C o m b i n i n g Case-Based a n d R u l e - B a s e d R e a s o n i n g : A Heuristic Approach* E d w i n a L. Rissland & D a v i d B. Skalak Department of Computer & Information Science University of Massachusetts Amherst, MA 01003 CSNET ADDRESS:
[email protected] Abstract In this paper we discuss a heuristically controlled approach to combining reasoning w i t h cases and reasoning w i t h rules. Our task is interpretation of under-defined terms that occur in legal statutes (like the Internal Revenue Code) where certain terms must be applied to particular cases even though their meanings are not defined by the statute and the s t a t u t o r y rules are unclear as to scope and meaning. We describe this problem, known as statutory interpretation, provide examples of i t , describe the need for melding case-based and rule-based reasoning, and discuss heuristics used in guiding reasoning on such problems. We conclude w i t h a discussion of our on-going work to model this mode of expert reasoning.
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Introduction
" S t a t u t o r y i n t e r p r e t a t i o n " is the process of t r y i n g to determine the meaning of a legal rule by analyzing its terms and then applying it to a particular set of facts. The difficulty presented to adjudicators, advocates and administrators by this exercise is t h a t critical terms are typically not defined completely (or at all) by a statute. Further, a rule taken as a whole may have unspoken qualifications and exceptions. Thus one must look outside a statute to other sources of knowledge for clues to its meaning and the meaning of its constituent elements. In particular, one tries to resolve i n t e r p r e t a t i o n problems by considering past applications of the rules and terms in question: by examining precedent cases, comparing and c o n t r a s t ing these w i t h the instant case, and arguing why a previous interpretation can (or cannot) be applied to the new case l [Levi, 1949; Llewellyn, 1960; T w i n i n g and *This work was supported in part by the Advanced Research Projects Agency of the Department of Defense, monitored by the Office of Naval Research under contract no. N00014-87-K-0238, the Office of Naval Research under a University Research Initiative Grant, contract no. N00014-86K-0764, and a grant from G T E Laboratories, Inc., Waltham, Mass. 1 Note, in the fullest sense, interpretation also requires consideration of whether a term or rule "should" be applied. In
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Miers, 1982]. The intepretation problem demands that one combine reasoning w i t h cases and reasoning w i t h rules (statutes). W h i l e the need to m i x case-based reasoning ( " C B R " ) and rule-based reasoning ( " R B R " ) is a prototypical feature of s t a t u t o r y legal reasoning, other domains also require i t . We believe our approach can be applied beyond the realm of law; in particular, to extend t r a d i t i o n a l expert system approaches to "soft" domains t h a t lack a strong domain model. For examples of underdefined terms in a legal rule, consider a section of the statute that governs the assessment of Federal income tax, the Internal Revenue Code (sometimes called just the " C o d e " ) . In stating the requirements for taking a home office deduction, Section 2 8 0 A ( c ) ( l ) of the Code employs such terms as "principal place of business", "convenience of the employer" and use on a "regular basis". 2 Nowhere are these elements defined in the statute; yet some scope must be afforded them in order to apply the statute to particular cases. While the meaning of such phrases is sometimes elucidated by official regulations issued thereunder by the Internal Revenue Service, a clear-cut definition (which does not itself use undefined terms) is almost never to be found. Often, the reach of the meaning of such phrases is fundamentally unclear, varies greatly according to the factual context in which they are used, and defeats precise definition by rules. For clues to their scope, practitioners rely on previously litigated tax cases that have construed these terms. 1.1 T h e I n t e r p r e t a t i o n P r o b l e m i n t h e L a w In statutory i n t e r p r e t a t i o n , ambiguous terms often this discussion, we leave aside these important normative aspects, which involve reasoning about legislative intent, policy and ethics[Fuller, 1958; Hart, 1958]. 2 §280A(c)(l) states that a deduction may be taken for "any item to the extent such item is allocable to a portion of the dwelling unit which is exclusively used on a regular basis — (A) [as] the principal place of business for any trade or business of the taxpayer, (B) as a place of business which is used by patients, clients, or customers in meeting or dealing with the taxpayer in the normal course of his trade or business, or (C) in the case of a separate structure which is not attached to the dwelling unit, in connection with the taxpayer's trade or business. In the case of an employee, the preceding sentence shall apply only if the exclusive use referred to in the preceding sentence is for the convenience of the employer."
