Showing papers by "Hector J. Levesque published in 2005"

Planning with loops

Abstract: Unlike the case for sequential and conditional planning, much of the work on iterative planning (planning where loops may be needed) leans heavily on theorem-proving. This paper does the following: it proposes a different approach where generating plans is decoupled from verifying them; describes the implementation of an iterative planner based on the situation calculus; presents a few examples illustrating the sorts of plans that can be generated; shows some of the strengths and weaknesses of the approach; and finally sketches the beginnings of a theory, where validation of plans is done offline.

Tractable reasoning with incomplete first-order knowledge in dynamic systems with context-dependent actions

Abstract: A basic reasoning problem in dynamic systems is the projection problem: determine if a formula holds after a sequence of actions has been performed. In this paper, we propose a tractable solution to the projection problem in the presence of incomplete first-order knowledge and contextdependent actions. Our solution is based on a type of progression, that is, we progress the initial knowledge base (KB) wrt the action sequence and answer the query against the resulting KB. The form of reasoning we propose is always logically sound and is also logically complete when the query is in a certain normal form and the agent has complete knowledge about the context of any context-dependent actions.

Semantics for a useful fragment of the situation calculus

Abstract: In a recent paper, we presented a new logic called ES for reasoning about the knowledge, action, and perception of an agent. Although formulated using modal operators, we argued that the language was in fact a dialect of the situation calculus but with the situation terms suppressed. This allowed us to develop a clean and workable semantics for the language without piggybacking on the generic Tarski semantics for first-order logic. In this paper, we reconsider the relation between ES and the situation calculus and show how to map sentences of ES into the situation calculus. We argue that the fragment of the situation calculus represented by ES is rich enough to handle the basic action theories defined by Reiter as well as Golog. Finally, we show that in the full second-order version of ES, almost all of the situation calculus can be accommodated.

Goal change

Abstract: Although there has been much discussion of belief change (e.g., [Gardenfors, 1988; Spohn, 1988]), goal change has not received much attention. In this paper, we propose a method for goal change in the framework of Reiter's [2001] theory of action in the situation calculus [McCarthy and Hayes, 1969; Levesque et al., 1998], and investigate its properties. We extend the framework developed by Shapiro et al. [1998] and Shapiro and Lesperance [2001], where goals and goal expansion were modelled, but goal contraction was not.

Only-knowing: taking it beyond autoepistemic reasoning

Abstract: The idea of only-knowing a collection of sentences has been previously shown to have a close connection with autoepistemic logic. Here we propose a more general account of only-knowing that captures not only autoepistemic logic but default logic as well. This allows us not only to study the properties of default logic in terms of an underlying model of belief, but also the relationship among different forms of nonmonotonic reasoning, all within a classical monotonic logic characterized semantically in terms of possible worlds.

Tractable reasoning in first-order knowledge bases with disjunctive information

Abstract: This work proposes a new methodology for establishing the tractability of a reasoning service that deals with expressive first-order knowledge bases. It consists of defining a logic that is weaker than classical logic and that has two properties: first, the entailment problem can be reduced to the model checking problem for a small number of characteristic models; and second, the model checking problem itself is tractable for formulas with a bounded number of variables. We show this methodology in action for the reasoning service previously proposed by Liu, Lakemeyer and Levesque for dealing with disjunctive information. They show that their reasoning is tractable in the propositional case and decidable in the first-order case. Here we apply the methodology and prove that the reasoning is also tractable in the first-order case if the knowledge base and the query both use a bounded number of variables.

The 2005 AAAI Classic Paper Awards

Abstract: Mitchell and Levesque provide commentary on the two AAAI Classic Paper awards, given at the AAAI-05 conference in Pittsburgh, Pennsylvania. The two winning papers were "Quantifying the Inductive Bias in Concept Learning," by David Haussler, and "Default Reasoning, Nonmonotonic Logics, and the Frame Problem," by Steve Hanks and Drew McDermott.