Temporalised normative positions in defeasible logic

Abstract

We propose a computationally oriented non-monotonic multi-modal logic arising from the combination of temporalised agency and temporalised normative positions. We argue about the defeasible nature of these notions and then we show how to represent and reason with them in the setting of Defeasible Logic. MOTIVATION AND LAYOUTAn increasing number of works on agents assume that in artificial societies normative concepts may play a decisive role, allowing for the flexible co-ordination of autonomous agents [5,20,15]. In particular, it seems crucial to model organisations of agents in terms of policy-based normative systems; accordingly an organisation should be characterised by specifying the normative positions relevant to design its structure. These positions include duties, permissions, but also powers, as for instance powers of creating further normative positions on the head of other agents. In this paper we will develop a formal machinery to account for several fundamental concepts that are required to model policy-based normative systems. These concepts will be embedded in a nonmonotonic and computationally-oriented framework based on Defeasible Logic (DL).From the conceptual standpoint, it is well known that the basic deontic qualifications (obligatory, forbidden, permitted and facultative) are not sufficient to capture all fundamental normative notions, such as the concepts of rights and power. For this reason, we will first provide an account in DL of the notion of other-directed obligation [14] to express, e.g., the first Hohfeldian set of fundamental concepts: duty, right, noright, and privilege. Second, we shall focus on different kinds of normative conditionals. This will enable us to characterise also the idea of normative power and articulate many potestative concepts such as the second Hohfeldian set of concepts: power, liability (or, subjection, to avoid confusion with the notion of liability, as used in tort law), disability, and immunity. * This work was partially supported by Australia Research Council under Discovery Project No. DP0558854 on "A Formal Approach to Resource Allocation in Web Service Oriented Composition in Open Marketplaces". Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. In general, we shall see how the analysis of normative conditionality and normative positions has to include temporal aspects in order to capture a number of nuances in the perspective of framing a fine-grained classification of such concepts. In fact, normative determinations take place along the axis of time: Normative preconditions hold or happen in certain temporally characterised occasions, and consequently their effects too hold or happen wi...

Rule-Based Agents in Temporalised Defeasible Logic

“…Here we concentrate only on the proof conditions to derive defeasible persistence of both rules with belief mode, and literals. The proof conditions given here are extensions of those given in [11] for the temporal aspects and can be used for goals and planning as in [6,5]. The proof conditions missing in this paper can be obtained from the corresponding conditions of [11,6,5] using the same intuition on which the proof conditions we are going to show illustrate.…”

“…The logical framework is based on DL, which is a simple and flexible sceptical nonmonotonic formalism that has proven able to represent various aspects of non-monotonic reasoning. We extend here the machinery developed in [11,6,5] to represent temporalised motivational attitudes of agents.…”

“…In [11] we showed how to derive temporal literals in a DL framework. But this is of limited use and what we need is a way to derive temporal rules.…”

“…But this is of limited use and what we need is a way to derive temporal rules. In this section we extend the framework developed in [11,6,5] with temporal rules with a view point. What this means is that we can reason about temporal rules that are valid at a particular instant of time.…”

“…Put in Bratman's terms to reason about intentions/goals/plans at t and decide for t. A complete solution would require the addition of a temporal variable to allow reasoning about the deliberation process after each time round [9]. A formal framework like the one developed in this paper is useful in the domain of legal information systems to reason about normative conditionals [11,10]. Another domain wherein the framework could be useful is with regard to policy-based rationality as outlined by Bratman [2] in his pursuit for a temporally extended rational agency.…”