arise from "open-textured" concepts, and these concepts are often the focus of case-based attack. By "opent e x t u r e d " concept we mean a concept that cannot be defined by necessary and sufficient conditions: one whose boundary is not sharp. Such concepts have been much discussed in jurisprudence [Hart, 1961; D w o r k i n , 1977] and also in philosophical discussions of " n a t u r a l k i n d " classes [Wittgenstein, 1958; P u t n a m , 1975]. Gardner's recent work [Gardner, 1987], for example, discussed how such legal open-textured concepts give rise to what are known as " h a r d " cases, t h a t is, cases over whose resol u t i o n experts (judges, scholars, etc.) disagree. Many concepts in domains like the law are open-textured and sometimes even familiar terms reveal a surprising opentextured l i n i n g , such as "contract" or "income". Concepts like "due care", which are used deliberately to i n dicate a variable standard of behavior, are clearly of this sort. So are "meeting or dealing" and "exclusive use" from the home office deduction rule. Their interpretation is the subject of numerous cases. 3 The need to do s t a t u t o r y interpretation is not necessarily the result of poor legal drafting. Rather it is a persistent problem that resists a legislature's best good-faith efforts at drafting tight statutes. Most generally, the persistence is due to the nature of the law and its relation to society; more particularly, to factual circumstances unanticipated at the time of drafting and a changing legal context [Levi, 1949; Sunstein, 1988]. This was one of the points of one classic discussion of the problem of statutory interpretation known as the "Hart-Fuller debate", between H.L.A. Hart and Lon Fuller in a Harvard Law Review dialogue [Hart, 1958; Fuller, 1958]. There they discussed, among other things, such deep jurisprudential issues as the nature and status of rules and the role of " o u g h t " (normative considerations) in statutory interpretation. 4 3
For instance, a case involving Max Frankel, The New York Times Managing Editor, Max and Tobia Frankel v. Commissioner, 82 USTC 318 (Filed February 28, 1984), addressed the former. Mr. Frankel maintained an office at his home in the Bronx, which he used for reading the morning papers, writing memoranda, clipping materials, and speaking by telephone to his employees, prominent politicians and community leaders. The Tax Court denied that Mr. Frankel met any of the three disjunctive requirements of the statute, ( A ) , (B), or (C). In particular, the use of the telephone to conduct business was held not to satisfy the meeting or dealing predicate, which was construed to require the physical presence of business contacts. 4 Two famous hypothetical statutory rules from this debate nicely illustrate the problems: (1) "No vehicles are allowed in the public park." and (2) " I t shall be a misdemeanor . . . to sleep in any railway station." Hart and Fuller were concerned with applying such rules to "hard" cases, where the puzzle is to interpret open-textured concepts, like "sleeping" or "vehicle", in light of a statute's purposes. For instance, does a tank which is part of a war veterans memorial statue count as a vehicle? What about a motorized baby carriage or wheelchair? What about a fire engine requiring access to a fire via the park? As for "sleeping", what should we decide about a bum who has obviously bedded down for the night but still has his eyes open? Should the result be any different as to a well-dressed commuter who has clearly dozed off?
