From the outset, the book focuses on grounding the reader in the biological context of human experience. The very first chapter discusses the human brain, explaining how it is structured and organized. With help of an illustration of an upside-down triangle segmented into four parts, Perry explains how the brain increases in complexity of function from the bottom to the top: The brainstem, responsible for automatic, regulatory functions; the diencephalon, which works with states of arousal; the limbic system, where one’s emotions are seated, as is the reward-response system; and the cortex, the uniquely human layer responsible for all higher-order cognitive processes, from language and memory to values, beliefs, and the ability to think about time. Perry explains the extreme malleability of the human cortex, allowing it to be shaped and reshaped by experience. This feature, the neuroplasticity of the human brain, is what has allowed generations of human beings to not only learn and accumulate skills, but also pass them down to future generations in a continuing sociocultural evolution.

Despite the cortex being the smartest part of the human brain, it is the last to develop; this is a function of its sequential nature.