A model theory of modal reasoning

This paper presents a new theory of modal reasoning, i.e. reasoning about what may or may not be the case, and, what must or must not be the case. It postulates that individuals construct models of the premises in which they make explicit only what is true. A conclusion is possible Ef it holds in at least one model, whereas it is necessary if ii holds in all the models. The theory makes three predictions, which are corroborated experimentally. First, conclusions correspond to the true, but not the false, components of possibilities. Second, there is a key interaction: it is easier to infer that a situation is possible as opposed to impossible, whereas it is easier to infer that a situation is not necessary as opposed to necessary. Third, individuals make systematic errors of omission and of commission. We contrast the theory with theories based on formal rules.