Patterns of Defeasible Reasoning

Various warrants (general defaults) for defeasible reasoning can be identified. The following ones are discussed by Pollock (1998, 2008):
General processes of human cognition, such as abduction and analogy (see Walton, Reed and Macagno 2008) can support further schemes for defeasible arguments, such as the following:
These defeasible warrants are not meant to substitute the logical, philosophical or psychological theories of the phenomena they address, such as perception, induction, abduction or analogy (see for instance, for analogy, Holyoak and Thagard 1996). They should be rather viewed as rules of thumb that may be supported, explained and constrained by such theories.
In the previous examples, I have considered further general defaults, such as those enabling the argument from good or bad consequences or the argument from expert testimony. I have also observed that more specific defaults may be used to construct defeasible arguments: empirical generalisations, as well as legal and moral norms, can be viewed as defaults. In fact, the set of the defaults that may be used in individual and social cognition cannot be reduced to an exhaustive list, since default warrants are justified pragmatically, i.e., because of how well they serve the needs of different practical or epistemic activity types (Walton and Sartor 2008). The successful use of a default warrant in a social activity (such as legal reasoning) critically depends on the extent to which the scheme enjoys shared acceptance, as providing valid support to its conclusions (since the default’s acceptance is a crucial precondition of its successful use in arguments meant to convince other people, or to converge with them into shared conclusions). Thus, even abstract legal principles, such as interpretive canons, only justify their conclusions in those legal systems in which they are in fact endorsed and deployed, so as to enjoy the status of social and institutional normative principle.
It is important to stress that defeasible arguments can include multiple steps. For instance, in an argument culminating in the conclusion of a rule, the rule may be supported by an interpretive argument, while rule’s factual antecedent may result from arguments assessing the available evidence. Consider the liability case illustrated in Figure 16. The argument for the liability of Doctor Mary includes the following: