Abu Zayd al-Balkhi was the first to differentiate between neurosis and psychosis, and the first to classify neurotic disorders and pioneer cognitive therapy in order to treat each of these classified disorders. He classified neurosis into four emotional disorders: fear and anxiety, anger and aggression, sadness and depression, and obsession. He further classified three types of depression: normal depression or sadness (huzn), endogenous depression originating from within the body, and reactive clinical depression originating from outside the body. He also wrote that a healthy individual should always keep healthy thoughts and feelings in his mind in the case of unexpected emotional outbursts in the same way drugs and First Aid medicine are kept nearby for unexpected physical emergencies. He stated that a balance between the mind and body is required for good health and that an imbalance between the two can cause sickness. Al-Balkhi also introduced the concept of reciprocal inhibition (al-ilaj bi al-did), which was re-introduced over a thousand years later by Joseph Wolpe in 1969.Amber Haque (2004), "Psychology from Islamic Perspective: Contributions of Early Muslim Scholars and Challenges to Contemporary Muslim Psychologists", Journal of Religion and Health 43 (4): 357–377 [362]
