Temporal Projection and Explanation

We propose a solution to problems involving temporal projection and explanation (e.g., the Yale shooting problem) based on the idea that whether a situation is abnormal should not depend upon historical information about how the situation arose. We apply these ideas both to the Yale shooting scenario and to a blocks world domain that needs to address the qualification problem. 1 Introduction The paper [1987] by Hanks and McDermott describing the Yale shooting problem has generated such a flurry of responses that it is difficult to imagine what another one can contribute. The points raised by Hanks and McDermott, both formal and philosophical, have been discussed at substantial length elsewhere. It is not our intention to enter here into the philosophical portion of this discussion. Rather, the contribution we hope to make is technical. In [Hanks and McDer-mott, 1987], the authors raise a specific technical question: How is a certain form of temporal reasoning to be modelled? They go on to demonstrate that none of the then existing descriptions of action using nonmonotonic reasoning is suitable, and proceed from this observation to draw more far-reaching conclusions regarding the role of formal methods in Al. A discussion of these philosophical issues is outside the intended scope of this paper. Since the initial circulation of Hanks' and McDer-mott 's ideas in 1985, a variety of researchers have pro-Dosed solutions to the technical challenge they raised Unfortunately, subsequent analysis (most of it over coffee tables) has shown most of these intended solutions to be flawed for one or another technical reason. We begin in the next section by presenting a brief description of the Yale shooting problem itself, and a variant that appears in [Baker, 1989]. The description given is in fact somewhat simpler than that presented in [Hanks and McDermott, 1987], but still retains all of the troublesome features of the original. In Section 3, we go on to describe proposed solutions, and investigate their technical shortcomings. The formal underpinnings of our own ideas are presented in Section 4, and we return to the Yale shooting scenario in Section 5, showing that our notions can be used to solve both the original problem and the variant presented in Section 2. In Section 6, we extend our ideas to deal with the qualification problem in a simple blocks world scenario. Concluding remarks are contained in Section 7. 2 The Yale shooting The Yale …