Final answer:
Emil Kraepelin classified mental disorders in the early days of psychiatry using the concept of monomania based on physical appearances.
Step-by-step explanation:
In the early days of psychiatry in the 1900s, Emil Kraepelin classified mental disorders based on the concept of monomania. Monomania referred to a specific fixation that led individuals to exhibit delusional behavior, such as imagining themselves to be a king. This classification system was based on the now discredited science of physiognomy, which held that physical appearances could be used to diagnose mental disorders. Kraepelin's classification system was influential in shaping the field of psychiatry in the early 20th century.