Two System Models of Reasoning

Abundant evidence suggests that there are two distinct systems of human reasoning, which can be referred to as intuition and deliberation. The evidence comes from studies of deductive and inductive reasoning, decision making, categorization, problem solving, probability and moral judgment, and planning. Most of the evidence is behavioral, although a little comes from cognitive neuroscience. The distinction rests on a set of properties that characterize each system (see Table 1). The intuitive system is designed to make quick and dirty assessments based on similarity and what can be directly retrieved from memory. It relies more on observable properties and well-ingrained schematic knowledge. The deliberative system is slower and more analytic. It depends directly on learned systems of rules, and its information processing is highly selective. We have conscious access not only to its products but also to its inner workings. The intuitive system is likely more evolutionarily primitive than the deliberative system, has more in common with other animals, and includes a greater proportion of older brain structures. This [Page 746]entry provides an overview of the evidence for this characterization.