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).