Ray Reiter's Knowledge in Action: A Review

نویسنده

  • Drew McDermott
چکیده

This is a beautiful book, in conception and execution. What Ray Reiter has done has taken a set of ideas worked out by him and his collaborators over the last 11 years and recrystallized them into a sustained and consistent presentation.1 This is not a collection of those papers but a complete rewrite that avoids the usual repetition and notational inconsistency that one might expect. It makes one wish everyone as prolific as Reiter would copy his example—but because that’s unlikely, we must be grateful for what he has given us. In case you haven’t heard, Reiter and his crew, starting with the publication of Reiter (1991), breathed new life into the situation calculus (McCarthy and Hayes 1969) that had gotten the reputation of being of limited expressiveness. The basic concept of the calculus is, of course, the situation, which we can think of as a state of affairs, that is, a complete specification of the truth values of all propositions (in a suitable logical language), although that’s closer to McCarthy’s and Hayes’s traditional formulation than the analysis Reiter settles on (which I describe later). Any atomic formula whose truth value can change over time must be written with a predicate that takes a situation as argument. We might write child(person1, person2) if we analyze a domain as not allowing the set of children or parents of a person to change. However, if the analysis must take into account time scales in which new children can be born, we The second problem is how to deal with the notorious frame problem and its various relatives (McCarthy and Hayes 1969; Pylyshyn 1987). The problem is that there is no implicit relationship between the truth values of a proposition in s and do(a, s), or do(a2, do(a1, s)). We can only state these relationships by adding axioms of some sort. Getting the axioms right, and getting algorithms based on them right, has not been easy. As a result, the basic situation calculus lived as a textbook curiosity for several years. Researchers interested in practical applications of temporal reasoning, such as automated planning, kept the basic ontology of the calculus but focused on efficient algorithms to the neglect of logical foundations. Researchers interested in the underlying logic looked for ways of getting the logic to work, most ingeniously by resorting to nonmonotonic techniques. Reiter began putting things back on track by addressing both these questions. He adopted and extended the solution to the frame problem of Haas (1987), Schubert (1990), and Davis (1990) and noticed that the basic do(a, s) notation does not in fact rule out the occurrence of interesting events between s and do(a, s). The result has been a stream of important papers that have given new life to the attempt to formalize realistic reasoning in temporal contexts. Knowledge in Action is a recapitulation of this work. It begins by explaining the framework and the solution to the frame problem. Chapters 4 and 5 then give a rigorous basis for the situation calculus and for logicprogramming reasoning in the important special case where the closedworld assumption holds. This is the assumption that any atomic formula that does not follow from a given set of axioms is false. A key result of Chapter 4 is the precise specification of how to regress a formula Q, that is, to derive the weakest formula P whose truth in a situation will make Q true in a later one. A formula can be regressed through any number of actions, so that to deduce whether Q is true in Sk = do(actionk, do(actionk–1, ..., do(action1, S0))), one can use reRay Reiter’s Knowledge in Action

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عنوان ژورنال:
  • AI Magazine

دوره 24  شماره 

صفحات  -

تاریخ انتشار 2003