A theory of planning should provide a model of how planning knowledge might be learned and stored in memory so as to be available and utilized in appropriate situations. This paper presents a content theory of the planning strategies and constraints on planning specific to joint planning situations. The categorization helps to explain what information is relevant to this general class of planning problems,namely goal pursuit situations where goals cannot be satisfied without the participation of another planner. The taxonomy of planning principles presented outlines the common problems in mutual goal pursuit situations, and provides strategies for resolving the problematic interactions. The principles apply to a variety of types of mutual goal pursuit arrangements such as business partners, a political coalition, or social relationships.

Solving the multiple goals problem has been a major issue in Artificial Intelligence models of planning (Sussman, 1975; Sacerdoti, 1975; Wilensky, 1978; Wilensky, 1980; Wilensky, 1983; Carbonell, 1979); however, most models have assumed that the best plan for a set of goals to be satisfied in coivj unction will zirise from a simple combination of the best individual plans for each goal. However, human planners seem to possess an ability to look at a set of goals, and charMterize them as a whole, instezui of as a collection of individual goals (Hayes-Roth and Hayes-Roth, 1979). In this paper, we introduce the notion of indexing complex multiple-goal plans in terms of the interactions between the goals that they satisfy. W e present the vocabulary requirements for representing the causality behii^ goal interactions, the general planning strategies used to resolve these interactions, and the specific plans based on these more general resolution strategies that are instantiated in the actual planning problem.

Previous research has found that when information stored in memory is discredited, it can still influence later inferences one makes. This has previously been considered as an editing problem, where one has inferences based on the information prestored in memory before the discrediting, and one cannot successfully trace out and alter those inferences. However, in the course of comprehending an account, one can potentially make inferences after a discrediting, which may also show influence from the discredited information. In this experiment, subjects read a series of reports about a fire investigation, and their opportunity to make inferences before a correction appeared in the series was manipulated. Subjects received a correction statement either directly following the information it was to discredit, or with several statements intervening. The results show that subjects w h o received the correction directly after the information it corrected made as many inferences based on the discredited information as subjects w ho received the correction later (and thus could presumably make many more inferences before the correction occurred). This suggests that discredited information can influence inferences made after a correction, as well as those made before. Several hypotheses accounting for this effect are proposed.

An important feature of human memory is the ability to retrieve previously unsolved problems, particularly when circumstances are more favorable to their solution. Zeiganiik (1927) has been widely cited for the finding that interrupted tasks are better remembered than completed ones; however, frequent replications and non-replications have been explained in terms of social psychological variables (Prentice, 1944). The present study examines differences in memory for tasks based on completion status by appealing to cognitive variables such as the nature of interruption, time spent during processing, and set size. In one experiment using word problems, subjects were interrupted on half of the problems after a short interval of active problem solving, and completed tasks were in fact better remembered than interrupted ones. However, less processing time was necessarily spent on problems that were interrupted. A second experiment held time constant, allowing subjects to abandon tasks they could not con^lete. In this experiment, the opposite result occurred, replicating Zeigarnik and showing better access to unsolved problems in free recall. However, enhanced memorability in this study may have resulted from a subject-generated impasse in problem solving rather than "interruption" per se. This successful replication also included set size differences in favor of incomplete problems. Under these conditions, the status of completion can serve as a useful index to past problem situations. These experiments are successful in identifying cognitive variables that explain when one can suspend effort on a failed problem, and recall it at a later time.

Over the past few years there has been a growing interest in the notion of using causal explanations in both learning ([Dejong and Mooney, 1986] [Mitchell et a/., 1986]) and planning ([Hammond, 1989], [Hammond, 1987] and [Simmonsand Davis, 1987]). The study of how complex causal explanations can be used in learning has turned into something of a cottage industry in AI; however, little attention has paid to how explanations may be constructed. In this paper, we will examine some of the current proposals concerning the process of explanation, augment them with a few ideas of our own, and suggest a new, more strategic level of knowledge about explanations that can be used to guide the explanation process. In particular, we are interested in the problems involved with integrating rule-based methods of explanation construction with memory-based approaches.

Several lines of research have suggested that information previously integrated into memory can influence inferences and judgments, even w h e n more recent information discredits it. A first experiment tested the prediction that information providing causal structure, versus being mentioned but otherwise unintegrated into the account, would lead to more influence, and found that subjects used both discredited and valid information affording causal structure to make inferences, but not incidentally mentioned information with the same content Experiment 2 found that w hen a plausible causal alternative accompanied the correction, subjects showed less influence from the discredited information than when the correction simply negated earlier information. The findingssuggest that the continued influence of discredited information depends on the causal structure it affords

Lack of transfer may result in part from a critical, though often ignored factor: the form of the initial representation of information during the process of analogical transfer. Using a Gick and Holyoak (1980, 1983) replication, in which subjects read a story in the guise of a memory experiment, subjects were later required to solve a problem which could be solved using an analogous strategy suggested by the story. Transfer performance was measured by the presence or absence of this target solution in subjects' protocols. The text of the original General story (from Gick & Holyoak) was modified slightly in one condition, where one role in the story was replaced by another type of actor. The changes were minor, as shown by the fact that the story modification did not affect similarity ratings between the story and problem. However the changes did appear to affect subjects' initial representation of the story and, as a result. Improve subsequent transfer to the problem. The results indicate that forming an initial representation of the story that is congruent with important features of the problem is critical for analogical transfer. Subjects' abstraction of a general problem solving schema is an inadequate explanation of these results.

The problem of access to prior cases in memory is a central Issue in the case-based reasoning Previous work on thematic knowledge structures has shown that using a complete exemplar of a thematic pattern allows access to the structure and related cases in memory. However, the knowledge and expectations provided by such structures can aid in planning and problem-solving. Therefore, to be most useful, the Information should become available before the Input pattern Is complete. Retrieval must therefore be possible based on only a subset of the features present In the full thematic pattern. This study investigated whether a pattern that contains elements predicting an outcome, but not the outcome itself, would result In access comparable to that found when a full pattern is used. The results showed that subjects were less successful accessing the thematic structure using partial patterns than they were when using full patterns. However, remindings based on partial patterns occurred more often than would be expected by chance. W e conclude that partial patterns contain some predictive features that can allow access to a thematic knowledge structure before the pattern Is complete.