As individuals, our ability to thrive depended on how well we navigated relationships in a group. If the group valued us, we could count on support, resources, and probably a mate. If it didn't, we might get none of these merits. It was a matter of survival, physically and genetically. Over millions of years, the pressure selected for people who are sensitive to and skilled at maximizing their standing. The result was the development of a tendency to unconsciously monitor how other people in our community perceive us. We process that information in the form of self-esteem and such related emotions as pride, shame, or insecurity. These emotions compel us to do more of what makes our community value us and less of what doesn't. And, crucially, they are meant to make that motivation feel like it is coming from within. If we realized, on a conscious level, that we were responding to social pressure, our performance might come off as grudging or cynical, making it less persuasive.

The common accounts of human nature that float around in society are generally a mixture of assumptions, tales and sometimes plain silliness. However, psychology is different. It is the branch of science that is devoted to understanding people: how and why we act as we do; why we see things as we do; and how we interact with one another. The key word here is ‘science.' Psychologists don't depend on opinions and hearsay, or the generally accepted views of society at the time, or even the considered opinions of deep thinkers. Instead, they look for evidence, to make sure that psychological ideas are firmly based, and not just derived from generally held beliefs or assumptions. In addition to this evidence-based approach, psychology deals with fundamental processes and principles that generate our rich cultural and social diversity, as well as those shared by all human beings. These are what modern psychology is all about.

Life is what physicists might call a ‘high-dimensional system,' which is their fancy way of saying that there's a lot going on. In just a single cell, the number of possible interactions between different molecules is enormous. Such a system can only hope to be stable if only a smaller number of collective ways of being may emerge. For example, it is only a limited number of tissues and body shapes that may result from the development of a human embryo. In 1942, the biologist Conrad Waddington called this drastic narrowing of outcomes canalization. The organism may switch between a small number of well-defined possible states, but can't exist in random states in between them, rather as a ball in a rough landscape must roll to the bottom of one valley or another. We'll see that this is true also of health and disease: there are many causes of illness, but their manifestations at the physiological and symptomatic levels are often strikingly similar.