Ostensibly, the theme of the book of what makes us "human", or, better, what makes humans so different from any other species. One can make a long list of features, but Gazzaniga believes that's not the correct answer. A "phase shift" must have occurred at some point that caused human brains to diverge dramatically from any other known brain.

He starts by discussing brain size: our brain is much larger than one would expect by extrapolating brain sizes in other mammals. Two genes have been found to be related to brain size: one first appeared 37,000 years ago, and one appeared about 5,800 years ago. Both these days are relevant for the history of human civilazione: the first one is roughly when humans started producing cultural artifacts, and the second one is roughly when urbanization, writing and agriculture began.

One of the most impressive features of the human brain is its ability to understand other brains, i.e. the ability to construct a "theory of mind" about other people's intentions and feelings. By observing someone's face and behavior, human brains can infer that person's invisible "state of mind", an expression originally introduced by USA psychologists David Premack and Guy Woodruff ("Does the chimpanzee have a theory of mind?", 1978). We don't just build a theory of our own self, but also a theory of other people's selves. Children cry when other children cry, a sign that they identify with their pain. We feel sorry for other people's misfortunes (although not always happy for other people's luck). Premack also discovered that children are prewired with the distinction between "minds" and non-minds, i.e. between sentient beings and inanimate matter. Children treat differently objects that move by themselves and objects that move only when someone moves them. Children tend to see a "motive" behind self-propelled objects. It is a built-in ability to guess the state of mind of another being.

Gazziniga is less credible when he indulges in the anatomical differences between humans and chimps. He notes that air and food share the same channel in the human throat: we risk death by choking but that oddity may have given us more powerful linguistic skills. That's pure speculation. Because of the small size of the human pelvis, birth is difficult and painful. Human newborns are helpless compared with pretty much any other mammal babies. Human babies require a lot of attention and training from their parents before they can "function" properly.

Gazzaniga then introduces his modular model of the brain, that was already the subject of previous books. He thinks that the brain has a modular organization, whereby many independent systems work in parallel. These "specialized" modules are evolutionary additions to the brain: it is no surprise that they perform different functions, and it is not surprising that those functions are useful to the overall functioning of the brain. Split-brain surgery had already proved that the human brain is made of at least two brains (the two hemispheres). Gazzaniga simply extended that idea. Basically, he thinks that many minds coexist in a confederation. Our behavior is due to the activity of these modules, rather than to conscious decisions. A special module, the "interpreter", located in the left hemisphere, interprets the actions of modules and provides explanations for our behavior. However, that happens after the unconscious modules have already determined our behavior. Beliefs do not precede behavior: they follow it. They are created by the interaction of the interpreter with the other modules. Behavior determines our beliefs, not the other way around. It is only by behaving that we conceptualize our selves, that we build a theory of our psychological state (in particular, beliefs).

Another unique human trait is the ability to create complex societies. Gazzinga explores the sources of human societies, from sexual selection to meat eating, hinting that the cognitive demands of the social life fostered an increase in brain size. Gazzaniga mentions Robin Dunbar's theory on what are the "cognitive demands" and constraints of social life. Given the size of the human neocortex, Dunbar believes that we are capable of about 150 close relationships. That's the natural size of a social group. Dunbar believes that language evolved from gossip, which in turn was an evolution for grooming.

Another unique human trait is morality. Moral judgments come natural. They don't require a lot of reasoning. Gazzaniga believes that morality is innate. In fact, it is instantiated in a handful of specialized brain modules. Paul Bloom thinks that morality is hard-wired in the human brain. Empathy is born with life: when babies hear crying, they start to cry themselves. They "feel" the pain of other babies. We feel the pain of other people. It is just natural that we desire for that pain to go away because it affects us, not only them. Morality and altruism are not learned. Humans are endowed at birth with a "moral sense".

Art is another human oddity. Gazzaniga thinks that it has evolutionary value because it helps us to classify the world and therefore it makes us better at learning (as Nicholas Humphrey originally argued). However, it is debatable whether reading a fantasy novel or an experimental novel truly increases our predictive power. It is more likely to confuse us and lead us to irrational behavior. An artist's work might also be valuable the way a peacock's tail is: to enhance one's sexual status among the other sex.

The book ends presenting the predictive-memory theory of Jeff Hawkins.