Defeasibility in Legal Theory

The idea of defeasibility remains highly controversial, as evidenced by the contributions contained in a recent collection (Ferrer and Ratti 2012).
Carlos Alchourron, a leading legal logician, has opposed the ideal of defeasible reasoning, arguing for a combination of systematic interpretation and deduction: systematic interpretation should merge rules and exceptions into a coherent whole to which deduction could be applied (see Alchourron 1996a, 1996b). Other legal theorists, such as Alexander Peczenik (Peczenik 2005, 115ff., Hage and Peczenik 2000) and Neil MacCormick (1995), have on the contrary argued that defeasibility plays a significant role in legal reasoning (see also Brozek .
It is no easy task to review the legal theorists’ approaches to defeasibility, since such theorists have advanced different understandings of defeasibility, which often do not comport with the idea of defeasibility as nonmonotonic reasoning. Brozek (2014) has pointed out different ways in which defeasibility is understood in Ferrer and Ratti (2012a), a collection of approaches to defeasibility in legal theory:
Ferrer Beltrán and Ratti consider, inter alia, the following formulation: “a norm is defeasible when it has the disposition not to be applied even though it is indeed applicable” (Ferrer Beltrán & Ratti 2012b: 31). Frederick Schauer, in turn, claims that “the key idea of defeasibility […] is the potential for some applier, interpreter, or enforcer of a rule to make an ad hoc or spur-of-the-moment adaptation in order to avoid a suboptimal, inefficient, unfair, unjust, or otherwise unacceptable, rule-generated outcome,” and concludes that “defeasibility is not a property of rules at all, but rather a characteristic of how some decision-making system will choose to treat its rules” (Schauer 2012, 81 and 87). Jorge L. Rodríguez says that “when we express a conditional assertion, we assume the circumstances are normal, but admit that under abnormal circumstances the assertion may become false”, and—transferring this characteristic of defeasibility into the domain of law—claims that “legal rules [are defeasible since they] specify only contributory, yet not sufficient, conditions to derive the normative consequences fixed by legal system” (Rodríguez 2012, 88). […] Finally, Riccardo Guastini claims that legal rules are defeasible since “there are fact situations which defeat the rule although they are in no way expressly stated by normative authorities in such a way that the legal obligation settled by the rule does not hold anymore.” (Guastini 2012, 183)
All the foregoing formulations point to interesting aspects of legal reasoning and to the practice of defeasible reasoning in the law. I would argue, however, that they fail to provide convincing redefinitions or clarifications of the notion of defeasibility. I have argued that defeasibility applies to three objects:
These three aspects are different faces of the same issue. A defeasible argument \(\mathcal{A}\) consists in a nonmonotonic inference: if we expand the argumentation basis from which \(\mathcal{A}\) is constructed with premises that enable the construction of a defeater \(\mathcal{B}\) to \(\mathcal{A}\), the conclusion of A will no longer justified relatively to the expanded argumentation basis, and in this sense, no longer derivable from it. Correspondingly, default conditionals make it possible to construct defeasible arguments, i.e., nonmonotonic inferences: the results obtained through defeasible modus ponens can be defeated by rebutters or undercutters.
According to this idea of defeasibility, a legal norm can be said to be defeasible whenever all the following conditions are jointly possible:
As we have seen in the examples above (for instance, in Figure 8), a defeasible norm \(N\) can be modelled as a default, i.e., in the logical form “if \(P(\mathbf{x})\) then presumably \(Q(\mathbf{x})\)”, i.e., as \(N\left(\mathbf{x}\right):\ P\left(\mathbf{x}\right)\Rightarrow Q\left(\mathbf{x}\right)\), where \(\mathbf{x}\) is the list of the variables in the norm (and the default would stand for the set of its ground instances). In fact., the inferences (arguments) warranted by that norm—i.e., arguments having the form (\(P\left(\mathbf{a}\right),\ N\left(\mathbf{x}\right):\ P\left(\mathbf{x}\right)\Rightarrow Q\left(\mathbf{x}\right),\ therefore\ Q\left(\mathbf{a}\right))\) where a is an individual case, namely, a list of values for variables \(\mathbf{x}\) —can be rejected, given appropriate conditions, without rejecting the norm or is antecedent. This would happen whenever the premise for building a rebutter (a stronger norm having the form\(\ N_{1}\left(\mathbf{x}\right):R\left(\mathbf{x}\right)\Rightarrow\neg Q\left(\mathbf{x}\right)\) or an undercutter (a norm having the form \(N_{2}\left(\mathbf{x}\right):R\left(\mathbf{x}\right)\Rightarrow\neg N_{1}\left(\mathbf{x}\right)\)) is available. This notion of the defeasibility also applies to the more abstract view of the applications of a norm as involving a meta-level warrant such as “If the norm ‘If \(N\left(\mathbf{x}\right):\ \)P \(\left(\mathbf{x}\right)\) then presumably Q \(\left(\mathbf{x}\right)\)’ is valid and P \(\left(\mathbf{a}\right)\) is the case, then presumably Q \(\left(\mathbf{a}\right)\)” (as in the model proposed by Hage 1997). In the following I will speak of argument warranted by a norm, to cover both models of norm-based reasoning.