1.2 T h e I n t e r p r e t a t i o n P r o b l e m O u t s i d e t h e L a w Although law is the focus of this discussion of mixed C B R / R B R paradigm reasoning, lawyers are by no means the only ones to combine these t w o different modes of reasoning. Mathematicians regularly combine reasoning deductively and reasoning w i t h examples. A l t h o u g h sometimes overshadowed by formal definitions and theorems and their proofs, examples, that is, cases, constitute a powerful aspect of expertise [Rissland, 1978]. Polya [Polya, 1965] speaks of the importance of interleaving these two modes of reasoning in the "alternating process" in which one switches to C B R (to find a counterexample) when deductive reasoning stalls and vice versa. The "dialectical" process discussed by Lakatos [Lakatos, 1976] depends critically on use of exemplar cases as much as it does on proof analysis. A n d in AI discovery systems, such as Lenat's A M , examples are a powerful source of control and focus of attention [Lenat, 1977]. Even in medicine, where heuristic rules have formed the core of the current generation of medical expert systems (e.g., rules of diagnosis as in M Y C I N ) , there is a rich body of specific cases of a phenomenon (e.g., particular cases in a particular practice such as "Mrs. Jones, the woman whose problem turned out to be borderline hypertension") which an expert might use, especially in cases requiring judgment calls. Equally i m p o r t a n t , even though one traditionally treats concepts like "hypertension" as well-defined in expert systems, such terms really are not so clear-cut as all that — for a large part of their meaning lies in how they were used in past cases.
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The Legalistic C h i l d : An Example of Interpretation
As an example of the need for statutory interpretation, consider the case of the "legalistic c h i l d " from the book by the British legal scholars T w i n i n g and Miers [ T w i n i n g and Miers, 1982] "Johnny, aged 7, is an only child. In recent months his mother has been mildly worried because he has developed a craving for sweet things and this has affected his appetite at meal times...Then one afternoon she finds that Johnny has gone into the larder and helped himself to half a pot of strawberry jam...she does not punish Johnny but instead says, 'That's naughty. In the future you are never to enter the larder without my permission/ 'What does enter mean, Mummy?' asks Johnny. l To go into', says his mother. 'O.K.' says Johnny, relieved that he has got off so lightly. Several incidents then follow. First, Johnny gets a broom and hooks the pot of jam from the larder and helps himself. 'I didn't enter the larder', he says. Next the cat enters the larder and attacks the salmon which mother has bought for a special occasion. Mother, upstairs, hears Johnny hooting with laughter. She comes down to see him standing outside the larder door watching the cat eating the fish. 'I may not go into the larder,' he says." Clearly one of the conflicts between Johnny and his mother concerns the meaning of enter, another is the
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scope of the rule itself (e.g., is there an unspoken exception t h a t allows entry in dire circumstances much like t h a t enabling fire trucks to r u n red lights?). We shall ret u r n to this example to demonstrate our computational approach to i n t e r p r e t a t i o n .
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Synopsis of C B R and M i x e d P a r a d i g m Approaches
Case-based reasoning ( " C B R " ) has grown rapidly in the last few years [Kolodner, 1988; Rissland and K i n g , 1988]. W i t h i n C B R there are two major classes of C B R that can be identified: problem-solving C B R [Hammond, 1986; Kolodner, 1987; Sycara, 1988] and precedent-based C B R [Ashley, 1988; Ashley and Rissland, 1988]. Precedentbased C B R is distinguished by its focus on the use of past cases ("precedents") to justify a solution and explain its rationale 5 . Anglo-American common law w i t h its doctrine of the binding nature of precedent is a paradigm of precedent-based C B R . On the other hand, in problemsolving C B R , the typical focus is on using past cases to find a a detailed problem solution (e.g., a plan, a course of action), where the new solution is generated by adapting a previous solution. Industrial design and planning are paradigmatic examples of problem-solving C B R [Barletta and M a r k , 1988]. B o t h types of C B R follow similar steps. Once a new case has been accepted for