The Emergence of the Idea of Defeasibility in Law and Ethics

Though it is only recently that formal logics for defeasible reasoning have been developed, we can may find references to it in the history of philosophical and legal reasoning.
A famous fragment by Aristotle apparently characterises legal reasoning as defeasible (Aristotle, Nicomachean Ethics , 1137b) in the sense that legal conclusions derived from general norms may have to be rejected in the face of particular cases having exceptional features that make those conclusions inadequate:
All law is universal, and there are some things about which it is not possible to pronounce rightly in general terms; therefore, in cases where it is necessary to make a general pronouncement, but impossible to do so rightly, the law takes account of the majority of cases, though not unaware that in this way errors are made. And the law is none the less right; because the error lies not in the law nor in the legislator, but in the nature of the case, for the raw material of human behaviour is essentially of this kind. So, when the law states a general rule, and a case arises under this that is exceptional, then it is right, where the legislator, owing to the generality of his language, has erred in not covering that case, to correct the omission by a ruling such as the legislator himself would have given if he had been present there, and as he would have enacted if he had been aware of the circumstances. (Aristotle, Nicomachean Ethics , 1137b)
Cicero distinguishes presumptive (probabilis) and necessary argumentation (Cicero, De inventione , Book 1, Section 44). He provides various patterns (warrants) for presumptive inferences: the (natural) meaning of a sign (e.g., blood traces indicate participation in a violent action), what happen usually (e.g., mothers love their children), common opinion (e.g., philosophers are atheists), or similarity (if it is not discreditable to the Rodians to lease their port-dues, then it is not discreditable even to Hermacreon to rent them). Moreover he considers how (defeasible) arguments may be refuted:
All argumentation is refuted when one or more of its assumptions is non granted, or when, the assumptions having been granted, it is denied that the conclusion follows from them, or when it is shown that the kind itself of the argumentation is faulty, or when against a strong argumentation another argumentation equally strong or stronger is put forward (Cicero, De inventione , Book 1, Section 79).
The second and the fourth items in Cicero’s list seem to correspond to what we called undercutting and rebutting, respectively, namely, those attacks that are peculiar to defeasible arguments.
The Aristotelian approach to the dialectics of rule and exception is developed by Aquinas:
[I]t is right and true for all to act according to reason: And from this principle it follows as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is true for the majority of cases: But it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance, if they are claimed for the purpose of fighting against one’s country. And this principle will be found to fail the more, according as we descend further into detail, e.g., if one were to say that goods held in trust should be restored with such and such a guarantee, or in such and such a way; because the greater the number of conditions added, the greater the number of ways in which the principle may fail, so that it be not right to restore or not to restore. (Aquinas, Summa Theologiae , I–II, q. 94, a. 4)
The idea of defeasibility in the legal domain is precisely outlined by G. W. Leibniz, who characterises legal presumption as defeasible inference, arguing that in presumptions
the proposed statement necessarily follows from what is established as true, without any other requirements than negative ones, namely, that there should exist no impediment. Therefore, it is always to be decided in favor of the party who has the presumption unless the other party proves the contrary. (Leibniz, De Legum Interpretatione, A VI iv C 2789)
Leibniz argues that all laws are defeasible: legal norms support presumptive conclusions, which are subject to exceptions established by other norms. He also points at the connection between defeasibility and burden of proof:
every law has a presumption, and applies in any given case, unless it is proved that some impediment or contradiction has emerged, which would generate an exception extracted from another law. But in that case the charge of proof is transferred to the person who adduces the exception. (Leibniz, De Legum Interpretatione, A VI iv C 2791)
Turning from law to morality, we can find a notion of defeasibility in the work of David Ross, an outstanding Aristotelian scholar and moral philosopher who developed a famous theory of prima facie moral obligations (Ross 1930; 1939). Espousing a pluralist form of moral intuitionism, Ross relates defeasibility to the possibility that, in concrete cases, moral principles may be overridden by other moral principles:
Moral intuitions are not principles by the immediate application of which our duty in particular circumstances can be deduced. They state […] prima facie obligations. […] [We] are not obliged to do that which is only prima facie obligatory. We are only bound to do that act whose prima facie obligatoriness in those respects in which it is prima facie obligatory most outweighs its prima facie disobligatoriness in those aspects in which it is prima facie disobligatory. (Ross 1939, 84–5)
Ross links the notion of defeasibility to the idea of outweighing, a key notion in reason-based approaches to practical reasoning (see the chapter on reasons). The ideas that moral reasoning consists in balancing reasons and the idea of defeasibility are indeed connected, under the assumption that we can legitimately make moral assessments also on the basis of partial knowledge of the situations we face., i.e., even when we are not guaranteed to have taken into account all relevant reasons. The fact that certain reasons support a certain action only provide a defeasible support to that action: these reasons justify that action in the absence of outweighing reasons to the contrary, but would fail to support the outcome in presence of the latter reasons. Consequently, if we believe that that the reasons justifying the actions are present and we are not aware of reasons to the contrary, we should conclude that the action is presumably justified (on the basis of the information we have). If we come to believe that outweighing reasons are present, we should withdraw this conclusion.
Indeed, defeasibility may make the appeal to general ethical principles compatible with the particularistic view that any moral principle or reason may be overridden or be inapplicable depending on the circumstances (Dancy 2004). As Horty (2007, 2012) has argued, moral principles should be viewed as defaults, that link reasons to actions (or obligations to act), and support such actions as long as they are not rebutted by reasons to the contrary or undercut by reasons against their application. Logics for defeasible reasoning provide formal accounts of the view that practical reasoning consists in the assessment of competing reasons for action by a bounded cogniser.
Although the notion of defeasibility is quite familiar in legal practice and in doctrinal work, it was not precisely and formally analysed it was brought to the attention of legal theorists by H. L. A. Hart (1951, 152):
When the student has learnt that in English law there are positive conditions required for the existence of a valid contract, [. . . ] he has still to learn what can defeat a claim that there is a valid contract, even though all these conditions are satisfied. The student has still to learn what can follow on the word “unless,” which should accompany the statement of these conditions. This characteristic of legal concepts is one for which no word exists in ordinary English. […] [T]he law has a word which with some hesitation I borrow and extend: This is the word “defeasible,” used of a legal interest in property which is subject to termination of “defeat” in a number of different contingencies but remains intact if no such contingencies mature.
References to the defeasibility of legal arguments can be found in important approaches to legal reasoning. For instance, Viehweg (1965) argued that lawyers approach specific problem situations, not by reasoning from a complete and consistent system of universal axioms, but by referring to an open, unordered, inconsistent, undetermined list of topoi (points of view, usually expressed as maxims) addressing the relevant features of the different situations that come up. Such topoi are usually defeasible, since they may fail to apply under particular situations. Consider, for instance, the legal topos that nobody can transfer to another person more rights than those he or she possesses (nemo plus juris in alium transferre potest quam ipse habet ). This rule does not apply to some exceptional cases in which a buyer in good faith can acquire property from an apparent seller that is not the actual owner.
Similarly, Perelman and Obrechts-Tyteca (1969 focus on the distinction between deductive demonstration and argumentation, affirmed that, contrary to demonstration, argumentation is always in principle open to challenge or reconsideration (see Blair 2012, 127).