Which individual provided a diagnostic classification system and model for explaining abnormal behavior?

Hippocrates is known for his significant contributions to the early understanding of medical and psychological conditions. He proposed a classification system for psychological and physical ailments that emphasized the balance of bodily fluids, or humors, as a critical factor in mental health. This early attempt at classification laid foundational ideas for how we conceptualize and categorize abnormal behavior in modern psychology.

His model suggested that imbalances in these humors could lead to various forms of mental disturbance, which reflects an early systematic approach to diagnosing and explaining abnormal behavior. This was a pioneering step toward developing diagnostic criteria in psychiatry, influencing later classification systems.

The contributions of other figures in psychology, such as Sigmund Freud with his theories of psychoanalysis, William James with his functionalist approach to psychology, and Carl Rogers with humanistic psychology, did not specifically focus on developing a formal diagnostic classification system for understanding abnormal behavior, making Hippocrates's work stand out in this context.