ABSTRACT
People understand a conditional, ‘if A then B’, such as ‘if’ Peg went swimming then she felt well’ by keeping in mind only true possibilities, e.g., A and B, not-A and not-B, not-A and B (Johnson-Laird & Byrne, 2002). Initially they think about only a few of them, e.g., A and B, because of working memory limitations. As a result, they make some inferences readily e.g., modus ponens (MP), given A, therefore B. But they find other inferences difficult, e.g., modus tollens (MT), given not-B, therefore not-A. The information does not correspond to their initial possibility and they must think about other true ones, e.g., not-A and not-B.
