Generating Legal Arguments

The most characteristic feature of the Common Law is the doctrine of precedent. According to this doctrine, each and every case decided by the court becomes a part of the law itself, not merely an explanation of the law which later judges may adopt or reject.¹

The danger in such an doctrine is that the law may become completely inflexible. In order to prevent this, there have been a number of devices developed to avoid "unwanted" outcomes. These devices include the obvious ones such as having regard to the seniority of the court which decides the case, but also less obvious ones which become a part of the legal reasoning process.

The process of reasoning with case law is so fundamental to the development of the Common Law that it forms a major part of the curriculum in law schools. Students are taught to read a body of case law, to apply that law to new fact situations and to use the legal technique of "distinguishing" previous cases.

This form of legal reasoning is in some respects similar to the general problem of reasoning by analogy, but there are some specific and difficult differences. First, there is a deontic aspect, for even those cases which are not strictly binding on a court have some legal force. A second difference is that the process contains formalised methods for "distinguishing" past cases. A case is distinguished by noting legally significant differences from previous cases which may be used to justify rejecting the outcome of the previous case insofar as its applicability to the problem at hand is concerned.

At one time it was thought that each decided case stood for a single rule of law which could be formulated in much the same way as a section of a statute, but this theory has been shown to be completely unworkable.² It is now clear that any interpretation of the legal significance of a case must be in the larger context of the legal material in which it is embedded.³ The meaning of a case can and does change as the body of knowledge surrounding it changes.

Perhaps for this reason, there has been little success in dealing with the general problem of reasoning with case law by means of the usual production rule formulation. It is not that it is theoretically impossible to write such rules, but that it is not the natural way in which lawyers reason with cases. If this is correct, then attempting to use the usual rule based systems will probably result in mediocre performance of the system and will certainly be wasteful of time in the knowledge engineering process.

It might be thought that systems which deal only with statute based law might avoid this problem of specialised common law reasoning, but that is not the case. This is because most statutes have received interpretation by the Courts which either clarify or change their meaning. Furthermore, the interpretation of statutes is itself a reasoning process which is unique to law.

Every lawyer has a favourite example of a statute which does not mean what it would appear (to a non-lawyer) to say. Here is ours: the statute reads "…every person shall..".⁴ It is clear that the "correct" representation is:

(all x)(if person(x) then ...)

However, the New Zealand Court of Appeal held that the correct representation was:

(all x)(if person(x) and not(solicitor(x)) then...)

It is important for the non-lawyer to note that this decision did not come as any great surprise to lawyers who were knowledgeable in the field. Any expert could have advised that the naive translation was risky.

The result was reached by applying well known and well understood techniques of legal reasoning. Some of those techniques involve the application of old case law; others are rules of interpretation used by lawyers when reading statutes. However, these rules of statutory interpretation are not in a form which allows them to be expressed directly in a production rule system.⁵ One of the recent Australian textbooks on the subject says:⁶

It is important at the outset to stress that these are nothing more than approaches and presumptions. To elevate them, as is so often done, to the level of "rules" is but to mislead as it invites the assumption they will be strictly applied by the courts…the so-called rules can only be regarded as aids to interpretation.

It is clearly possible to write rule based systems which would assist in applying that reasoning to assist in the interpretation of statutes, but the point is that a simple translation of a statute into executable code is likely to produce a machine lawyer that is a candidate ripe for disbarment.

In spite of these difficulties, there have been some useful rule based legal systems developed.⁷ Although most of these deal with statute law, there are a few which have attempted to deal with case law related subjects.⁸ For the reasons mentioned above, we believe that an alternative approach to the problems of case law will produce better systems at a cheaper cost.

One such area is the dispute which arises when a person finds an object which is then claimed by another party. In the proto-typical case, the item is found on premises which are occupied by the other claimant.

