Law and Defeasibility

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by Jaap Hage



Lawyers sometimes feel that logic has little to offer them. If the rule that thieves are punishable is applicable to the thief of a bike, logic seems to force the conclusion that this thief is punishable. However, as lawyers know, this need not be the case. If the thief was not accountable, he will, in most legal systems, not be punishable. Does this mean that the law is not logical, that the rule does not run as it seems to run, or that lawyers do not know how to reason? The answer is none of these, but instead that legal reasoning is defeasible. Somehow conclusions that at first sight seem to follow from the rules of law and the facts of a case do not follow after all. This entry deals with two issues. The first issue is what this defeasibility amounts to, and the second whether the law, or legal reasoning is defeasible.

Defeasibility as distinguished from nonmonotonicity

Before dealing with the question what defeasibility is, a possible source of confusion must be discarded. In logical theory there is a characteristic that pertains to some logics, most notably to systems of deductive logic, and does not belong to other systems. This characteristic, monotonicity, boils down to the following: A logic is monotonic if the conclusions that follow from a set of Premises, also follow from all supersets of Premises. This means that a derivable conclusion cannot become underivable through the addition of one or more premises.
A logic is nonmonotonic if and only if it is not monotonic. Nonmonotonic logics can be used to model defeasible reasoning in the law. However, nonmonotonicity is a characteristic of systems of logic, not of the law or informal legal reasoning. This entry only deals with defeasibility, not with nonmonotonicity. (But see the entry Nonmonotonic logic and defeasible legal reasoning in this encyclopedia.)

The nature of defeasibility

‘Defeasibility’ originally was a technical legal term, standing, according to Collins English Dictionary, for the capability of an estate or an interest in land of being defeated, or – what boils down to the same thing – being rendered void. In The ascription of responsibility and rights, Hart extended the use of this notion to allconcepts that have the property that there are a number of conditions of application, but also one or more circumstances that, if they occur, end the prima facie applicability of the concept. The concept of a contract is a typical example. A contract that has come into existence after an offer and acceptance can be invalidated if one of the parties involved invokes a defeating condition, such as fraudulent misrepresentation, or undue influence. Since this kind of defeasibility concerns the (retro-active) change of the facts, and not our beliefs about the facts, we may call it ontological defeasibility.
Hart connected defeat not so much to facts as to concepts. Concepts such as ‘(valid) contract’ would be defeasible, because they cannot be adequately characterised without reference to the conditions that would defeat their applicability. One may introduce a special term, conceptual defeasibility, for this defeasibility of concepts. Conceptual defeasibility is obviously closely connected to ontological defeasibility. Most, if not all, of our beliefs are amenable to revision. Some changes in the set of all our beliefs occur spontaneously, for instance because of sensory perception, or because we forget things that we used to know. Other changes are generated by the insight that beliefs should rationally be accepted or rejected given what else we believe. This insight may lead us to accept new beliefs that should rationally be accepted, or to reject beliefs that we held, because they should rationally be rejected. It is possible to call the revisability of our beliefs ‘defeasibility’, and to use the term epistemic defeasibility for this kind of defeasibility.
We accept some of our beliefs because it is justified to accept them given our other beliefs. For instance, we believe that John is punishable, because we both believe that John owns pornography and that owning pornography is punishable. If we stop believing that owning pornography is punishable, the belief that John is punishable loses its justification. The same holds if we acquire the beliefs that Johns owns pornography purely for scientific purposes and that owning pornography for scientific purposes is not punishable. To say it differently, the belief that John is punishable, which used to be justified given the original belief set, is not justified anymore given the new belief set. We may call this kind of defeat, which results from changes in the beliefs that underlie another belief, justification defeat.
Sometimes the notion of defeasibility is also used in connection with conditionals (logical operators) and with rules. A conditional p ® q can be said to be defeasible if one or more of the following are the case:

1. if p ® q is true, then it is not necessarily the case that p & r ® q is true (no ‘strengthening of the antecedent’);

2. if both p ® q and p are true, then it is not necessarily the case that q is true;

3. if both p ® q and p are true, then it is not necessarily the case that q can be derived validly (where validity is taken in a broader sense than deductive validity, because otherwise this third possibility would coincide with the second).

The ‘defeasibility’ of conditionals is not primarily a phenomenon outside logic that can be modelled by means of some logical theory, but rather a characteristic of some logical theories. In this respect it is closely related to nonmonotonicity.
A rule ‘if conditions, then conclusion’ may be said to be defeasible if it is not necessarily the case that if the rule conditions are satisfied, the conclusion holds (Prakken and Sartor 1996). The rule that thieves are punishable might be considered defeasible in this sense, because it may be the case (if there is a ground of justification) that somebody is a thief, and nevertheless not punishable. When a rule is defeasible in this sense, making an exception to the rule does not mean that the rule is not applied correctly. It is part of the ‘logic’ of defeasible rules that it is possible to make exceptions to these rules even when the rule’s conditions are satisfied, and without reinterpreting the rule to make the absence of an exception part of the rule conditions.

