Irrationality Book Summary - Irrationality Book explained in key points
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Irrationality summary

Stuart Sutherland

The Enemy Within

4.3 (61 ratings)
23 mins

What is Irrationality about?

Irrationality (1991) is a guide to illogical decisions, unreasonable actions and irrational behavior as a whole. These blinks reveal how people tend to be more irrational than rational, examines several reasons why and offers solutions as to how we can become a little more logical in our decision making.

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    Key idea 1 of 10

    Irrationality is more common than people think, and it’s the result of ignoring knowledge.

    We tend to think that human beings are deeply rational beings, and the basis for this idea can be traced back thousands of years. From Aristotle’s claim that man is a “rational animal,” to Descartes’ famous statement, “I think, therefore I am,” to Kant’s “have the courage to use your own reason,” a common belief throughout history has been that humans are fundamentally rational.

    But this might not be true after all.

    When we meet a new person, we often judge them in an instant, based solely on their looks. This is both exceedingly common and totally irrational, as people often turn out to behave very differently than they appear.

    So, clearly we’re prone to irrationality – but what does that word actually mean?

    In a nutshell, irrationality means deliberately forming conclusions that aren’t based on knowledge. So, it might be rational for a young boy to climb a tree to try to touch the moon. But the same action would be absolutely irrational if performed by an adult astronomer who knows how far away the moon is.

    Basically, the breadth of our knowledge plays a role in determining the rationality of our actions. But, while rational thinking is based on knowledge, rational conclusions can also be false. For example, people long believed that all swans were white; this was a completely rational assumption until the Australian continent was discovered, where black swans are common.

    This is an example of how insufficient or false knowledge can produce false conclusions, despite rational thinking. But false rationality is not to be confused with irrationality. Irrational thinking differs in the sense that it’s deliberate.

    Inadvertently forgetting to carry the one when doing math will produce an error – but it wouldn’t be the result of irrationality. Irrationality would be to ignore knowledge despite its ample presence, such as by thinking someone is unsuited for a job despite her résumé being a perfect fit for the requirements.

    So, making an irrational decision requires deliberate action. But why do humans do this?

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    Best quote from Irrationality

    Everybody is irrational some of the time, and the more complex the decisions to be taken, the more irrational they are.

    —Stuart Sutherland
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    About the Author

    Stuart Sutherland was a renowned psychologist and writer who taught at Oxford University and the University of Sussex. He is best known for his book Irrationality and a personal account of his struggle with manic depression, titled Breakdown.

    Who should read Irrationality?

    • People who think of themselves as entirely rational beings
    • Anyone who struggles to make decisions, especially rational ones

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