The law which governs such disputes is contained almost entirely in the decided cases.¹¹ Although there are not a large number of cases, they are complicated and difficult to resolve. Since each of the parties is seeking a windfall, policy is difficult to formulate and there is little sense of justice to guide the court. Of these cases, one judge has said:¹²

These cases…have long been the delight of professors and text writers, whose task it often is to attempt to reconcile the irreconcileable.

Although the cases go back to 1721, the most recent one is Parker v British Airways.¹³ Parker was a passenger on British Airways. While waiting in the VIP lounge, he found a gold bracelet lying on the floor. He took the bracelet to lost and found and left it with instructions that it was to be returned to him if the owner of it was not found. Although BOA did not find the owner, they did not return it to Parker but instead sold it for some £800.

This paper describes the FINDER system which was designed as an experimental testbed using the finder cases. The Parker case is not included in the knowledge base of the FINDER system. An opinion written by the system is included as Appendix A.

The form of the output of FINDER illustrates an important feature of legal expert systems. Justification plays a different role in a legal expert system than in other advising systems. Usually the justification mechanisms are rudimentary and are clearly designed to give the user confidence in the decision reached by the system. A system may advise "take two aspirin and go to bed"; the justification provided for that advice will usually consist of a listing of the rules used to derive the conclusion.

By contrast, the justification in a legal system is the main product, for the justification is no more and no less than the legal arguments which support the suggested outcome. It is in the nature of legal reasoning that these arguments must also address the support for the opposite outcome. It is these arguments which, if the matter goes to court, must be presented for adjudication.

In this regard, a legal expert system is more akin to design systems than to the traditional diagnostic systems of medicine. The FINDER system uses a simple but effective model for the construction of legal arguments and presents them in a form which is familiar to lawyers.

The reasoning model used in the FINDER system has been incorporated as a separate reasoning model in a more general legal expert system shell called XSH.¹⁴ XSH supports the usual procedural and rule based reasoning, but also allows for case-based legal reasoning on the FINDER model. The module is called PANNDA, an acronym for Precedent Analysis by Nearest Neighbour Discriminant Analysis.

The law relating to finders is entirely case law. Indeed, the area was chosen for experimentation for precisely that reason. It is an unusual area in this respect, for almost all serious legal problems involve both statute and case law.

There does not seem to be any direct method of using the PANNDA approach for the analysis of statute law except for the somewhat artificial method of concocting hypothetical cases. If this approach were used, it would be necessary to change the REPORT subsystem very substantially, for the current format does not really make sense for hypothetical cases. Perhaps the case summaries could be replaced by some sort of argument structure which would reveal why the hypothetical case was chosen and why it is thought that the outcome is that suggested.

It seems more logical to handle "pure" statute law by means of rule based systems. But even in the FINDER model, integration with a rule based system would make a great deal of sense. One immediate suggestion is a "screening" rule which would take care of the problem of a finder who is a trespasser on the land of the other party. In such a circumstance, there is scarcely any reason to continue with the other questions or to invoke the FINDER inferencing model.

This is just one example of what lawyers call the "policy" issues involved in finders cases. There are certain circumstances which have never arisen in reported cases for the simple reason that the answer is so obvious that only a fool would attempt to defend the issue in court.

Another example might be that the place where the object is found is a bank vault; although the finder may not be a trespasser, the occupier of the premises, the bank, clearly permits the entry only for very limited purposes and it would scarcely make any sense to suppose that the finder of an article in such a place would be entitled to it in preference to the occupier.

Although lawyers refer to these as policy issues, they are actually rather closely tied to fact situations which can be described in the knowledge base language. Again, these situations could be the basis of hypothetical "policy" cases in the knowledge base. The cases could then be given a very high weighting in the nearest neighbour algorithm to ensure that "policy" was not lightly overridden.

However, it seems more straightforward to phrase these policy situations as rules. If that is done, then the problem becomes one of devising a smooth interface between the rule based knowledge system and the PANNDA model.

