Thinking, Fast and Slow

By Daniel Kahneman

(Farrar, Straus and Giroux, 499 pp., $30)

Humans are rational animals. In addition to the rich perceptual, cognitive, and motivational systems that they share with other creatures, they have a unique way of forming beliefs and preferences that is not instinctive but deliberate. They can form beliefs by logical inference, assessment of evidence, and statistical judgments of likelihood; and they can form preferences and make choices by the principled evaluation of alternative courses of action, on the basis of their rationally formed beliefs about the likely outcomes of those alternatives. The results of all these forms of reasoning can be preserved, recorded, and passed on to others, making possible the growth of knowledge and civilization. Reason has created our world.

But reason depends on a constant supply of material from our pre-rational, animal nature—from perception, feeling, and natural desires. There is also an intermediate level of automatic judgment, some of it learned through experience, that operates more quickly than conscious reasoning and is essential for navigating the world in real time. The relation among these faculties is complicated. Even when we think we are using reason to arrive at the right answer to some factual or practical question—taking the relevant data consciously into account—our reasoning may be influenced more directly, without our knowledge, by the instinctive forces with which it coexists.

Daniel Kahneman is one of the psychologists who has done the most to advance our understanding of how this complex set of mental factors works and some of the problems that arise in the interaction among its components. His name will always be linked with that of Amos Tversky, whose early death in 1996 at the age of fifty-nine brought their long intellectual collaboration to a close. This book is dedicated to Tversky’s memory, and it is a tribute to him and an admirable account of their work together. It is also a clear, comprehensive, and often witty introduction to an interesting area of research, written by a leading contributor with exceptional expository gifts.

REASON IS traditionally divided into two types, theoretical and practical, which control the formation of beliefs and the determination of choices, respectively. It is a scarce resource in each of us. Most of the time, in most respects, we have to operate on autopilot, because we cannot spare the conscious attention to identify and weigh up the pros and cons for everything we do or think. Kahneman’s aim in his book is not just theoretical but also practical. He wants to provide us with a degree of self-understanding that will permit us to manage our thoughts and choices better, both individually and collectively. He says he cringes when his work with Tversky “is credited with demonstrating that human choices are irrational.” This may seem an odd reaction, since the pair certainly showed that humans are systematically prone to make certain mistakes. But mistakes are not irrational until one has, and fails to use, the means to avoid them. Kahneman’s aim is not to criticize human nature, but to identify features of the way human beings function that had not been recognized by influential theories of rational choice, and to suggest how we can guard against some of their untoward consequences.