The document discusses definitions and classifications of mental health disorders. It describes how the World Health Organization (WHO) defines health and mental health. Mental illness can occur when the brain is not functioning well, disrupting thinking, perception, emotion, behavior, physical functioning or signaling. The document also discusses the differences between neurosis (minor mental disorders) and psychosis (major personality disorders), and how psychiatric disorders are classified based on symptoms and etiology. It provides a brief history of classification systems and describes the International Classification of Diseases (ICD) published by WHO and the Diagnostic and Statistical Manual of Mental Disorders (DSM) published by the American Psychiatric Association.