguments in Ambient Intelligence

The imperfect nature of context in Ambient Intelligence environments and the special characteristics of the entities that possess and share the available context information render contextual reasoning a very challenging task. The accomplishment of this task requires formal models that handle the involved entities as autonomous logic-based agents and provide methods for handling the imperfect and distributed nature of context. This paper proposes a solution based on the Multi-Context Systems paradigm in which local context knowledge of ambient agents is encoded in rule theories (contexts), and information flow between agents is achieved through mapping rules that associate concepts used by different contexts. To handle imperfect context, we extend Multi-Context Systems with nonmonotonic features: local defeasible theories, defeasible mapping rules, and a preference ordering on the system contexts. On top of this model, we have developed an argumentation framework that exploits context and preference information to resolve potential conflicts caused by the interaction of ambient agents through the mappings, and a distributed algorithm for query evaluation.

ANGLE: An autonomous, normative and guidable agent with changing knowledge

Some emerging computing systems (especially autonomic computing systems) raise several challenges to autonomous agents, including (1) how to reflect the dynamics of business requirements, (2) how to coordinate with external agents with sufficient level of security and predictability, and (3) how to perform reasoning with dynamic and incomplete knowledge, including both informational knowledge (observations) and motivational knowledge (for example, policy rules and contract rules). On the basis of defeasible logic and argumentation, this paper proposes an autonomous, normative and guidable agent model, called ANGLE, to cope with these challenges. This agent is established by combining beliefs–desires–intentions (BDI) architecture with policy-based method and the mechanism of contract-based coordination. Its architecture, knowledge representation, as well as reasoning and decision-making, are presented in this paper. ANGLE is characteristic of the following three aspects. First, both its motivational knowledge and informational knowledge are changeable, and allowed to be incomplete, inconsistent/conflicting. Second, its knowledge is represented in terms of extended defeasible logic with modal operators. Different from the existing defeasible theories, its theories (including belief theory, goal theory and intention theory) are dynamic (called dynamic theories), reflecting the variations of observations and external motivational knowledge. Third, its reasoning and decision-making are based on argumentation. Due to the dynamics of underlying theories, argument construction is not a monotonic process, which is different from the existing argumentation framework where arguments are constructed incrementally.

Well-founded semantics for defeasible logic

Fixpoint semantics are provided for ambiguity blocking and propagating variants of Nute’s defeasible logic. The semantics are based upon the well-founded semantics for logic programs. It is shown that the logics are sound with respect to their counterpart semantics and complete for locally finite theories. Unlike some other nonmonotonic reasoning formalisms such as Reiter’s default logic, the two defeasible logics are directly skeptical and so reject floating conclusions. For defeasible theories with transitive priorities on defeasible rules, the logics are shown to satisfy versions of Cut and Cautious Monotony. For theories with either conflict sets closed under strict rules or strict rules closed under transposition, a form of Consistency Preservation is shown to hold. The differences between the two logics and other variants of defeasible logic—specifically those presented by Billington, Antoniou, Governatori, and Maher—are discussed.