If the coexistence of the three conditions above is impossible, then the norm at issue can be said to be strict or indefeasible, and can be modelled as a material conditional, having the form: “for all \(\mathbf{x}\), if \(P(x)\) then \(Q(x)\)”, i.e. \(\forall x(P(x)\rightarrow Q(x))\). More plausibly an indefeasible norm \(N\) could be represented through a universal strict conditional “for all \(x\), if \(P(x)\), then necessarily \(Q(x)\)”, i.e, \(\forall x(P(x)\twoheadrightarrow Q(x))\) (or possibly as a strict rule, which may not allowing for contraposition, see Prakken 2010). The strict conditional, which is here denoted with the arrow, \(\twoheadrightarrow\) expresses the idea that the correlation between \(P(x)\) and \(Q(x)\) does not depend on the present factual situation (on the actual world), but would rather hold in every possible factual situation (the norm being unchanged). If we accept the indefeasible norm and also accept that its antecedent holds in any possible context, we must accept that also the norm’s consequent holds in that context. could not be the object of exceptions in a strict sense, namely, of provisions stating that the unmodified norm does not apply when an impeding circumstance E(a) is established. To avoid the effect of \(N\) to be triggered in circumstance E(a) , we would have to substitute it with the new norm \(\forall\mathbf{x}\ (P(\mathbf{x})\ \bigwedge\neg E(\mathbf{x})\ \twoheadrightarrow Q(\mathbf{x})\). Because of this change the norm’s effect could be established in a case only when both predicates \(P\) and \(\neg E\) are established in that case. Rather than \(E\) being an impeditive fact capable of blocking the application of the norm, \(\neg E\) would become a constitutive fact that must be established, for that effect to be triggered.
Let us consider for instance a norm linking the causation of harm to the obligation to compensate the victim. If the norm were defeasible, it would mean that if any individual \(x\) culpably harms another individual \(y\), then presumably \(x\) must compensate \(y\), and it could be modelled in the logical form\(:\ N\left(x,\ y\right):\ CulpablyHarms(x,y)\Rightarrow MustCompensate(x,y)\). If the norm were indefeasible, it would rather mean that for all individuals \(x\) and \(y\), if \(x\) harms \(y\) then necessarily \(x\) has to compensate \(x\), and it could be modelled in the logical form: \(\forall x,\ y(CulpablyHarms\left(x,y\right)\twoheadrightarrow MustCompensate\left(x,y\right))\).
Thus, from the perspective here developed, the defeasibility of a norm pertains to its content, as expressible in its logical form, and therefore is not affected by the fact that the norm may be declared invalid: this may happen, under appropriate conditions, for both defeasible and indefeasible norms. Similarly, the defeasibility of a norm is not affected by the fact that the norm may be modified or substituted through judicial interpretation or through legislation. Both defeasible and indefeasible norms can be modified by new legislation or case law. The difference rather pertains to the necessity of a modification to introduce an exception:
In their analysis of the notion of defeasibility in the law Ferrer and Ratti (2012b, 36) distinguish three cases: (1) the norm’s validity is defeasible, in the sense that it depends on defeasible criteria, (2) the norm is externally defeasible, in the sense that the “conditions of applications contain implicit exceptions whose scope has not been determined”, and (3) the norm’s normative content is defeasible in the sense that it specifies operative facts that are contributory conditions for the production of the norm’s legal effect. In particular, they say that in the third case “the norm s antecedent contains implicit exceptions which may not be exhaustively identified.” They also affirm that in cases (1) and (2), it is not the norm itself which is defeasible, but rather the criteria for its validity or application, while the norm should be represented as a material conditional.
As Ferrer and Ratti (2012b, 36) rightly observe, only their third case of defeasibility is really significant: the first two cases depend on meta-norms on validity or application are defeasible in the third sense, namely, according to their content. However, the way in which they describe defeasibility by content differs from the approach here adopted in three regards.
Firstly, it makes the implicitness of the exceptions to a norm a necessary condition for the defeasibility of the norm. On the contrary, I have argued that even explicit exception to a norm presuppose the defeasibility of that norm, since they give rise to the pattern that characterises defeasible argumentation: the absence of the exception is not needed to construct an argument warranted by the norm, though that argument can be defeated by arguments warranted by the exception.
Secondly, the characterisation of the antecedent of a defeasible norm as providing contributory conditions for the norm’s conclusion fails to capture the idea of defeasible connection between the antecedent and the conclusion of that norm. As I observed in Section 3, a contributory condition for a conclusion may fail to provide any presumptive support for that conclusion. This is the case when a norm has a conjunctive antecedent so that its application results in a linked argument (see Figure 2). On the other hand, the antecedent of defeasible norm can be described as a contributory reason for the norm’s conclusion. A genuine contributory reason for a legal conclusion should indeed provide on its own sufficient presumptive support to that conclusion, i.e., it should match the antecedent of a legal default, and so enable the construction of a separate defeasible argument. Separate defeasible argument sharing the same conclusion contribute to a stronger convergent argument supporting the same conclusion (see Figure 3 and Figure 4).