analysis, C B R proceeds by ( 1 ) analysing it (e.g., by c o m p u t i n g features, relations and indices) to retrieve a set of relevant cases from case memory; ( 2 ) f r o m these selecting a subset of best cases from which to craft a solution or interpretation for the problem case; ( 3 ) derivation of a solution or intepretation complete w i t h supporting arguments in the case of precedent-based C B R and w i t h implementation details in the case of problem-solving C B R ; ( 4 ) testing of the the interpretation (e.g., w i t h h y p o t h e t i c a l ) or solution (e.g., w i t h simulations) w i t h an eye to assessing its correctness, strengths, weaknesses, generality, etc.; and ( 5 ) storing the newly solved or interpreted case i n t o case memory and appropriately adjusting indices and other C B R mechanisms such as similarity metrics. Note, in assessing relevancy in Step 1, and all the other steps of C B R as well, one must view cases f r o m the point of view of the case and task at hand. So, for instance, j u s t because a known case was a landmark case does not necessarily make it i m p o r t a n t for the present case since the t w o m i g h t not share any relevant similarities. Furthermore, in s t a t u t o r y interpretation the C B R must address the requirements of the statute. It is not enough simply to argue about the meaning of legal concepts; one must tie the arguments to the statute. This latter remark shows why our past work on H Y P O is insufficient 5 As in previous precedent-based systems of our group, H Y P O and T A X - H Y P O , the key idea is to reason from cases similar to the current case in order to argue for a particular decision in the current case and to justify the reasoning in terms of the past cases. A large part of the effort is on selecting and arguing about the relevancy of cases: showing similarity with supporting cases and distinguishing contrary cases.
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in itself for modelling s t a t u t o r y interpretation [Rissland and Skalak, 1989]. At this p o i n t , researchers have only recently begun to write about the integration of C B R w i t h other reasoni n g paradigms [Goel and Chandrasekaran, 1988; K o t o n , 1988a; K o t o n , 1988b; Marques et a/., 1988; Walker et a/., 1988]. We feel t h a t such mixed-paradigm approaches are natural and shed light on both the cognitive skills i n volved in such reasoning and on questions of architecture and control of their computational models.
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Heuristics for M i x e d P a r a d i g m Reasoning
In our study of s t a t u t o r y i n t e r p r e t a t i o n , we have gathered a collection of 30 or so heuristics that we believe experts use for controlling and interleaving reasoning w i t h rules and reasoning w i t h cases. These heuristics can be divided into a number of categories: 6 1. Ways to Begin Reasoning 2. Rule-based Near Miss 3. Rule-based Near Hit 4. Ways to Broaden a Rule 5. Ways to Discredit a Rule 6. Ways to Confirm a Hit 7. Ways to Confirm a Miss 8. Ways to Confirm Reasoning: ''Sanity Checks" 9. Ways to Deal with Results Opposite from that Desired 10. Ways to Deal with Failure of Reasoning to Yield a Definite Conclusion 11. Ways to Focus the Reasoners 12. Open-Textured Elements Some of our heuristics, like those in groups 8 and 12, are very similar to those employed by Gardner [Gardner, 1987]. We are currently exploring the use of these heuristics in controlling a mixed paradigm system, combining case-based and rule-based reasoning, called C A B A R E T (CAse-BAsed REasoning T o o l ) . The main features of C A B A R E T ' S architecture are (Figure 1): • There are two co-reasoners (CBR and RBR ). t Each co-reasoner is capable of running in a standalone manner. • Each co-reasoner has a dedicated r e p o r t e r process that reports the end results of a reasoner and certain aspects of its intermediate processing. • A c o n t r o l l i n g process uses reporters' observations and its library of control heuristics to decide how the system as a whole and the individual reasoning processes are to proceed. • The ultimate goal for which the system is working (an argument for a given side or a neutral explanation) is specified by the user, as part of the intitial input to C A B A R E T . 6
A hit refers to the establishment of the antecedent of a rule, on the rule-based side, or the presence of all the prerequisites of a dimension (index), on the case-based side. A miss is the opposite of a " h i t " . Near miss and near hit are discussed below.
