New Generation Computing, 13 (1995) 435-439 OHMSHA, LTD. and Springer-Verlag

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WORKSHOP REPORT: Application of Logic Legal Reasoning

OHMSHA. LTD. 1995

Programming to

Rober t K O W A L S K I , Imperial College, Department of Computing, 180 Queen's Gate, London SW7 2BZ, UK

w Introduction The Workshop on the Application of Logic Programming to Legal Reasoning

was a one day workshop organised by Dr. Katsumi Nitta from ICOT and chaired by Professor Hajime Yoshino from Meiji Gakuin University. It covered a wide range of topics, most of which concerned one or more of the following three main problems: 1. the need for a natural and expressive knowledge representation formalism for

representing legal concepts; 2. the importance of an appropriate theory of legal argumentation; 3. the nature of the interaction between case-based and rule-based reasoning in the

law.

w Deductive Object Oriented Database The first talk by Chie Takahashi and Kazumasa Yokota described an applica-

tion of the deductive object oriented database language/system Quixote developed at ICOT to the representation of the Vienna Sales Convention, also known more formally as the United Nations Convention on Contracts for the International Sale of Goods (GISG). The application is a demanding one and gives a good indication both of the knowledge representation power and of the problem solving capabilit ies of Quixote.

Articles in the CISG are represented as conclusion-condition rules, where conditions and conclusions are attributes of object terms or are object terms labelled by the modules to which they belong. Module labels and attribute names behave like predicate symbols. The object-orientation of the representation seems to correspond quite naturally to objects in the domain, such as facts about events that take place in the world and legal concepts concerning the legal effect of world events and various legal capabilities.

The paper indicates that one of the most important obstacles to employing such systems for query answering in practice lies in the difficulty of establishing that all the conditions of the rules needed to answer the query actually hold in the case under consideration. The solution employed in the paper is to return condit ional answers to queries, noting that more work needs to be done to reduce the number of conditions in answers to a reasonable level.

w Argumentation The talk by Masato Shibasaki and Katsumi Nitta from ICOT investigated an

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extension of the argumentation theory, developed by Giovanni Sartor, and applied the extension to problems in Japanese criminal jurisprudence. Articles of law are represent- ed by means of rules in extended logic programs, with both classical negation and negation as failure. Rules are related to another by means of a priority relation, used to resolve conflicts when different rules lead to competing conclusions.

The paper shows how to distinguish between justified, plausible and defeated arguments. It uses a number of examples from Japanese criminal law to illustrate the applicability of the approach. These examples are very convincing, and indicate thin argumentation theory has a great deal to contribute not only to legal reasoning and defeasible reasoning, but to the formalisation of practical reasoning in general. Undoubtedly, this will continue to be an increasingly important area of research in the future.

w Logical Analysis of Legal Language The following talk by Hirosato Nomura concerned the linguistic analysis and

logical representation of the Japanese text of the CISG. I found the talk fascinating for two reasons. First, I have argued, myself, that many legal documents are written in a style which is intermediate in formality between ordinary natural language and precise, artificially formal languages. Thus, it has seemed to me that the language of legal documents might help to identify, more clearly than ordinary natural language, the knowledge representation structures needed to represent the meaning of natural lan- guage in general. Second, I have recently begun to study Japanese and was especially interested, therefore, to learn about the relationships between the syntax of sentences in Japanese and the underlying semantic and logical structure of such sentences.

I was especially pleased to learn, therefore, that, for the purpose of representing the logical meaning of the Japanese text of the CISG, Nomura has developed a uniform formal framework for representing both linguistic and logical constructions. For lack of space, the paper is able to give only brief overview of the framework. However, one interesting feature of the framework, which is highlighted in the paper, is the fact that a single notion of dependency structure is used to represent both linguistic dependency structures, such as semantic cases, as well as logical and legal dependencies, such as that between a legal requirement and a legal effect.

w Case-based Reasoning Edwina Rissland gave an invited talk about the natural synergy, which exists

between case-based reasoning (CBR) and rMe-based reasoning (CBR) in the law and outlined the CABARET system, which combines the two modes of reasoning. The architecture of CABARET is perfectly general and applies to any domain which is characterised in terms of rules and cases. However, it has been instantiated for the particular domain of the so-called home office deduction defined in the United States Internal Revenue Code.