Is legal reasoning defeasible?

The next question to deal with is whether defeasibility plays a role in legal reasoning. I assume without further argument that ontological and conceptual defeat play a role in the law (see MacCormick 1995 for examples), and confine my discussion of this question to the defeasibility of legal rules and to justification defeat in connection with the division of the burden of proof. (Bayon 2001 discusses some other reasons why legal reasoning might be defeasible.)
One reason why legal reasoning might be defeasible is based on the assumption that legal rules are defeasible in the sense that it is possible to find implied exceptions that can often not be specified in advance. This assumption is broadly shared. (Bayón 2001 gives a number of references in footnote 21.) It may, however, be argued (as Bayón 2001 does) that it is not a necessary characteristic of legal rules that they are thus defeasible. A legal system might make it impossible to allow exceptions to rules, even if they are over-inclusive (apply to cases to which they were not intended to apply), or if some relevant principle was not taken into consideration in drafting the rule.
There are two extreme ways to deal with exceptional cases in which rules give ‘wrong’ solutions for cases. One extreme way is to ignore the rule in such cases and fall back on the principles, values or policies underlying the rule, and on all other principles, values or policies that might turn out to be relevant, and decide on the best outcome on the basis of all relevant factors. On this approach, the presence of the rule does not make any difference to legal decision making, because the rule is only applied if its outcome agrees with the outcome of the underlying principles. The rule is then superfluous next to the principles; it is merely a ‘rule of thumb’. (Schauer 1991, p. 77) The other extreme way is to apply the rule, without any regard to whether its outcome is correct in the light of the relevant principles. On this ‘entrenched’ model (Schauer 1991, p. 52.), the applicability of a rule makes principles that deal with the case at hand superfluous. By describing the two mentioned ways as extreme, I suggested that a middle way is possible. This middle way is to take applicable rules as the starting point in legal decision making, but to leave the possibility open to deviate from the rule’s outcome if this is desirable in the light of the relevant principles. Whether this possibility to deviate should be used, does not only depend on the outcome of balancing the relevant principles, but also on the facts that deviating from the rule diminishes legal certainty and the legislator’s authority, both of which are reasons not to deviate from the rule. The amount of weight that is attached to the applicability of a rule as an independent reason for the rule’s conclusion, determines whether this middle way runs closer to the first, or to the second extreme. (This way to deal with rules is at least similar to what Schauer calls rule-sensitive particularism. See Schauer 1991 p. 97 and the literature mentioned there.) It is imaginable that there are legal systems which do not allow exceptions to any rule. However, there are very good reasons why a legal system should sometimes allow exceptions to at least some rules, and the possibility of a legal system that does not allow exceptions to rules seems to be a mere theoretical one.

Defeasibility and the burden of proof

At least some legal conclusions can turn out to be unjustified in the light of new information that was not taken into account in drawing them. Suppose that Violet speeded and is prosecuted. If the prosecutor succeeds to prove the speeding and nothing else happens, the judge is justified in her conclusion that Violet is punishable. However, if Violet defends herself by pointing out that her child was seriously ill and that she speeded under force majeure to bring her child in time to the hospital, acceptance of this defence by the judge would take the justification of this conclusion away. In other words, the information that Violet acted under force majeure functions as a justification defeater for the conclusion that Violet is punishable for speeding.
The division of the burden of proof can also be explained by means of procedural rules that allow a judge to convict a defendant if a transgression was proved, while force majeure was not proved (see Bayón 2001). Under this interpretation, which focuses on procedural aspects rather than on the question whether the conclusion that the suspect is punishable is justified, no defeat seems to be at stake. Arguably, however, justification defeat plays a role under this interpretation of the burden of proof too. The procedural rules that indicate under which circumstances a judge can convict a suspect, reflect the circumstances under which a judge is justified in believing that the suspect is punishable. By default a judge is not justified in assuming that somebody is punishable (presumption of innocence). Therefore it must be proved that the suspect committed a fact that is punishable. When this has been proved, the judge is pro tanto justified to believe that the suspect is punishable. However, if it has also been proved that there was a ground of justification, the belief that the suspect is punishable is not justified anymore. Therefore the procedural rules only allow the judge to convict a suspect if it was proved that he committed a crime, and if it was not proved that there was a ground of justification. Both references in the procedural rules to proof, rather than to fact, are signs that some form of defeasible reasoning is at stake.
In general defeasibility plays a role when conclusions are not based on which facts obtain, but on which facts have been proved. Facts about the past do not change, but beliefs about the past, including what has been proved about the past, tend to evolve in the course of time. As a consequence, beliefs that are based on what was proved are liable to lose their justification if relevant facts that did not count as proved before, come to count as having been proved.