g u m e n t a t i v e stances: c o n f i r m i n g t h a t a r u l e s h o u l d f i r e , confirming t h a t a rule should not fire, broadening the scope o f a r u l e ( e n a b l i n g i t t o b e a p p l i e d even w h e n i t seems n o t t o a p p l y ) , a n d d i s c r e d i t i n g ( l i m i t i n g t h e scope o f ) a r u l e . T h e s e are t r i g g e r e d as f o l l o w s :
F o r i n s t a n c e , i f R u l e 1 has f i r e d , b u t y o u d o n ' t l i k e some consequence o f R u l e 1 , y o u ( a n d C A B A R E T ) m a y l o o k for w a y s t o d i s c r e d i t t h a t r u l e . C A B A R E T k n o w s , for e x a m p l e , several w a y s to " d i s c r e d i t " a r u l e : find cases w h e r e t h e consequent was d e e m e d n o t t o h a v e been est a b l i s h e d , even t h o u g h t h e r u l e f i r e d ; n a r r o w t h e reach of the open-textured words in the rule, and so f o r t h . T h e C o n t r o l D e s c r i p t i o n L a n g u a g e also c o n t a i n s d e s c r i p t o r s such as: near miss, near hit, open texture, point of view, most on point cases a n d p r i m i t i v e t a s k d i r e c t i v e s , such as forward chain, backward chain, filter cases, confirm a hit, confirm a miss, broaden, discredit. T y p i c a l tasks for the C B R process are analogize t w o cases ( p o i n t o u t t h e d i m e n s i o n s i n c o m m o n a n d l i k e values a l o n g such d i m e n s i o n s ) a n d distinguish cases ( p o i n t o u t d i m e n s i o n s n o t i n c o m m o n o r d i s s i m i l a r values a l o n g s h a r e d d i m e n sions) [ A s h l e y , 1988]. T h e C D L d e s c r i p t o r s near miss a n d near hit are a p p l i c a b l e t o b o t h t h e R B R side a n d the C B R side. G e n e r a l l y , a near miss is h a d w h e n a r e s u l t (say, one t h a t y o u w a n t ) i s m i s s i n g one c o m p o n e n t i n o r d e r t o o b t a i n . A r u l e - b a s e d near miss o c c u r s w h e n a l l b u t one c o n j u n c t o f a r u l e c a n b e e s t a b l i s h e d . A case-based near miss h a p p e n s w h e n a l l b u t one prer e q u i s i t e of a d i m e n s i o n ( i n d e x ) are p r e s e n t in t h e case k n o w l e d g e base. 8 E x a m p l e s o f rules i n t h e " n e a r m i s s " g r o u p are: • If you have all but one conjunct of the antecedent C A B A R E T uses a n a g e n d a - b a s e d C o n t r o l l e r i n w h i c h the heuristic c o n t r o l rules d i r e c t a n d interleave the t w o m o d e s o f r e a s o n i n g b y p o s t i n g a n d p r i o r i t i z i n g tasks for each t o d o . T h e c o n t r o l r u l e s are w r i t t e n i n C A B A R E T ' s Control Description Language, ( " C D L " ) , which provides a v o c a b u l a r y w i t h w h i c h t o express ( i ) h i g h e r level control descriptors t h a t d e s c r i b e at a f a i r l y h i g h level t h e s t a t e o r r e s u l t o f t h e case-based reasoner a n d t h e r u l e based r e a s o n e r , a n d ( i i ) tasks for t h e C o n t r o l l e r t o suggest for each r e a s o n e r . F o r i n s t a n c e , since w h a t a n exp e r t , o r C A B A R E T , does i n a p a r t i c u l a r s i t u a t i o n m a y d e p e n d o n w h a t side i s b e i n g a r g u e d f o r , t h e C D L has a d e s c r i p t o r for point-of-view. D e p e n d i n g o n w h e t h e r t h e user w a n t s t h e c o n s e q u e n t o f a r u l e t o b e e s t a b l i s h e d , point-of-view m a y be pro ( f o r ) or con t h e r u l e . 7 C h a n g i n g t h e p o i n t o f v i e w enables e x p l o r a t i o n o f a s i t u a t i o n f r o m various a r g u m e n t a t i v e vantage points. Four groups ( # 4 - 7 a b o v e ) o f C A B A R E T ' S h e u r i s t i c s c o n c e r n such arT
I n tax l a w , for instance, the Commissioner of I n t e r n a l Revenue may argue against a s t a t u t e t h a t gives a taxpayer a deduction f r o m his gross income, w i t h the taxpayer arguing for i t .