CABARET uses various types of control rules to combine the two modes of reasoning. Examples of these include: 1. When one mode of reasoning fails, switch to the other. 2. Once a conclusion has been reached using one form of reasoning, check that it also

holds in the other mode. 3. If all but one condition of a rule can be established, then use CBR to try to establish

the condition.

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4. If all but one condit ion of a rule can be established, then use CBR to try to broaden the application of the rule to cover the condition.

5. Use CBR to establish conditions which are deliberately open-textured.

w Temporal Relations in Case-based Reasoning The topic of case-based reasoning was taken up again in the talk by Satoshi

Tojo and Katsumi Nitta, which dealt with the similarity between the temporal features of affairs reported in different cases. They proposed a sophisticated classification of the temporal features of the affairs of a case, utilising results about the classification of verbs developed in linguistics. They formalise these features in terms of the various intervals and time points associated with affairs.

Based upon this formalisation of the temporal features of the affairs in a case, they employ a number of default rules to propose temporal relationships between the affairs, taking into account the order in which the affairs are presented in the descrip- tion of the case. These default rules sometimes give rise to erroneous results, which are partially corrected by means of meta-rules.

The paper describes how similarity between cases can be decided by comparing the temporal relations between affairs in the two cases. The approach is illustrated by a number of interesting examples.

w Analogical Reasoning Using Generalisation The following talk by Makoto Haraguchi and Tokuyasu Kakuta presented a

different approach to overcoming the limitations of rule-based reasoning. It investigat- ed an approach to analogical reasoning based upon generalising rules rather than on determining similarities between cases.

Given a new case, to which no existing rules apply, appropriate generalisations of the rules are applied instead. The generalisation of rules is controlled by using a sorted logic and sort hierarchy. A term of a given sort in a rule which fails to unify with a term in a current goal is replaced by a variable of a more abstract sort, higher in the hierarchy, which does unify. The failure of rules to apply to a goal can also be repaired by generalising predicates. This is sometimes necessary when generalising the sort of a term gives rise to ill-sorted predicates.

This approach to analogical reasoning has been applied to a complex example from the Japanese Civil Code. Compared with a previous study of the same example, the use of sorts to control the generalisation of rules greatly reduces the computat ional costs.

w More Argumentation The invited talk by Bob Kowalski presented his work with Francesca Toni on

argumentation, based on their work with Bondarenko and Dung. The main accomplish- ment of the theory until now has been to provide a unifying framework for many previously distinct approaches to default reasoning.

The argumentation theory is concerned with determining what assumptions can acceptably be used to extend a given set of facts. The notion of acceptability goes beyond the simple requirement of consistency and requires that a set of assumptions be able to defend itself against attack from other sets of assumptions. One set of assump- tions attacks another if together with the facts it implies the contrary of some assump- tion in the other.

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With this notion of attack, stable models in logic programming and the standard semantics of default logic, auto-epistemic logic and non-monotonic modal logic can all be understood as regarding a set of assumptions as acceptable if and only if consists of all the assumptions it does not attack. The argumentation theory helps to identify other, more appropriate notions of acceptability. An important direction for further work, touched upon in the paper, is to investigate how the theory can help to reconcile conflicts between different sets of assumptions.

w Developing Legal Ontologies The paper by Masaki Kurematsu, Masayoshi Tada and Takahira Yamaguchi

investigates the use of a general ontology of concepts, such as that incorporated in the Japan Electronic Dictionary, to aid a user in developing a legal ontology for a legal expert system. One benefit of such an approach is that the general ontology can help to refine and improve an initial user-defined ontology for a given legal domain. Another is that using a general ontology to help with the construction of legal ontologies for different domains can help to ensure consistency, compatibility and inter-operability between different systems using the different ontologies.

