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“Deduction is that mode of reasoning which examines the state of things asserted in the premisses, forms a diagram of that state of things, perceives in the parts of the diagram relations not explicitly mentioned in the premisses, satisfies itself by mental experiments upon the diagram that these relations would always subsist, …and concludes their necessary, or probable, truth.” – Charles Sanders Peirce

At the Mental Models and Reasoning website, we report studies of the strengths and failures of human reasoning. We focus on basic research and experimental data, as well as on developing computational models of higher-level cognitive processes. The webpage was set up, and continues to be maintained, by Sunny Khemlani (, in 2006 when he was a graduate student in the Mental Models and Reasoning Lab at Princeton. The lab, but not the webpage, was closed on July 1st 2012 when Phil Johnson-Laird became emeritus and a visiting scholar at NYU. Many former members of the lab, however, continue to carry out research on models and reasoning, and thanks to the good offices of Princeton and of Dr. Khemlani, the site continues to be a repository of results, theories, and computer models.

Experimental data in cognitive psychology suggest that humans reason and solve problems through the use of internal representations that can be mentally scrutinized and processed. The theory that posits the existence of such models and the mechanization thereof, known as mental model theory, was established by Philip Johnson-Laird (1983) and has proven extremely powerful in predicting and explaining higher-level cognition in humans.

This theory departs from the standard view, which assumes that there is a formal logical or probabilistic calculus in the mind. “Formal rules of inference play no part in inferences,” says Johnson-Laird, “though from Piaget onwards, psychologists have proposed theories based on them.”