Grant begins chapter 1 with the story of Mike Lazaridis, a tech-savvy young man who decided to drop out of college months before finishing his electrical engineering degree. Lazaridis went on to invent the BlackBerry phone, which at one point dominated the US smartphone market. Unfortunately, BlackBerry’s failure to adapt led to its market share dropping drastically in 2014. Grant maintains that although Lazaridis’s mode of thinking was once revolutionary, his failure to rethink in a rapidly evolving tech landscape led to the BlackBerry’s obsolescence.

In this new age of information and technology, Grant asserts, it is integral for us to learn to adapt to accelerating changes to widely accepted knowledge and assumptions—such as Pluto’s status as a planet. Grant and his colleague, Phil Tetlock, discovered that we usually take on one of three roles when we think and talk: that of a preacher, prosecutor, or politician. People become preachers when they advocate for beliefs and assumptions they hold sacred, while they become prosecutors when arguing against the perceived flaws of others. Finally, they become politicians when they are trying to court the favor or approval of their audience.

Grant takes as an example the case of Stephen Greenspan, who, against the warnings of his financial adviser friend, invested nearly a third of his savings in a retirement fund which turned out to be a Ponzi scheme managed by Bernie Madoff. He maintains that each of the three modes (preacher, prosecutor, and politician) contributed to Greenspan's ill-fated decision to invest in the fund. The key to eluding such gullibility, Grant asserts, is to adopt the mode of a scientist.

Apart from a profession, being a scientist is also a frame of mind that prioritizes testing and rethinking hypotheses. In an experiment conducted by European researchers, more than a hundred Italian startup founders were gathered for an entrepreneurship training program wherein one group was taught “scientific thinking” while the other wasn’t. In the following year, they found that the “scientific thinking” group gained much, much more in revenue than that of the control group. Instead of preaching, prosecuting, and politicking, Grant asserts that entrepreneurs would benefit more from science-based business models. While entrepreneurs are often seen as strong-minded figures, the most successful ones adopt the carefulness and flexibility of scientists.

Grant returns to Mike Lazaridis, who refused his engineers’ call to incorporate certain features in the BlackBerry, such as an Internet browser, encrypted text messages, and entertainment apps. Although Lazaridis had been a tech prodigy since childhood, he was only able to rethink in terms of engineering and design—and not market conditions. Following this, Grant asserts that intelligence is often an obstacle to mental dexterity. He cites confirmation bias (seeing what one expects to see) and desirability bias (seeing what one wants to see) as the two culprits, compounded by the fact that intelligence often leads to the inability to perceive the limits of one’s understanding. To adopt the mode of the scientist is to become actively open-minded, foregoing intuition for evidence. Grant then cites examples and asserts that what great scientists and great US presidents have in common is their cognitive flexibility.

Finally, Grant explains that the process of rethinking begins with intellectual humility. This humility leads to doubt, which leads to curiosity and, finally, discovery. Often, however, people indulge in overconfidence cycles instead of rethinking cycles—as in the case of Mike Lazaridis. His pride in his prized invention led to his company’s downfall, as he was convinced that the BlackBerry’s keyboard was too iconic to forego in favor of touchscreen models. Conversely, Apple’s team of engineers was able to successfully advocate for building a mobile phone model of their iPod after assuring Steve Jobs that it wouldn’t alter the core identity of the company. The iPhone turned out to be a massive success, accounting for almost half of Apple’s revenue only a few years after its release. Grant ends the chapter by emphasizing the importance of open-mindedness to the practice of rethinking.