The Rationale for Defeasibility

Pollock (1998) argues that defeasibility is a key aspect of human cognition (and more generally, of the cognition of any boundedly rational agent). We start with perceptual inputs and proceed by inferring beliefs from our current cognitive states (our percepts plus the beliefs we have previously inferred). A process so described must satisfy two apparently incompatible desiderata:
According to Pollock, the only way to reconcile these requirements is by defeasible reasoning. We must adopt beliefs on the basis of a small set of perceptual inputs, but then must be ready to retract these beliefs in the face of additional perceptual inputs, whenever these additional inputs conflict with the initial basis for our beliefs.
Thus, defeasible reasoning appears to have different, but related, functions (see Sartor 2005, Section 2.2, 2.3). The first function consists in providing us with provisional beliefs, on which basis we can reason and act, until we gain information to the contrary.
The second function consists in activating a structured process of inquiry that consists in drawing pro tanto conclusions, looking for their defeaters, for defeaters of defeaters, and so on, until stable outcomes are obtained. This process has two main advantages: (1) it focuses the inquiry on relevant knowledge, and (2) it continues to deliver provisional results while the inquiry moves on.
A third function of defeasibility consists in enabling our collective knowledge structures to persist in time, i.e., to continue to work as a shared communal asset, even though each of us is exposed to new information, often challenging the information we already have.
We indeed have two basic strategies for coping with the provisional nature of human knowledge: revision and defeasibility.
Revision assumes that our general knowledge is a set of universal laws. When we discover a case where such universal laws lead us to a false (unacceptable or absurd) conclusion, we must conclude that our theory (or the subsets of it entailing the false conclusion) has been falsified, becoming thus unacceptable (Popper 1959). Thus, we must abandon some propositions in that theory and replace them with new universal propositions, from which the false conclusion is no longer derivable. Rational strategies for revising a theory have been the object of several studies (see, for instance, Alchourrón, Gärdenfors, and Makinson 1985; Gärdenfors 1987). In the legal domain, this idea was originally proposed Alchourrón and Makinson (1981) and was subsequently developed by Maranhao (2013).
The other strategy, defeasibility , assumes that general propositions are defaults, that are meant govern most cases or the normal cases. Thus, we can consistently endorse such propositions and deny that they apply to certain cases: the exception serves the rule, or at least it does not compromise the rule. To deal with an anomalous case on a defeasibility strategy, we do not abandon the default or change its formulation, but instead we assume that the default’s operation is limited on grounds that are different from those that support the use of the default itself. As we saw in the previous example, these grounds may provide an argument that undercuts or rebuts the argument warranted by the default. The idea that legal norms are defaults (rather than strict rules) makes possible a certain degree of stability in legal knowledge: we do not need to change our norms whenever their application is limited through subsequent exceptions or distinctions. However, this perspective does not exclude the need to abandon a norm, when it no longer reflects a “normal” connection, being superseded by subsequent norms (as in implicit derogation), or when it is explicitly removed from the knowledge base (as in explicit derogation: see Governatori and Rotolo 2010).