The paper describes a computer environment to help a user construct a legal ontology from a general ontology. Before using the environment, a user must construct an initial legal ontology, which consists of a legal concept hierarchy and a collection of legal concept definitions. The environment matches these concepts with concepts in the general ontology, identifies differences between matching concepts and suggests changes to the legal ontology to reduce the differences it has found.

In addition .to the static analysis of the legal ontology in relation to the general ontology, the environment also includes an inductive logic programming tool for dynamic analysis and improvement both of the legal ontology and of the rules formulat- ed in the ontology. The static and dynamic analysis tools have been applied to the CISG and have given encouraging results.

w Legal Meta-reasoning In his talk, Hajime Yoshino argued that the process of legal reasoning is largely

concerned with the use of legal meta-reasoning to determine the validity of object-level legal rules. A typical use of such meta-reasoning is to determine the priority between lower object-level rules. For example, the statements that (1) an upper law has priority over a lower law, (2) a particular law has priority over a general !aw, and (3) a new law has priority over an old law, can all be understood naturally as legal recta-rules, Moreover, legal meta-meta-rules can be used to assign meta-rule (1) priority over recta-rule (2) and (2) priority over (3).

Professor Yoshino has developed a sophisticated approach to legal knowledge representation using "compound predicate formulas". These formulas facilitate a natural representation of complex legal rules and states of affairs; for example, to represent the meaning of such sentences as "Bernard's acceptance of the offer by Anzai reaches Anzai on November 17". There is a systematic and simple way of transforming such formulas into formulas of ordinary first-order logic. Professor Yoshino demon- strated the application of meta-rules to a problem in the domain of Japanese contract law. The meta-rules determine what object-level rules can validly be applied to the given problem.

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w The TAXMAN Project The final talk was an invited talk by Thorne McCarty about the TAXMAN

project he has pursued since the early 1980s. He described his early work on the representation of legal concepts by means of prototypes and transformations, applied to the example of Eisener v. Macomber, an early stock dividend case. He discussed the case in detail, analysing the conflicting arguments of justice Pitney and justice Brandeis.

The paper discusses two problems with the early TAXMAN theory and sketches some of the later develoments of the theory aimed at overcoming these problems. The first problem was the inadequacy of the semantic foundation of the original frame-based language. This has been addressed in later work by the development of LLD, a Language for Legal Discourse. LLD has facilities for the representation of states, events, actions and various modalities over actions such as permission and obligation. It also provides a systematic treatment of sorts and subsorts and it includes both count terms and mass terms. The language is based on intuitionistic rather than classical logic.

The second problem was to clarify the criteria and constraints for the theory of prototypes and transformations itself. This has been addressed in part by constructing the definitons of concepts using only definite rules and then using circumscription to express indefinite information.

w Conclus ions The final discussion session was dominated by a lively debate focused on the

differences between the C A R A R E T and T A X M A N approaches to case-based reason- ing. Although both approaches agree that legal reasoning involves an intimate connec- tion between cases and arguments, they seem to disagree on the need for a deep conceptual model to link reasoning by means of cases with reasoning by means of rules, There seemed to be general agreement, however, that this is an important area that merits continued research.

My general impression of the workshop was very positive. My conviction of the importance of work in this field was reinforced, not only because of its applications in the law, but also because of its impact on the development of more powerful knowledge representation and problem solving techniques in computing. I was also pleased to learn more about the important contributions to this field being made in Japan, much of which is led by Nitta's work at ICOT and Yoshino's work with LESA (the Japanese Legal Expert System Association). I look forward to following the continued progress of the important work in this area in the future.