of a rule, and you want the rule to fire, broaden the rule. • If you have all b u t one conjunct of a rule, and you want the rule to fail, confirm the miss. C A B A R E T , in t u r n , knows a number of ways to " b r o a d e n " a r u l e , for e x a m p l e : • Use C B R to find cases where the rule did not fire, b u t the consequent of the rule s t i l l held. ( T h a t is, show t h a t the missing conjunct is not necessary to fire the rule.) t Use C B R to find cases where the rule did fire, and point out the similarities between those cases and the present case. ( Show that effectively you have the missing conjunct.) • Use C B R to find similar cases where the rule did not fire, but the u l t i m a t e disposition of the case was consistent w i t h the user's point of view. (Show 8
A C D L "near h i t " has analogous meanings for C B R and R B R . T h e term generally applies when there are many possible ways to establish a result, and all b u t one of t h e m have failed.
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that the rule firing is not necessary for the ultimate result the user wants.) • Expand the scope of any open-textured predicates in the missing conjunct.
5
bright-line sufficient condition for the s t a t u t o r y element "substantially late." • May-Not-Enter-Rule: Johnny may not enter the larder. • Reasonable-Rule: Johnny may reasonably enter the larder. • Late-Supper-Rule: If supper is substantially late, and Johnny is hungry, and Johnny receives permission his Mother, then Johnny may enter the larder. • When-Is-Late-Rule: Supper is substantially late if it is more than 30 minutes late.
A D e t a i l e d E x a m p l e of M i x e d P a r a d i g m Reasoning
To illustrate the above control heuristics in action, consider a further incident i n v o l v i n g our legalistic child Johnny: Johnny is home with his rapidly aging grandmother. His parents are out at the Royal Shakespeare Company's performance of "A Midsummer Night's Dream". Grandmother forgets about dinner. Johnny is hungry. It's 55 minutes past the usual dinner hour. He asks his grandmother for permission to enter the larder. She turns and smiles at him. She is off in another world. Johnny goes to the larder and feasts on bread and jam. His parents return. What result? 5.1 T h e C a s e K n o w l e d g e B a s e Let us assume t h a t Johnny has been involved in several incidents of entering the larder in addition to the " b r o o m pole" and " s a l m o n " cases, which he lost. T h u s , assume Case Knowledge Base contains: • The Broom Pole Case: Johnny used a broom pole to hook a pot of jam. He stood outside the larder. He was punished. • The Salmon Case: Johnny did not enter the larder but watched the household cat ravage the salmon. He was scolded. • The Babysitter Case: Johnny's parents were out and Johnny was watched by his usually iron-willed babysitter, Maggie. Johnny's parents forgot to tell Maggie anything about dinner. Supper was late and Johnny was hungry. Johnny asked permission from the babysitter to enter the larder. She said OK. Johnny feasted on scones and jam. When his parents returned, they said it was understood that Maggie, in lieu of Mother, could give permission to enter the larder. • Willful Disobedience Case: Johnny, who was peeved with his parents, went into the larder. He ate all the jam in the larder. He was not permitted to watch Dr. Who for month. • WeaseVs Case: Johnny convinced his pal, Weasel, to enter the larder. They ate. Mother sent Weasel home and Johnny to his room. • Case of the Glasgow Cousins: Johnny's poor cousins from Glasgow passed through. Johnny was the only one home. They asked for a bite before they went on their way. Johnny said O K . He asked them to hand him the j a m . They did. Johnny was rewarded for his generosity to his poor relations when his Mother returned. 5.2 T h e R u l e B a s e A simple rule base — w i t h several representative types of rules — suffices for our current i l l u s t r a t i o n . The first rule is the background p r o h i b i t o r y rule; the second provides a "reasonable m a n " standard often seen in AngloAmerican t o r t and c r i m i n a l law; the t h i r d provides a more detailed statute w i t h a number of open-textured elements, some deliberately so; and the last rule is a
