The ability to make logical inferences is one of the key components of human reasoning. There has been a great deal of empirical and theoretical attention given to models of how humans make deductions. However, there is one very basic aspect of logical reasoning that has curiously received relatively little attention. One of the critical aspects of logical reasoning is the explicit requirement to disregard the empirical truth of the premises used to make inferences, a requirement that is often expressed by the instructions to "suppose that the premises are true." Yet, although we know that following such instructions is not always easy for even educated reasoners, there is no clear model of just what is involved in doing so, and why this might be difficult. This paper attempts to suggest such a model and to examine it by looking at a paradoxical phenomenon concerning reasoning with causal conditionals.Studies of deductive reasoning in both adults and children have clearly shown that people often make deductions that are not consistent with the rules of formal logic. In the context of the focus of this paper, the most interesting of these cases concerns the logical rule known as modus ponens (MP). This requires a reasoner to suppose that a given "If P then Q" conditional relation is true, and then to deduce that "P is true" implies that "Q is true." Modus ponens is the simplest of the deductive inferences that can be made in conditional logic. Developmental studies have shown that very young children can reliably make the MP inference (Ennis, 1976) before any of the other conditional inferences, such as modus tollens (MT), and so on. Adults generally tend to endorse the MP inference at a very high rate (Evans, 1993). In fact, both of the major theories of human reasoning consider that making the MP inference requires a minimal amount of cognitive effort. Rules theorists (e.g., Braine & O'Brien, 1991;Rips, 1994) consider MP to be one of the most basic of the inference rules that they claim characterize how people make deductive inferences. Braine (1990) has even proposed that the MP inference rule should be considered a logical primitive with biological origins. Mental model theory considers that the MP inference requires the least costly of the possible representations of the conditional (the "iniPreparation of this manuscript was supported by grants from the Fonds pour la Formation de Chercheurs et l'Aide à la Recherche (FCAR) and the Natural Sciences and Engineering Council (NSERC) to H.M. The authors thank David Over and Denise Cummins for their helpful comments on a previous version of this manuscript. Correspondence should be addressed to H. Markovits, Department of Psychology, University of Quebec, C.P. 8888, Succ. "A," Montreal, PQ, H3C 3P8 Canada (e-mail: markovits.henry@uqam.ca).
Suppression of valid inferences and knowledgestructures: The curious effect of producing alternative antecedents on reasoning with causal conditionals HENRY MARKOVITS and FRÉDÉRIC POTVIN University of Quebec, Montreal, Quebec, C...
