Brooks and Winfrey explain how metacognition, or the act of “thinking about thinking” (32) can lead to greater happiness, for although people cannot control external events or circumstances, they can control their own internal thoughts and reactions. The authors describe a concept known as the “triune brain,” which was developed by neuroscientist Paul D. MacLean in the 1970s. This model suggests that three distinct parts of the human brain contribute to the formation of both thoughts and behaviors: the brain stem, or reptilian brain; the limbic system, or paleomammalian brain; and the neocortex, or neomammalian brain. MacLean believed that the reptilian brain was the most primal part, sharing commonalities with the brains of reptiles, and that the neocortex was the most human. While modern research has largely invalidated the triune brain theory, Winfrey and Brooks contend that it is still useful to contemplate the brain as having three functions—detection, reaction, and decision—that work together to keep a person safe. Detection and reaction are split-second, instinctual responses, while decision-making involves conscious thought and reasoning.

The authors explain that emotions are nothing more than “signals to your conscious brain that something is going on that requires your attention and action” (38).