Related entries

ArgumentationExclusionary reasonsFormalism in the lawJustificationLegal theory constructionNon-monotonic logic and defeasible legal reasoningPrinciples and rulesWeighing and balancing in the law.

Annotated bibliography

Bayón, J.C., Why is legal reasoning defeasible?. In A. Soeteman (ed.), Pluralism and law. Dordrecht: Kluwer 2001, p. 327-346.

A good discussion of the possible reasons why legal reasoning might be defeasible. Bayón is in general sceptical about these reasons.

Brozek, B., Defeasibility of legal reasoning, Zakamyce: Kantor 2004.

Contains an extensive discussion of the use of several kinds of nonmonotonic logic to model legal reasoning.

Hage, J.C., Reasoning with rules, Dordrecht: Kluwer 1997.

Contains a philosophical discussion of reasons and applies this discussion to analyse many variants of defeasible legal reasoning. Reason-based Logic is proposed as a logical tool to deal with this kind of reasoning.

Hage, J.C., Law and Defeasibility, Artificial Intelligence and Law 11 (2003), p. 221-243.

The paper on which this entry was based. Deals more extensively with the various forms of defeasibility, with the defeasibility of legal reasoning, and with the question whether it is useful to model defeasible legal reasoning by means of a nonmonotonic logic.

Hart, H.L.A., The Ascription of Responsibility and Rights. Proc. of the Aristotelian Society, 49 (1949), p. 171-194.

The paper in which Hart introduced the notion of defeasibility in legal theory.

Loui, Ronald P., Hart’s Critics on Defeasible Concepts and Ascriptivism. Proceedings of the Fifth International Conference on Artificial Intelligence and Law, New York: ACM 1995, p. 21-30.

A useful overview of the impact of Hart’s views on defeasibilty on the work in jurisprudence, ethical theory and logic.

MacCormick, D.N., Defeasibility in Law and Logic, Z. Bankowski e.a., Informatics and the Foundations of Legal Reasoning, Dordrecht: Kluwer 1995, p. 99-118.

A jurisprudential account of ontological defeasibility in the law, in which the connection with institutional legal facts receives attention.

Peczenik, A., On Law and Reason, Dordrecht: Kluwer 1989.

A comprehensive study of legal justification that assigns a central role to defeasibility under the headings of ‘prima facie statements’, ‘weighing and balancing’, and ‘jumps’.

Prakken, H. and G. Sartor. A Dialectical Model of Assessing Conflicting Arguments in Legal Reasoning. Artificial Intelligence and Law, vol. 4 no. 3/4. (1996) p. 331-368. Also in H. Prakken and G. Sartor (eds.), Logical Models of Legal Argumentation. Kluwer Academic Publishers, Dordrecht 1997.

Contains an influential logical model of defeasible legal reasoning.

Prakken, H., Logical Tools for Modelling Legal ArgumentA Study of Defeasible Reasoning in Law, Dordrecht: Kluwer 1997.

A thorough, but relatively technical treatment of logical tools to deal with defeasible reasoning, especially in the law.

Prakken H. and G. Sartor (eds.), Logical Models of Legal Argumentation, Dordrecht: Kluwer 1997.

A set of papers that provides a good overview of work in the field of Artificial Intelligence and Law that deals with modelling defeasible legal reasoning. It also includes an insightful paper by Peczenik that connects this work to jurisprudence.

Prakken H. and G. Sartor, The three faces of defeasibility in the law, Ratio Juris 17 (2004), vol. 1, 118-139.

In this paper, Prakken and Sartor distinguish three ways in which defeasibility plays a role in the law, namely in connection with inferences, with the legal process and with legal theory construction.

Sartor, G., Defeasibility in legal reasoning, Z. Bankowski e.a., Informatics and the Foundations of Legal Reasoning, Dordrecht: Kluwer 1995, p. 119-148.

A jurisprudential discussion of defeasibility in legal reasoning with attention for the implications for logical theory.

Schauer, F., Playing by the rules. Oxford: Clarendon Press 1991.

A jurisprudential discussion of the ‘logic’ of rule application, in which the defeasibility of (reasoning with) rules plays a central role.

Toulmin, S.E., The Uses of Argument, London: Cambridge University Press 1958.

The classic work that includes the paper (The layout of arguments) in which legal reasoning is adduced as a bench-mark for the evaluation of logical theories, and in which one of the earliest accounts of defeasible reasoning is given.

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