One way of doing this has been implemented in XSH.¹⁷ In essence, there is a single rule which initiates the session:

IF POLICY is OK and PRECEDENT is OK THEN\\
OUTCOME is FINDER WINS\\
ELSE\\
OUTCOME is FINDER LOSES

The initiating goal is to find a value for OUTCOME. Determination of POLICY is done by means of backward chaining production rules. Once the determination of the value of POLICY is complete the system proceeds to the PANNDA module to determine the case law aspects of the problem.

This approach has worked well in the finders cases, eliminating the asking of irrelevant questions when the situation is one which falls within the policy considerations of the finders cases. The rule based procedures also manage to filter out some anomalous cases which would otherwise need to be handled by the consistency checks in the PANNDA module.

The mixed model also provides another valuable consistency check, for if the POLICY and PRECEDENT variables indicate different outcomes, then the case is clearly "difficult" for the system.

The following is an opinion written by FINDER given the facts of Parker v British Airways [1982] 1 All E R 834. Parker found a gold bracelet in the public area of the VIP Lounge of British Airways. He turned it in to lost and found with instructions that BA should attempt to find the true owner of the bracelet. Failing that, Parker asked that the bracelet be returned to him. BA sold the bracelet when they were unable to find the true owner. BA leased the terminal where the lounge was located.

The consultation begins by the system soliciting relevant facts from the user. When sufficient information has been gathered, the system writes an advice. The opinion below is presented exactly as it comes from the computer.

In my opinion, the outcome is the same as that in Bridges v Hawkesworth, that is, the finder should win.

In Bridges v Hawkesworth (1851) 21 LJQB 75 the plaintiff found a bundle of banknotes on the floor of the public area of a shop. He handed the notes to the shopkeeper in order that the true owner of the notes might be found. Although the owner never was found, the shopkeeper refused to return the notes to the finder. The court found for the finder, holding that there is a "general right of [a] finder to any article which has been lost as against all the world except the true owner". It was further noted that the notes had never been in the custody of the defendant nor within the protection of his house as might be the case had they intentionally been deposited there.

There are many similarities: the finder is not the occupier, the chattel is not attached, there is a bailment of the chattel and the chattel is not hidden.

Of course, the present case is not on all fours with Bridges v Hawkesworth since in that case the non-finder was the owner of the real estate.

The opposite result was reached in Yorkwin v Appleyard.

In Corporation of London v Appleyard [1963] 1 WLR 982 workmen employed by Wates Ltd were engaged in cutting a key-way into a cellar wall for the purposes of securing a foundation when they found an old wall-safe built into a recess of the old wall. Inside was a wooden box which contained a large number of Bank of England notes. The notes were handed over to the City of London police who sought interpleader proceedings to determine who was entitled to the possession of the notes.

Wates Ltd were an independent contractor engaged by Yorkwin Ltd for a construction project. Yorkwin were lessees in possession of the property which was owned in fee simple by the Corporation of London.

The Court followed the decision in South Staffordshire Water Co v Sharman [1896] 2 QB 44 in holding that the occupier is, in the absence of a better title elsewhere, entitled to the possession of objects which are attached to or under the land. Consequently, since the notes were in a wooden box within a safe built into the wall of the old building, the safe formed part of the demised premises. Yorkwin, being in lawful possession of the premises, were in de facto possession of the safe, even though ignorant of its existence.

Although Yorkwin was entitled to possession as against the finders, they in turn were displaced by the Corporation of London which relied successfully on a term in the lease which granted them the right to certain objects found on the premises.

But Yorkwin v Appleyard has some significant differences: the chattel was attached, there was not a bailment of the chattel, there was a master/servant relationship between the parties and the chattel was hidden. Consequently, I think there is nothing in Yorkwin v Appleyard to warrant any change in my conclusions.

Return to Alan's Home Page. Please send inquiries & questions to Alan L Tyree.