This introduction presents an overview of key concepts covered in the subsequent chapters of this book. The book articulates the basic strands of human nature that can both resonate with the personal and yet be objective. It explains the basis of human suffering as arising from maladaptive deviations in the expression of our individual humanness. The book examines various subdivisions and psychobiological mechanisms of the defence and safety systems, which have special-purpose capabilities for dealing with different contexts. It argues that there are different kinds of threats that social animals have to deal with such as predators and conspecific threats. The book also examines peripheral physiological systems and shows that the pre-existing physiological state of an animal has a major impact on the response it emits to excitatory stimuli. It presents a view called the defensive drift hypothesis which suggests how various symptoms tend to become locked together.