Defeasibility in the Law
Defeasible reasoning characterises the law at different levels.
First, clues to the defeasibility of legal reasoning are embedded in the
very language of legal sources. As we saw in the previous example, the
legislator itself often suggests how to construct defeaters to certain
arguments. For example, to indicate that liability in tort can be
excluded by appealing to self-defence or a state of necessity, the
legislator may use any of the following formulations:
-
Unless clause . One is liable if one voluntarily causes damage,
unless one acts in self-defence or in a state of necessity.
-
Explicit exception . One is liable if one voluntarily causes
damage. One is not liable for damages if one acts in self-defence or
in a state of necessity.
-
Presumption . One is liable if one voluntarily causes damage and
one does not act out of self-defence or a state of necessity. The
absence of both is presumed.
According to all these formulations, to build an argument to the effect
that one must make good some damage, it is normally sufficient to
ascertain that one voluntarily caused that damage, but this argument is
defeated by counterarguments appealing to the fact the person turns out
to have acted either out of necessity or in self-defence.
Defeasibility is also an essential feature of conceptual
constructions in the law. Legal concepts must be applied to such a
diverse range of instances that they can at best offer a tentative and
generic characterization of the objects to which they apply, a
characterization that must be supplemented with exceptions. General
legal concepts presuppose defeasibility: the requirement of absolute
rigour in defining and applying concepts—the demand that all features
which are included in, or entailed by, a concept apply to each of its
instances—would paradoxically run counter to the very possibility of
being “logical” in the sense of using general concepts. In fact, even
the definitions of the legal concepts that can be found in statutes and
codes reflect the stepwise defeasible process of establishing legal
qualifications: first, a general discipline is established for a certain
legal genus (e.g., the genus “contract”); special exceptions are then
introduced for species within this genus (e.g., the species contract of
sale); finally, further exceptions may be introduced for specific
subspecies (e.g., the sale of real estate). Consequently, when using
conceptual hierarchies, we must apply to a certain object the rules
governing the category in which it is included, but only insofar as no
exceptions emerge concerning a subcategory in which that object is also
included.
Defeasibility can be deliberately established by the legislator, but it
may also result from the evolution of legal knowledge: after a general
rule has been established, exceptions are often provided for those cases
where the rule appears to be inadequate.
This is typically the evolution of judge-made law, where general
rationes decidendi are often limited by way of
distinctions , that is, by way of exceptions introduced for
specific contexts (on defeasibility and precedents, see Prakken and
Sartor 1998; Horty 2012). In such cases, judges often leave the original
default rule unchanged and add a new, prevailing rule that addresses the
specific situations requiring a distinction. For instance, in the
Monge case (US Supreme Court, 28 Feb. 1974, No. 6637), the judges
introduced an exception to the idea that contracts of employment at will
(lacking any set term) could be terminated by both parties regardless of
the reason (“for any reason or no reason at all”). They stated that
“a termination by the employer of a contract of employment at will
which is motivated by bad faith or malice or based on retaliation
[…] constitutes a breach of the employment contract.
Correspondingly, on the basis of this rule the dismissed employee Olga
Monge could build an argument (her dismissal was a breach of contract,
being based on malice and retaliation) that could defeat the employer’s
argument that she could be legitimately dismissed on the ground that her
contract was at will. Note that the judges could also have revised the
original rule into a new rule: “a contract can be terminated by both
parties for any reason unless the employer is terminating the contract
motivated by bad faith or malice or based on retaliation.” The new rule
would have triggered the same dialectical exchange, as long as the
unless clause was interpreted as attributing to the employee the burden
of proving bad faith, malice, or retaliation.
Finally, we need to also consider the procedural aspect of
defeasibility. As noted, this aspect concerns the fact that defeasible
reasoning activates a structured process of inquiry in which we draw
prima facie conclusions, look for their (prima facie) defeaters, look
for defeaters of defeaters, and so on, until stable results can be
obtained. A process like this one reflects the natural way in which
legal reasoning proceeds. This is especially the case in the law’s
application to particular situations, when we have to consider the
different, and possibly conflicting, legal rules that apply to such
situations and must work out conflicts between these rules.
The defeasibility of legal reasoning also reflects the dialectics of
judicial proceedings, where each party provides arguments supporting his
or her position, and these arguments conflict with the arguments made by
the other party. The debate of the parties is usually transferred to the
judicial opinion that takes in the results of the dispute and determines
its output. To convincingly justify a judicial decision in a case
involving genuine issues, it is not sufficient to state a single
argument; it is necessary to establish that the winning argument
prevails over all arguments to the contrary, especially those that have
been presented by the losing party, or that the latter arguments have to
be rejected on other grounds.
Finally, doctrinal work cannot avoid being contaminated by the
dialectics of legal proceedings, since its main function consists in
providing general arguments and points of view to be used in judicial
debates. From this perspective, doctrinal reasoning may be viewed as
consisting in an exercise in unilateral dialectics , understood as
a disputational model of inquiry in which “one develops a thesis
against its rivals, with the aim of refining its formulation, uncovering
its basis of rational support, and assessing its relative weight”
(Rescher 1977, 47).
The significance of defeasibility in legal reasoning has been recently
confirmed by the psychological experiments by Gazzo et al (2016), which
show how both lawyers and laypersons reason defeasibly when applying
legal norms. When presented with a legal conditional, in its usual
formulation (If somebody kills another human, then this person should be
punished for manslaughter), and with an instance of the antecedent
condition (Bert killed another human), most participants in the
experiment conclude for the conditional’s conclusion (Bert should be
punished for manslaughter), but withdraw this conclusion when told that
an exculpatory circumstance (because of a psychological disorder, Bert
was not able to control his actions) also obtains. The experiments also
show that lawyer are better than laypersons in withdrawing legal
conclusion when faced with legally recognised exceptions, having a more
precise knowledge of such exceptions and of their role in legal
reasoning.