ABSTRACT
One of the central proposals of the mental model theory of deduction is that people make deductions according to a semantic principle of validity: An argument is valid if there are no counterexamples to it, that is, if there are no situations in which the premises are true but the conclusion is false (Johnson-Laird, 1983; Johnson-Laird & Byrne, 1991). How is the search for counterexamples carried out? Not much is known about the process and our aim in this chapter is to examine some of the issues surrounding counterexample discovery (see also Byrne, Espino, & Santamaria, 1998). Is the search for counterexamples an unconscious automatic component of the reasoning mechanism? Failures to search for counterexamples may reflect constraints on reasoning in general, such as working memory limitations on the number of models that can be kept in mind. Alternatively, is the search for counterexamples a deliberate conscious strategy (as suggested by Evans, chap. 1, this volume; see also Handley, Dennis, Evans, & Capon, chap. 12, this volume)? Failures to search for counterexamples may reflect a deliberate decision made by reasoners who consider that a plausible, possible rather than necessary, conclusion is sufficient for their current purposes. As a first attempt
toward answering such questions, we consider in this chapter how people deal with counterexamples that have been made explicitly available to them.
