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«The missing link revisited: The role of teleology in representing legal argument T. J. M. BENCH-CAPON Department of Computer Science, The University ...»

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Artificial Intelligence and Law 10: 79–94, 2002.


© 2002 Kluwer Law International. Printed in the Netherlands.

The missing link revisited: The role of teleology in

representing legal argument


Department of Computer Science, The University of Liverpool, Liverpool, UK

E-mail: tbc@csc.liv.ac.uk

Abstract. In this paper I recapitulate the ideas of Berman and Hafner (1993) regarding the role of

teleology in legal argument. I show how these ideas can be used to address some issues arising from more recent work on legal argument, and how this relates to ideas associated with the “New Rhetoric” of Perelman. I illustrate the points with a discussion of the classic problem of which vehicles should be allowed in parks.

1. Introduction The first morning of ICAIL 1993 in Amsterdam concerned argument. The first two papers presented, Henry Prakken’s logical framework for modelling legal argument (Prakken 1993) and Tom Gordon’s paper on using dialogue games to model legal argument (Gordon 1993), have been enormously influential and there has followed a great volume of work articulating, developing and combining these approaches.

Just before lunch on that morning a no less interesting paper with an entirely different approach was presented, Don Berman and Carole Hafner’s Representing Teleological Structure in Case Based Legal Reasoning: The Missing Link (Berman and Hafner 1993). Unfortunately this has not produced anything like the same degree of subsequent research. In this paper I want to recapitulate the ideas in that paper and to re-examine them in the light of what we have learnt about modelling legal argument since 1993. In doing so I hope to establish that the time is ripe to pay some very serious attention to their ideas.

This paper was originally written in December 1999. Since then it has received responses from Henry Prakken and Giovanni Sartor (in this volume), and has been developed in on-going work, both by Henry Prakken and by myself in collaboration with Giovanni Sartor. The most recent expression of my views can be found in Bench-Capon and Sartor (2001). In preparing this paper for publica- tion I have attempted to remain faithful to my original views, and not to anticipate too much later developments, although I have taken the opportunity to correct some mistakes in the original version.


2. Hunting, shooting and fishing The paper presented in Amsterdam (Berman and Hafner 1993) was based on three cases, commonly used in teaching American students law. In the first, Pierson v Post, the plaintiff was hunting a fox, on open land, in the traditional manner using horse and hound when the defendant killed and carried off the fox. The plaintiff was held to have no right to the fox because he had gained no possession of it. In the second case, Keeble v Hickeringill, the plaintiff owned a pond and made his living by luring wild ducks to the pond with decoys, shooting them and selling them. Out of malice the defendant used guns to scare the ducks away from the pond. Here the plaintiff won. In a third case, Young v Hitchens, both parties were commercial fisherman. The plaintiff spread a net, some half a mile in length, and began to close it. When the opening was no more than a few dozen feet wide, the defendant sped into the gap, spread his own net and caught the fish which had been trapped by the plaintiff as he closed his net. In this case the defendant won.

Berman and Hafner then constructed an argument for the defendant in Young, against the background of Pierson and Keeble. They say that they are following the approach used in HYPO (Ashley 1990). They do indeed use the three-ply style of argument developed in HYPO, in which a party to the case cites a precedent, the opponent responds and the original party attempts to rebut the response. Their representation of the cases, however, differs somewhat from the original conception of HYPO (e.g., Ashley and Rissland 1988). Berman and Hafner use factors, which are features of the case which may be present or absent and which, if present, favour either the plaintiff (pro-plaintiff factor) or the defendant (pro-defendant factor).

HYPO in contrast used dimensions, which are features of the case which can take a range of possible values, ordered according as to the extent to which they favour a particular side. Consider the question of whether the hunter had possession of the animal. Seen as a factor, we simply ask whether the animal was caught or not. Seen as a dimension, we can have a range of possibilities progressively more favourable to the defendant, running from caught to no contact at all, and passing through some intermediate positions such as mortal wounding, wounding, hot pursuit, started, and seen. Using dimensions permits a richer representation of the case situation, and allows us to avoid some awkwardness in choosing factors, such as whether the factor should be caught (pro-plaintiff), or not caught (pro-defendant), or perhaps both, as well as not requiring us to make “all or nothing” decisions.

Using factors, on the other hand, does make a useful simplification. In fact the use of factors has in recent years become rather more common than dimensions: the description of cases in CATO (Aleven 1997) and the reconstruction of case based reasoning of Prakken and Sartor (1998) both use factors rather than dimensions.

In the remainder of this paper I shall use factors in this way, although I think dimensions remain important and interesting, and I hope to explore the difference that using them makes in some future work.



Berman and Hafner can be seen as identifying five factors. Two are proplaintiff: that the plaintiff was following his livelihood (LIVELIHOOD) and that the land was owned by the plaintiff (OWNLAND). Three are pro-defendant: that the animal was not caught (NOTCAUGHT), that the land was open (OPEN) and that the defendant was in competition with the plaintiff (COMPETE). Pierson contains NOTCAUGHT and OPEN, Keeble OWNLAND, LIVELIHOOD and NOTCAUGHT and Young NOTCAUGHT, OPEN and COMPETE. Actually Berman and Hafner speak of four factors, one of which, the status of the land, takes two values. This makes it look rather like a dimension (although if it were a dimension we might expect more than these two values, and include, for example, the possibility of the defendant owning the land). I think it is more consistent to see five factors, but to bear in mind that OWNLAND and OPEN are mutually exclusive.

With these five factors, Pierson looks rather clear: the plaintiff does not own the land, did not catch the beast and was looking for pleasure rather than business, so only pro-defendant factors are present. Keeble, although the plaintiff was not in possession of the ducks, had the pro-plaintiff factors that the land was owned and he was engaged in a commercial pursuit, and we know that these were sufficient to overcome the pro-defendant factor NOTCAUGHT.

If we wish to make an argument for the defendant in Young, we can propose that Pierson is followed. The plaintiff can reply by distinguishing on the grounds that the plaintiff is making his living, and cite Keeble to show that not having captured the prey is not fatal to the plaintiff’s case. The defendant can now give a rebuttal, since he can distinguish Keeble, on the grounds that in Young the water is not owned by the plaintiff, and further add that the motive of the defendant was not malice but business competition. So the issues are identified, but the question remains as to whether are persuaded (or are able to persuade a judge) that the extra pro-defendant factor and the missing pro-plaintiff factor in Young are sufficient to cause us to reject Keeble.

And this is where the reasoning in this model runs out. Although we have identified some candidate cases, the question remains as to which should govern Young, and why it should do so. To find clues as to whether we can give reasons for following Pierson rather than Keeble we need to examine the texts of the decisions.

Importantly these texts refer to the purposes that the judges saw as being promoted by their decisions. Pierson was found in favour of the defendant For the sake of certainty, and preserving peace and order in society. If first seeing, starting or pursuing such animals... should afford the basis of actions... it would provide a fertile source of quarrels and actions. (Quoted in Berman and Hafner 1993, italics mine).

One judge dissented: for him the pursuit and destruction of foxes was of sufcient social value to be encouraged and protected by law. Two points should be noted: first that the reason why capturing the animal matters is because only that is considered sufficiently clear evidence of a right to the animal, and second that what seemed to be a clear case containing only pro-defendant factors was in fact 82 T. J. M. BENCH-CAPON disputed. Perhaps we should have included in our analysis an extra, pro-plaintiff, factor, relating to the social value of the plaintiff’s activity, to reflect that the case was in fact decided by weighing competing social values.1 In the second case, Keeble, the social utility is greater, and the evidence for this is that the plaintiff could earn a living from hunting ducks. Although the right to make one’s living

undisturbed is alluded to, the crucial point is that:

When decoys have been used... in order to take a profit for the owner of the pond... and whereby the markets of the nation may be furnished; there is great reason to give encouragement thereunto (Quoted in Berman and Hafner 1993, italics mine).

In other words here (unlike in Pierson) we can be sure that the plaintiff’s activity is valuable because we know he can make a living from it, and so people are prepared to pay for his activity. We can assuage our doubts with respect to certainty because we have a clear criterion for saying that the activity is one to be encouraged. When we come to Young, the social benefit is neutral – the same fish furnish the markets of the nation whether they are delivered by Young or Hitchens.

Having removed this reason, the appeal to the need for certainty can prevail, just as it did in Pierson. Arguably, in addition in this case the decision can also be seen as encouraging vigorous competition, which may have even greater economic benefits.

Seen from this teleological perspective, and informed by the reasons for the

rules as well as the rules themselves we can see two things:

• That we can come to a rational2 decision as to the case to follow;

• That apparent similarities and differences (the open land in Pierson and the owned land in Keeble, and the engagement in making a living in both Keeble and Young) may be more or less useful. Distinguishing on the ownership of the land weakens the effect of Keeble for the plaintiff in Young, but does little to positively promote a decision for the defendant, unless we wish to argue that potential presence of ducks on a pond confers possession of the ducks on the pond owner.

Berman and Hafner then proceed to an analysis of an example of reasoning performed by Branting’s GREBE system (Branting 1991). Here they clearly show how relying on rules manifested in cases without reference to the purposes of these rules leads the reasoner astray, through pursuing false similarities and differences between the cases.

The last section of the paper gives some suggestions for augmenting a representation of cases in terms of factors with the legal purposes which explain why each of the factors favours the plaintiff or the defendant. Now we can judge competing arguments not only on the importance of the factors themselves, but on the value we accord to the purposes from which they arise. This both makes the arguments more realistic, and the choice between competing arguments less arbitrary.



3. Theory construction and theory coherence I have recapitulated Berman and Hafner (1993) at some length, both because it is an excellent paper, and because it provides a strong argument that working from decisions without their reasons will often leave us with no reason to prefer one precedent to another, and can even lead us into error. In this section I shall consider one of the leading approaches to modelling legal argument that has been developed since 1993, and suggest that it also is unable to resolve disputes because it ignores the purpose of the rules it employs.

In a series of papers, of which we can take Prakken and Sartor (1998) as representative, Henry Prakken and Giovanni Sartor have articulated a model of legal reasoning based on a logical formalisation, and the notion of a dialogue game.

Their main aim is to allow conflicting norms to be reconciled, and the essential idea is that to win a case one must put forward an argument which can be defended against any attack. Attacks may either be undercutting, attacking a premise in the original argument, or defeating, establishing the negation of the conclusion of the original argument. An attack can be defended against either by attacking the attacking argument, or, in the case of a defeater, claiming a higher priority for the rule grounding the original argument than that grounding the putative defeater. As the debate proceeds, arguments introduce new rules, and their coherence with other rules is tested. What is happening here is that we are constructing a theory (in the logical sense), which is considered coherent if its sentences are able to withstand attack from within the theory. This is excellent as an ex post reconstruction of the reasoning, because here we can use the decision itself to reveal priorities. It, is however, less useful ex ante, since it is unclear where these priorities come from.

In Prakken and Sartor (1998) they make use of their account to reconstruct a style of factor based reasoning, such as we have seen above. Here each case gives

rise to three rules:

• A rule of the form if conjunction of all pro-plaintiff factors present then plaintiff

• A rule of the form if conjunction of all pro-defendant factors present then defendant

• A rule expressing that one of these rules has a higher priority that the other, depending on the way the case was decided.

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