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The mental model theory postulates that reasoners build models of the situations described in premises. A conclusion is possible if it holds in at least one model of the premises; it is probable if it holds in most of the models; and it is necessary if it holds in all of the models. The theory also postulates that reasoners represent as little information as possible in explicit models and, in particular, that they represent only information about what is true. One consequence is that there should be a category of illusory inferences: they will have conclusions that seem obvious, but that are wholly erroneous. Experiment 1 established the existence of such illusory inferences about probabilities. Overall, 88% of the intelligent adult subjects chose as more probable an outcome that was impossible for at least one of the illusory problems. Experiment 2 corroborated the phenomenon and showed that illusory inferences include a wide variety of problems. Finally, the paper argues that current theories based on formal rules of inference are unlikely to be able to explain the illusions.

Reasoning about possibilities is fundamental in daily life and inartificial intelligence. It is formalized in modal logics, of whichthere are infinitely many. Two experiments showed thatindividuals make inferences that are parsimonious aboutpossibilities, and that they reject conclusions referring topossibilities that the premises do not support. Both sorts ofinference contravene modal logics, i.e., the simplest system ofmodal logic and the infinite number of systems based on it.

Many theories of reasoning and many experiments presupposethat human ability is stable over time, and so people usuallydraw the same conclusion from the same premises. The as-sumption has hitherto had little or no empirical investigation.We therefore analyzed a study in which 20 participants drewtheir own conclusions to the 64 sorts of syllogisms on twooccasions separated by roughly a week. We report the na-ture of the changes in the participants’ conclusions includingtheir spontaneous improvement in logical accuracy, and use amodel-based program, mReasoner, to explain the results.

The mental model theory postulates that reasoners build models of the situations described in premises, and that these models normally make explicit only what is true. A computer program revealed an unexpected consequence of the theory: it predicts that certain inferences should have compelling but erroneous conclusions. Two experiments corroborated the existence of such illusions in inferences about what is possible given quantified assertions, such as 'At least some of the plastic beads are not red.' Experiment 1 showed that, as predicted, participants erroneously inferred that impossible assertions were possible, and that possible situations were impossible, but they performed well with control problems. Experiment 2 demonstrated the existence of similar illusions in inferences from dyadic assertions, e.g. 'All the boys played with the girls'.

The mental model theory of reasoning postulates that individuals construct mental models of the possibilities in which the premises of an inference hold and that these models represent what is true but not what is false. An unexpected consequence of this assumption is that certain premises should yield systematically invalid inferences. This prediction is unique among current theories of reasoning, because no alternative theory, whether based on formal rules of inference or on probabilistic considerations, predicts these illusory inferences. We report three studies of novel illusory inferences that depend on embedded disjunctions--for example, premises of this sort: A or else (B or else C). The theory distinguishes between those embedded disjunctions that should yield illusions and those that should not. In Experiment 1, we corroborated this distinction. In Experiment 2, we extended the illusory inferences to a more stringently controlled set of problems. In Experiment 3, we established a novel method for reducing illusions by calling for participants to make auxiliary inferences.

When individuals detect an inconsistency in a set of propositions, they tend to change their minds about at least one proposition to resolve the inconsistency. The orthodox view from William James (1907) onward has been that a rational change should be minimal. We propose an alternative hypothesis according to which individuals seek to resolve inconsistencies by explaining their origins. We report four experiments corroborating the explanatory hypothesis. Experiment 1 showed that participants' explanations revised general conditional claims rather than specific categorical propositions. Experiment 2 showed that, when explanations did revise the categorical proposition, participants also tended to deny the consequences of a second generalization. Experiment 3 showed that this tendency persists when participants previously affirmed these consequences explicitly. Experiment 4 showed that, when participants could easily explain an inconsistency by revising a generalization, they were more likely to accept the consequences of a second generalization. All four results contravene minimalism but support the explanatory hypothesis.

The theory of mental models postulates that individuals infer that a spatial description is consistent only if they can construct a model in which all the assertions in the description are true. Individuals prefer a parsimonious representation, and so, when a description is consistent with more than one possible layout of entities on the left--right dimension, individuals in our culture prefer to construct models working from left to right. They also prefer to locate entities referred to in the same assertion as adjacent to one another in a model. And, if possible, they tend to chunk entities into a single unit in order to capture several possibilities in a single model. We report four experiments corroborating these predictions. The results shed light on the integration of relational assertions, and they show that participants exploit implicit constraints in building models of spatial relations.

A hyper-emotion theory of psychological illnesses is presented. It postulates that these illnesses have an onset in which a cognitive evaluation initiates a sequence of unconscious transitions yielding a basic emotion. This emotion is appropriate for the situation but inappropriate in its intensity. Whenever it recurs, it leads individuals to a focus on the precipitating situation and to characteristic patterns of inference that can bolster the illness. Individuals with a propensity to psychological illness accordingly reason better than more robust individuals, but only on topics relevant to their illness. The theory is assessed in the light of previous research, a small epidemiological study of patients, and 3 empirical studies. Keywords: psychological illnesses, emotions, reasoning, cognitive therapy, unconscious processes

Relations can hold between relations, as in assertions such as: Cordelia loves Lear more than Goneril does. Naive reasoners can make inferences that depend on these higher order relations, which are vital for science and mathematics, but no existing theory explains such inferences. The present paper presents a theory based on mental models of the situations under description, and it reports four experiments corroborating the theory. Experiment la showed that the difficulty of such inferences from two premises depends on the integration of the information from the premises into a single model. The same result held in Experiment 1b, even when individuals were not permitted to make written workings. Experiment 2 required the participants to think aloud, and their protocols revealed that they developed three main strategies. Experiment 3 biased the development of these strategies, showing that individuals assemble them 'bottom up' from various tactical steps.

Inferences about spatial. temporal, and other relations are ubiquitous. This article presents a novel model-based theory of such reasoning. The theory depends on 5 principles. (a) The structure of mental models is iconic as far as possible. (b) The logical consequences of relations emerge from models constructed from the meanings of the relations and from know ledge. (c) Individuals tend to construct only a single, typical model. (d) They spontaneously develop their own strategies for relational reasoning. (e) Regardless of strategy, the difficulty of an inference depends, on the process of integration of the information from separate premises, the number of entities that have to be integrated to form a model. and the depth of the relation. The article describes computer implementations of the theory and presents experimental results corroborating its, main principles.

This article presents a theory of how individuals reason from inconsistency to consistency. The theory is based on 3 main principles. First, individuals try to construct a single mental model of a possibility that satisfies a current set of propositions, and if the task is impossible, they infer that the set is inconsistent. Second, when an inconsistency arises from an incontrovertible fact, they retract any singularly dubious proposition or any proposition that is inconsistent with the fact; otherwise, they retract whichever proposition mismatches the fact. A mismatch can arise from a proposition that has only mental models that conflict with the fact or fail to represent it. Third, individuals use their causal knowledge--in the form of models of possibilities--to create explanations of what led to the inconsistency. A computer program implements the theory, and experimental results support each of its principles.

Co-reference occurs when two or more noun phrases refer to the same individual, as in the following inferential problem: Mark is kneeling by the fire or he is looking at the TV but not both. / Mark is kneeling by the fire. / Is he looking at the TV? In three experiments, we compared co-referential reasoning problems with problems referring to different individuals. Experiment 1 showed that co-reference improves accuracy. In Experiment 2, we replicated that finding and showed that co-reference speeds up both reading and inference. Experiment 3 showed that the effects of co-reference are greatest when the premises and the conclusion share co-referents. These effects led the participants to make illusory inferences--that is, to draw systematically invalid conclusions. The results are discussed in terms of the mental model theory of reasoning.

How do logically naive individuals determine that an inference is invalid? In logic, there are two ways to proceed: (1) make an exhaustive search but fail to find a proof of the conclusion and (2) use the interpretation of the relevant sentences to construct a counterexample--that is, a possibility consistent with the premises but inconsistent with the conclusion. We report three experiments in which the strategies that individuals use to refute invalid inferences based on sentential connectives were examined. In Experiment 1, the participants' task was to justify their evaluations, and it showed that they used counterexamples more often than any other strategy. Experiment 2 showed that they were more likely to use counterexamples to refute invalid conclusions consistent with the premises than to refute invalid conclusions inconsistent with the premises. In Experiment 3, no reliable difference was detected in the results between participants who wrote justifications and participants who did not.

This article presents a theory of how individuals detect whether descriptions of an entity are consistent or inconsistent. The theory postulates that individuals try to construct a mental model of the entity in which all the propositions are true. If they succeed, they infer that the description is consistent; otherwise, they infer that it is inconsistent. We report three experiments that corroborated the theory. Experiment 1 confirmed that evaluating consistency is easier when an initial model suffices than when reasoners have to find an alternative model. Experiment 2 established the occurrence of illusory inferences about the properties of entities. Experiment 3 showed that the illusions correspond to mental models of the assertions, even when these models are wrong because they fail to represent what is false.

The authors outline a theory of conditionals of the form If A then C and If A then possibly C. The 2 sorts of conditional have separate core meanings that refer to sets of possibilities. Knowledge, pragmatics, and semantics can modulate these meanings. Modulation can add information about temporal and other relations between antecedent and consequent. It can also prevent the construction of possibilities to yield 10 distinct sets of possibilities to which conditionals can refer. The mental representation of a conditional normally makes explicit only the possibilities in which its antecedent is true, yielding other possibilities implicitly. Reasoners tend to focus on the explicit possibilities. The theory predicts the major phenomena of understanding and reasoning with conditionals.

Four experiments examined the strategies that individuals develop in sentential reasoning. They led to the discovery of five different strategies. According to the theory proposed in the paper, each of the strategies depends on component tactics, which all normal adults possess, and which are based on mental models. Reasoners vary their use of tactics in ways that have no deterministic account. This variation leads different individuals to assemble different strategies, which include the construction of incremental diagrams corresponding to mental models, and the pursuit of the consequences of a single model step by step. Moreover, the difficulty of a problem (i.e., the number of mental models required by the premises) predisposes reasoners towards certain strategies. Likewise, the sentential connectives in the premises also bias reasoners towards certain strategies, e.g., conditional premises tend to elicit reasoning step by step whereas disjunctive premises tend to elicit incremental diagrams. Keywords: Strategies; Reasoning; Mental models