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5.3 M i x e d P a r a d i g m S c e n a r i o s How rule-based and case-based processing might proceed depends on the point of view of the user, so posit t h a t Johnny and his Mother are at odds for this purpose: he wants to enter the larder; she wants to keep h i m out. C A B A R E T is being designed to leave processing traces like the following. 9 For convenience, we bracket the rule and italicize its category. Reasoning f r o m Johnny's Point of View • The Controller begins processing on the RBR side [ Ways to Begin Processing - Begin with RBR, backward chaining] • RBR begins with Reasonable-Rule (as it would yield a conclusion with Johnny's point of view.) • Controller uses [Deliberate Open Textured Predicate Use CBR] to find relevant cases construing "reasonably". • CBR finds no close relevant cases. • Controller uses [ Ways to Deal with Failure of Processing to yield a Definite Conclusion - Toggle] to switch back to RBR. t RBR backchains on Late-Supper-Rule. • RBR establishes conjunct "supper is substantially late" by application of When-Is-Late-Rule. • RBR establishes the conjunct "Johnny is hungry" by reference to the facts of the case. • RBR's reporter reports that it cannot establish the conjunct "Johnny receives permission from Mother" but it has established the other two. t Controller uses [Rule-based Near-Miss - show that you effectively have the missing conjunct] to suggest CBR on cases where the Late-Supper-Rule fired and the permission conjunct was effectively established. • CBR finds The Babysitter permission case as a most on point case and points out similarities between it and the current case. Also cites Glasgow Cousins case as being relevant. • Controller reports that a conclusion has been reached: it can be argued that Johnny may enter the larder, using the Late- Supper- Rule, modulo using The Babysitter Case to show satisfaction of permission predicate. Reasoning f r o m Johnny's Mother's Point of
View t The Controller applies [Ways to Begin Processing - Begin with RBR, backward chaining]. • RBR looks for rules with Mother's point of view, and seizes on May-Not-Enter-Rule. • RBR reports success on May-Not-Enter-Rule (there are no 9
For simplicity, assume here that the prioritizing mechanism for the agenda is the default one: that the agenda is a stack, and the most recently posted control task is the first one to run.
prerequisites to satisfy). • C o n t r o l l e r posts [ W a y s to C o n f i r m Processing: Sanity Check - Use C B R to c o n f i r m R B R satisfaction of t o p levelgoal] to the agenda. • C B R searches for cases where (the M a y - N o t - E n t e r - R u l e was fired a n d ) J o h n n y was not p e r m i t t e d to enter the larder. • C B R reports t h a t the Willful-Disobedience Case is a relevant case ( i n v o l v i n g a s t r a i g h t f o r w a r d application of MayN o t - E n t e r - R u l e , w i t h M o t h e r ' s point of view) b u t t h a t on the other hand The Babysitter and Glasgow Cousins cases are also o n - p o i n t and are cases against her position. • C o n t r o l l e r reports t h a t a conclusion can be reached w i t h M a y - N o t - E n t e r - R u l e and Willful-Disobedience Case as a supp o r t i n g case b u t t h a t the conclusion is suspect because of existing contra cases. T h e r e are several a l t e r n a t i v e w a y s t h i s s c e n a r i o c o u l d h a v e p r o c e e d e d . T h e C o n t r o l l e r c o u l d h a v e suggested " T r y t o e s t a b l i s h a n a r g u m e n t for M o t h e r ' s p o i n t o f v i e w b y s h o w i n g t h e f a i l u r e o f a l l rules e s t a b l i s h i n g J o h n n y ' s point of view." O r , n o t i n g t h a t C A B A R E T gives t h e user t h e o p t i o n t o choose t h e i n i t i a l reasoner, t h e user m a y h a v e s t a r t e d o f f w i t h C B R . So, d i f f e r e n t c o n c l u s i o n s a n d w a y s o f r e a c h i n g t h e m c a n b e u n e a r t h e d , even w i t h t h e s a m e r u l e s , case base, a n d c o n t r o l h e u r i s t i c s .
