The mental model theory of reasoning postulates that reasoners build models of the situations described in premises, and that these models normally make explicit only what is true. The theory has an unexpected consequence: it predicts the occurrence of inferences that are systematically invalid. These inferences should arise from reasoners failing to take into account what is false. We report an experiment that corroborated the occurrence of these illusory inferences, and that eliminated a number of altemative explanations for them. Results illuminate the controversy among various current theories of reasoning.

The mental model theory postulates that reasoners build models of the situations described in premises, and that each model represents a possibility. The present paper proposes that causal relations, such as A causes B and A allows B. have meanings that concern only possibilities and a temporal constraint that B cannot precede A. This theory predicts that causes and enabling conditions differ in meanings, contrary to a long tradition in philosophy and psychology that they are logically indistinguishable. It also predicts that individuals should reason about causation on the basis of mental models rather than on fully explicit models. Three experiments corroborated these predictions.

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. A conclusion is possible if it holds in at least one mental model, whereas it is necessary if it holds in all the models. The theory makes a crucial prediction, which we corroborated experimentally. 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.

The mental model theory postulates that reasoners build models of the situations described in premises. A conclusion is possible if it occurs in at least one model; and it is impossible if it occurs in no models. According to the theory, reasoners can cope with what is true, but not with what is false. A computer implementation predicted that certain inferences should yield cognitive illusions, i.e. they have conclusions that should seem highly plausible but that are in reality gross errors. Experiment 1 showed that, as predicted, participants erroneously inferred that impossible situations were possible, and that possible situations were impossible, but they performed well with control problems. Experiment 2 replicated these results, using the same premises for both the illusory and the control inferences: the participants were susceptible both to illusions of possibility and to illusions of impossibility, but they coped with the control problems.