Reasoning about necessity and possibility: A test of the mental model theory of deduction.

Abstract

This article examined syllogistic reasoning that differs from previous research in 2 significant ways: (a) Participants were asked to decide whether conclusions were possible as well as necessary, and (b) every possible combination of syllogistic premises and conclusions was presented for evaluation with both single-premise (Experiment 1) and double-premise (Experiment 2) problems. Participants more frequently endorsed conclusions as possible than as necessary, and differences in response to the 2 forms of instruction conformed to several predictions derived from the mental model theory of deduction. Findings of Experiments 2 and 3 showed that some fallacies are consistently endorsed and others consistently resisted when people are asked to judge whether conclusions that are only possible follow necessarily. This finding was accounted for by the computational implementation of the model theory: Fallacies are made when the first mental model of the premises considered supports the conclusion presented. Traditional applications of logic concern the validity of arguments, that is, proving that some conclusion is necessary given some premises. However, in everyday reasoning it may be just as important to decide whether some proposition is possible in light of the given information. Inferences of possibility occur whenever rules and regulations constrain a person's behavior rather than determine what it must be. For example, students choosing a degree program within a modular course structure will have many degrees of freedom but will have to respect constraints owing to timetabling restrictions, availability of teaching staff, prerequisite and corequisite relations between modules, and so on. Their decision making here involves inferring what is possible and then deciding between the possibilities identified. Most of the rules by which people live their lives in society, including criminal law, operate in a similar way. It is rarely determined by such rules that people must follow a particular course of action, but they are frequently constrained to act within a set of legal possibilities. The psychological study of deductive reasoning has been steadily increasing in recent years, and there are now many hundreds of experimental studies reported in the literature (see Evans, Newstead, & Byrne, 1993, for a review). In view of the above remarks, however, there is a curious limitation