System 1 represents the portion of the human mind that is primarily intuitive, effortless, unconscious, and fast. System 1 embodies what Kahneman means by “fast thinking.” It operates through associative reasoning, seeks causal connection between events, understands groupings as the average individual of the grouping, and follows other psychological processes to produce predictable errors, despite being fairly accurate much of the time. System 1 shapes reality in the sense that it generates much of what a person becomes consciously aware of, including recognition of incongruity that triggers more deliberative thought.

System 2 is the complement to System 1 and represents the slow thinking of the rational mind. It requires effort to perform, as exemplified by the process of solving math problems. System 2 can override the quick-thinking System 1, but only if a person becomes aware of a reason to do so. System 1 can sometimes create such strong impressions that System 2 cannot effectively override them. Further, System 2 is generally very lazy and will avoid expending effort whenever possible.

A heuristic is similar to a rule of thumb. Kahneman provides a “technical definition” of the term: A heuristic is “a simple procedure that helps find adequate, though often imperfect, answers to difficult questions” (98).