6
[ G o e l a n d C h a n d r a s e k a r a n , 1988] A s h o k Goel and B. Chandrasekaran. I n t e g r a t i n g Model-Based Reasoni n g a n d C a s e - B a s e d R e a s o n i n g . I n Proceedings o f the A A A I Workshop o n A I i n Design. A m e r i c a n Associat i o n for A r t i f i c i a l I n t e l l i g e n c e , A u g u s t 1988. S t . P a u l . [ H a m m o n d , 1986] K r i s t i a n 3 . H a m m o n d . CHEF: A M o d e l o f Case-based P l a n n i n g . I n Proceedings A A A I 86. A m e r i c a n A s s o c i a t i o n for A r t i f i c i a l I n t e l l i g e n c e , A u g u s t 1986. P h i l a d e l p h i a , P A . [ H a r t , 1958] H . L . A . H a r t . P o s i t i v i s m a n d t h e Separation of L a w and M o r a l s . H a r v a r d L a w Review, 71:593629, 1958. [ H a r t , 1961] H . L . A . H a r t . The C o n c e p t o f L a w . C l a r e n d o n Press, O x f o r d , 1 9 6 1 . [ K o l o d n e r , 1987] J a n e t L . K o l o d n e r . E x t e n d i n g P r o b l e m Solver C a p a b i l i t i e s T h r o u g h C a s e - B a s e d I n f e r ence. I n Proceedings o f the F o u r t h A n n u a l I n t e r n a t i o n a l M a c h i n e L e a r n i n g W o r k s h o p , I r v i n e , C A , 1987. Morgan Kaufman. [ K o l o d n e r , 1988] J a n e t L . K o l o d n e r , e d i t o r . Proceedings o f the D A R P A C a s e - B a s e d R e a s o n i n g W o r k s h o p 1988, C l e a r w a t e r B e a c h , F L , M a y 1988.
Conclusions
I n t h e l a w a n d o t h e r d o m a i n s , g o v e r n i n g rules o f t e n have w o r d s o r phrases t h a t c a n n o t b e d e f i n e d precisely. I n ord e r t o a p p l y s u c h rules one m u s t reason w i t h past cases i n o r d e r t o c l a r i f y a m b i g u o u s r u l e t e r m s . W e have d e v e l oped a heuristic approach to c o m b i n i n g reasoning w i t h rules a n d r e a s o n i n g w i t h cases i n o r d e r t o solve t h i s k i n d of interpretation problem. Our approach is grounded in a set o f c o n t r o l h e u r i s t i c s t h a t d e t e r m i n e s w h a t reasoni n g tasks t o p e r f o r m , g i v e n t h e s t a t e s o f t h e co-reasoners. T h e s e h e u r i s t i c s are a p p l i e d b y t h e C o n t r o l l e r m o d u l e o f C A B A R E T , a n a r c h i t e c t u r e w e are c o n s t r u c t i n g t o experiment w i t h heuristic approaches to the interpretation p r o b l e m and to p e r f o r m i n g m i x e d - p a r a d i g m reasoning in general.
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Cognitive Models
