Another theme of Kahneman’s book, proclaimed in the title, is the existence in our brains of two independent sytems for organizing knowledge. Kahneman calls them System One and System Two. System One is amazingly fast, allowing us to recognize faces and understand speech in a fraction of a second. It must have evolved from the ancient little brains that allowed our agile mammalian ancestors to survive in a world of big reptilian predators. Survival in the jungle requires a brain that makes quick decisions based on limited information. Intuition is the name we give to judgments based on the quick action of System One. It makes judgments and takes action without waiting for our conscious awareness to catch up with it. The most remarkable fact about System One is that it has immediate access to a vast store of memories that it uses as a basis for judgment. The memories that are most accessible are those associated with strong emotions, with fear and pain and hatred. The resulting judgments are often wrong, but in the world of the jungle it is safer to be wrong and quick than to be right and slow.

System Two is the slow process of forming judgments based on conscious thinking and critical examination of evidence. It appraises the actions of System One. It gives us a chance to correct mistakes and revise opinions. It probably evolved more recently than System One, after our primate ancestors became arboreal and had the leisure to think things over. An ape in a tree is not so much concerned with predators as with the acquisition and defense of territory. System Two enables a family group to make plans and coordinate activities. After we became human, System Two enabled us to create art and culture.

The question then arises: Why do we not abandon the error-prone System One and let the more reliable System Two rule our lives? Kahneman gives a simple answer to this question: System Two is lazy. To activate System Two requires mental effort. Mental effort is costly in time and also in calories. Precise measurements of blood chemistry show that consumption of glucose increases when System Two is active. Thinking is hard work, and our daily lives are organized so as to economize on thinking. Many of our intellectual tools, such as mathematics and rhetoric and logic, are convenient substitutes for thinking. So long as we are engaged in the routine skills of calculating and talking and writing, we are not thinking, and System One is in charge. We only make the mental effort to activate System Two after we have exhausted the possible alternatives.

Freeman Dyson on Daniel Kahneman. Or, Freeman Dyson explains internet commenting.