Finally. it is not clear to me why the issue of determining where a norm is defeasible or not should be specifically addressed as a “matter of interpretation”. It is a matter that pertains to the determination of the logical structure of the norm at issue, an issue that could pertain to interpretation or not depending on how one understand the notion of interpretation, i.e., as concerning every ascription of meaning to a text, or only the ascription of meaning meant to address some doubts (see Dascal and Wroblewki 1986). Interpretation in the first sense obviously covers the determination of every aspect of the content of a norm having a textual source, and therefore it also covers also the determination of its logical structure. In fact, to determine whether a norm is defeasible or not, we have to consider —depending on whether we are approaching the norm from a socio-legal or from doctrinal perspective— (a ) the way in which the norm is comprehended and used by the community of its users (those who endorse/follow/apply it) or (b ) the way in which the norm should correctly be comprehended and used by the same community. This determination would involve an empirical assessment according to (a ) or a normative assessment according to (b ). This assessment would be no less (and no more) dependent on interpretation than the determination of any other aspects of the norm’s content, such as the structure and the components of its antecedent and its consequent. Finally, the issue of determining whether a legal norm is defeasible would be utterly trivial if we were to adopt —both empirically and normatively— the view that all legal norms are defeasible in this abstract sense —i.e., the assumption that no strict legal norm exists, as a matter of fact— following Leibniz’s suggestion.
It seems to me that we need to distinguish clearly two aspect concerning a norm’s defeasibility. The first aspect, to which we have referred in this contribution by using the term “defeasible”, pertains the intrinsic logical structure of a norm: is the norm meant to establish a presumptive or a conclusive link between its antecedent and its conclusion? Defeasibility so understood is a counterfactual property: to say that a norm N, having antecedent P and conclusion Q (I leave the variables implicit, for simplicity’s sake), is defeasible just means that it is in principle possible to reject an argument for Q warranted by N , while accepting both N and P : we can imagine a system \(\mathbb{L}\) and a factual constellation \(\mathbb{F}\), such that with regard to the argumentation basis \(\mathbb{L\cup F}\) both N (unchanged) and P are justified (e.g. being unchallenged), but the argument that delivers Q on the basis of N and P is rebutted or undercut, by arguments constructible from \(\mathbb{L\cup F}\).
Once we have determined that N is intrinsically defeasible, we can address further issues. One issue concerns determining whether N- warranted arguments can be defeated in the legal system \(\mathbb{L}\) containing N , i.e., whether a rebutting or undercutting counterargument to an \(N\)-warranted argument can be mounted by using only norms in \(\mathbb{L}\), plus appropriate operative facts (see Section 11). A different issue concerns determining whether N could be defeated given the possible (permitted or empowered) judicial modifications of the current legal system L, namely, whether judicial construction/interpretation could introduce in \(\mathbb{L}\) new norms that enable the construction of defeaters against the application of N . Obviously answering either of these issues may or will require the interpretation of the legal system \(\mathbb{L}\) under consideration (on the connection between the possibility that a norm is defeated and interpretation, see Duarte 2011, 135).
Finally, the idea of a norm’s defeasibility as pertaining to the logical structure of that norm, leads me to address one further claim by Ferrer and Ratti, namely, the view that whenever a norm’s validity or application is determined by defeasible metarules, the norm itself must be indefeasible. Since a norm’s defeasibility only concerns the logical structure of that norm, the fact that a norm is defeasible does not exclude (nor require) that its validity as well as the domain of its intended application are governed by defeasible criteria. Inapplicability rules, however, may presuppose the defeasibility of the norm that they address: a rule stating that norm N is inapplicable under exceptional circumstances E , is usually meant enable the construction of undercutters to N- warranted arguments, namely, arguments having the following form: \(E\) is the case, if \(E\) than \(N\) does not apply (does not warrant its conclusion), therefore \(N\) does not apply: \(E,E\Rightarrow\neg N\), therefore \(\neg N\) (see argument \(C\) in Figure 8).
In conclusion, I think that, notwithstanding the multifarious creative ways in which legal theorists have framed the idea of defeasibility, it would better to stick to the more limited and precise concept on which other disciplines —such as logic, philosophy, and computing— converge, namely, the view that defeasible reasoning is nonmonotonic and that the antecedent of a defeasible norm provides only presumptive support to the norm’s conclusion. The considerations that have been presented as alternative analyses of the concept of defeasibility should rather be rephrased are pertaining to the ways in which (a) arguments applying defeasible legal norms can be rebutted or undercut, or (b) existing legal norms, both defeasible or indefeasible ones, can be abrogated or modified.