Critical thinking skill is exemplified by asking questions about alternative possibilities in order to reliably achieve some objective. Asking and answering questions is a skill of dialogue. Alternative possibilities are represented by mental models. A process of questioning mental models is (or should be) adopted because of its reliability for achieving the purposes of the participants within the available time. Thus, the theory of critical thinking draws on and synthesizes research on three separate topics:

1. Theories of reasoning according to which people represent information about a problem or situation by means of mental models of alternative possibilities, evaluate the models in the light of relevant background knowledge, update the models by adding new information as it becomes available, revise models to resolve internal inconsistencies, and draw conclusions by inspecting the surviving possibilities (adapted from Johnson-Laird, 1983; Johnson-Laird & Byrne, 1991).

2. Theories of critical discussion in which a proponent must defend a claim against challenges of various kinds by an opponent or critic (adapted from Rescher, 1977; Walton & Krabbe, 1995; van Eemeren & Grootendorst, 1992; Walton, 1998).

3. Theories of the cognitive mechanisms and processes involved in belief formation and decision making, which vary in their reliability or their association with proficient performance in a domain (adapted from Simon, 1997; Gigerenzer & Selten, 2001; Ericsson & Smith, 1991; Klein et al., 1993; Payne, Bettman, & Johnson, 1993).

Critical thinking has a multi-layered structure . The three aspects of the theory form a spectrum from internal ratinality or coherence (mental model theory) to intersubjective dialogue to correspondence with external reality (reliability). The concept of critical thinking as internal or external dialogue forms the crucial brdige.

A model of critical thinking with three embedded layers: mental models, critical dialogue, and control based on reliability.

All three of these aspects involve both empirical and normative elements. In particular, each of the three layers is associated with distinctive criteria of performance, which progress from internal to external in their focus:

1. At its innermost core critical thinking involves representation of alternative possible states of affairs, or mental models. The key metric of performance at this level is the explanatory coherence of mental models and the coherence between mental models and background beliefs. Errors occur when cognizers overlook alternative possibilities or fail to properly assess the relative plausibility of different mental models, including their comprehensiveness and simplicity as explanations.

2. At the intermediate level, mental models are embedded within a layer of critical questioning which motivates the generation and evaluation of possibilities. Critical questioning may take place within a single mind among different individuals, but is evaluated by reference to norms for conducting the appropriate kinds of critical dialogue. Dialogue types are differentiated by the purposes they serve, by the types of challenges that are permitted to the opponent, and the scope of the permitted responses by the proponent. At this level of analysis, errors occur when cognizers fail to ask or answer questions associated with the appropriate argumentation scheme, use argumentation schemes that obstruct the purpose of the dialogue, or inappropriately shift from one dialogue type to another (Walton, 1998).

3. At the outermost layer, critical thinking is a judgment about the reliability of a cognitive process or faculty, hence, the degree of trust that should be placed in its outputs. A critical dialogue is only one of various cognitive or social processes that might be utilized alone or in combination to generate beliefs and decisions. Non-deliberative processes, such as pattern recognition, may be more reliable under some conditions and can almost always be used to verify the results of reasoning - just as reasoning is used to check the results of intuition. At this level, errors occur when cognizers use inappropriate or inefficient strategies, and when they terminate a process too soon or continue it too long.

In sum, critical thinking skill is exemplified by asking and answering critical questions about alternative possible states of affairs, with the intent of achieving the purpose of an on-going activity.