Plato (Greek, 420–347 B.C.) was the first disciple of Socrates who suggested that the soul can be divided into three (3) parts: appetite, reason, and temper. Plato also discussed the concept of health as being related to the harmony between the body and the mind. Thus, he has been credited as being the first to propose the concept of mental health.
Lexicon of Psychology - Terms, Treatments, Biographies,