INTERVIEWER: Can you describe the two–system theory?

KAHNEMAN: Many of us who study the subject think that there are two thinking systems, which actually have two very different characteristics. You can call them intuition and reasoning, although some of us label them System 1 and System 2. There are some thoughts that come to mind on their own; most thinking is really like that, most of the time. That’s System 1. It’s not like we’re on automatic pilot, but we respond to the world in ways that we’re not conscious of, that we don’t control. The operations of System 1 are fast, effortless, associative, and often emotionally charged; they’re also governed by habit, so they’re difficult either to modify or to control. There is another system, System 2, which is the reasoning system. It’s conscious, it’s deliberate; it’s slower, serial, effortful, and deliberately controlled, but it can follow rules. The difference in effort provides the most useful indicator of whether a given mental process should be assigned to System 1 or System 2.

Empirical controversy about the reality of cognitive illusions dissolves when viewed in the perspective of a dual-process model. System 2 'knows" some of the rules that intuitive reasoning is prone to violate, and sometimes intervenes to correct or replace erroneous intuitive judgments. Thus, errors of intuition occur when two conditions are satisfied: System 1 generates the error and System 2 fails to correct. In this view, the experiments in which cognitive illusions are "made to disappear" do so by facilitating the corrective operations of System 2.