Responses to inconsistent premisses cannot count as suppression of valid inferences.

Byrne (1989) claims to have demonstrated that context can suppress valid inferences like modus ponens. If substantiated, the claim would refute the idea that valid inferences cannot be countermanded without contradiction, although implicatures and invited inferences can be (e.g., Braine & Rumain, 1981; Geis & Zwicky, 1971; Grice, 1975). That, in turn, would invalidate one of the few available methods for distinguishing between inferences that are intuitively necessary and those that are merely implicated or invited. Thus, on conditional reasoning problems, Rumain, Connell, and Braine (1983) found that the common fallacies known as “denying the antecedent” and “asserting the consequent” can be suppressed by a kind of modification of the problem that leaves modus ponens untouched. Markovits (1984, 1985) has reported similar results. Byrne claims that by an essentially similar maneuver one can cause subjects to reject instances of modus ponens. She concludes that this sort of experimental maneuver provides no more reason for thinking that subjects have an inference rule for modusponens than that they have inference rules that generate the fallacies. We think that there are important differences between the problem modifications of Rumain et al. and of Byrne, and these lead us to dispute Byrne’s claims. In the experiment of Rumain et al. subjects reasoned about the content of a closed box. Some of the problems were ordinary conditional reasoning problems in that they presented a single conditional like: