Mental models represent possibilities, and the theory of mental models postulates three systems of mental processes underlying inference: (0) the construction of an intensional representation of a premise's meaning -a process guided by a parser; (1) the building of an initial mental model from the intension, and the drawing of a conclusion based on heuristics and the model; and (2) on some occasions, the search for alternative models, such as a counterexample in which the conclusion is false.

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... stem 0 is linguistic, and it may be autonomous. System 1 is rapid and prone to systematic errors, because it makes no use of a working memory for intermediate results. System 2 has access to working memory, and so it can carry out recursive processes, such as the construction of alternative models. However, it too is fallible when the limited processing capacity of working memory becomes overburdened. The three systems are embodied in a unified computational implementation of the model theory, called mReasoner, which is a recent departure in the theory. We review its three systems as they apply to reasoning about the properties of sets of individuals, and we explore how these systems can be extended to other domains of reasoning.