Learning to Understand Contractual Situations

In the field of law, decisions in previous cases often play a significant role in the presentation and outcome of new cases. Lawyers are constantly recalling old cases to aid them in preparing their own briefs. How do lawyers remember cases? What are the features they use to organize and retrieve past decisions? How do lawyers learn which features are important? To address these Questions we are constructing a model of legal novices (i.e. first year law student) and the processes by which they learn contract law. Our model is embodied in a computer program called STARE (from the latin, stare decisis which refers to the principle of using past cases to decide current disputes). STARE will read descriptions of contractual situations and attempt to predict the decision based on its general commonsense knowledge of agreements, the previous cases stored in an episodic memory, and knowledge of some basic legal concepts. I . I N T R O D U C T I O N We are interested in constructing a mode) of first year law students learning contract law. Law students already have a great deal of commonsense knowledge about the social interactions between people (agreements), right and wrong (justice), and the world in general. They are taught new concepts with specific legal meanings and how to relate these concepts to their commonsense knowledge. They are also taught how to reason analogically from new cases to previous cases and use these reasoning chains to predict and support the decisions in the new cases. To better understand what STARE will face, let's consider an example case that a law student might encounter: Case-l:LaRue a police officer, promises Frank, a merchant who owns a store on LaRue's beat, that he will keep an eye on Frank's store during his lunch hour if Frank will pay him $50 a month. Frank agrees. Is there a contract in CASE-1? Answering this question requires knowledge about social roles (role tnemes) such as policemen, merchants, and public servants, what it means to promise to do something, to keep an eye on someone else's property, and to agree to something. In addition, we infer certain facts from the situation: LaRue has a goal of earning extra money, Frank believes that LaRue has the authority to perform his promise, and there is an agreement between Frank and LaRue. We can now tentatively answer the question based on whether we believe that LaRue is free to moonlight during his lunch hour. If we know that policemen are obligated to act if they witness a crime, even if they are not on duty, then we would probably say there is not a contract. We have a commonsense heuristic which * This research supported in part by a grant from the Keck Foundation. The first author was also supported in part by a Rand Corporation AI Fellowship, the second author by an IBM Faculty Development Award, and the third author by a Hughes AI Center grant. states that one cannot assume a new obligation if it conflicts with an existing obligation. Assume that we have the above case in memory along with the decision that there was no contract and the supporting reasoning chain for the decision. Now we read the following case: Case-2:Fred, a police officer, promises Barney, a merchant who owns a store which is not on Fred's beat, that he will keep an eye on Barney's store instead of walking his beat if Barney will pay him $50 per month. Barney agrees. Given that CASE-1 is in episodic memory, we have no trouble deciding that there is no contract in CASE-2. This assumes that we were able to get to CASE-1 to use it for the current situation. How cases are organized and indexed in episodic memory is crucial to our ability to understand new situations. The reminding of CASE-1 could have occurred from the role themes (both situations involved a policeman and a merchant), the nature of the agreement (protection), or because the same commonsense knowledge was used to understand both cases. In reality it is a combination of these elements. Our memory now contains CASE-1 and CASE-2 which are indexed together. Now we read next situation: Case-3:0'Hara, a police officer on vacation in Florida, promises Alfred, a store owner, that he will keep an eye on Alfred's store during Alfred's lunch hour if Alfred will pay him $10 a day. Alfred agrees. We should be reminded of either CASE-1, CASE-2, or both (since we only have two cases in memory) and must now decide if CASE-3 is similar. We know that there was no contract in the previous cases because of a conflict with a pre-existinff obligation. In CASE-3 no such obligation exists so we should conclude that there is a contract. What kind of knowledge is necessary to 1) recognize the presence or absence of an obligation, and 2) build the appropriate indices into episodic memory? n . K N O W L E D G E A N D M E M O R Y As we have just seen, understanding and reasoning about contractual situations requires interaction between a variety of knowledge sources. The following abstract constructs are essential to understanding CASE-1, CASE2, and CASE-3: goals (Wilensky, 1978a), plans (Schank & Abelson, 1977), themes (Wilensky, 1978b), interpersonal relationships (Schank & Abelson, 1977), events (Dyer, 1981), social acts (Schank & CarboneII, 1979), and role themes (Dyer, 83). We must also be able to represent legal concepts corresponding to OFFER, OBLIGATION, PROMISE, and ACCEPT. In the following sections we shall address the issues of representing general knowledge, legal knowledge, and their integration in memory. 292 S. Goldman et al A. General Knowledge In the introduction, we identified some of the commonsense knowledge we must represent. There is knowledge about social roles. We represent this knowledge using role themes (RTs) (Dyer, 1083). RTs are organized in an inheritance hierarchy and contain knowledge about societal relationships. Consider what we know about policemen: Policemen are public servants, paid by our tax dollars, whose task is to enforce our laws. They are often assigned a particular area to patrol (beat) and are responsible for protecting the people and property within that area. Actually we know much more, for example, we know that policemen, firemen, and paramedics all have a responsibility to the public. Therefore, in addition to RTPOLICEMAN, RT-FIREMAN, and RT-PARAMEDIC, we have a more eeneral role theme, RT-PUBLIC-SERVANT which supersedes these three. We conclude that it is the knowledge contained in RT-PUBLIC-SERVANT which tells us that there is a pre-existing obligation in CASE-1 and CASE-2. Once we recognise the obligation, we apply commonsense rules such as: