 During the 18th century, the humoral theory of melancholia was increasingly being challenged by mechanical and electrical explanations; references to dark and gloomy states gave way to ideas of slowed circulation and depleted energy.German physician Johann Christian Heinroth, however, argued melancholia was a disturbance of the soul due to moral conflict within the patient.Eventually, various authors proposed up to 30 different subtypes of melancholia, and alternative terms were suggested and discarded.